1<!DOCTYPE Article PUBLIC "-//Davenport//DTD DocBook V3.0//EN"> 2 3<Article> 4 5<ArtHeader> 6 7<Title>The extended-2 filesystem overview</Title> 8<AUTHOR 9> 10<FirstName>Gadi Oxman, tgud@tochnapc2.technion.ac.il</FirstName> 11</AUTHOR 12> 13<PubDate>v0.1, August 3 1995</PubDate> 14 15</ArtHeader> 16 17<Sect1> 18<Title>Preface</Title> 19 20<Para> 21This document attempts to present an overview of the internal structure of 22the ext2 filesystem. It was written in summer 95, while I was working on the 23<Literal remap="tt">ext2 filesystem editor project (EXT2ED)</Literal>. 24</Para> 25 26<Para> 27In the process of constructing EXT2ED, I acquired knowledge of the various 28design aspects of the the ext2 filesystem. This document is a result of an 29effort to document this knowledge. 30</Para> 31 32<Para> 33This is only the initial version of this document. It is obviously neither 34error-prone nor complete, but at least it provides a starting point. 35</Para> 36 37<Para> 38In the process of learning the subject, I have used the following sources / 39tools: 40 41<ItemizedList> 42<ListItem> 43 44<Para> 45 Experimenting with EXT2ED, as it was developed. 46</Para> 47</ListItem> 48<ListItem> 49 50<Para> 51 The ext2 kernel sources: 52 53<ItemizedList> 54<ListItem> 55 56<Para> 57 The main ext2 include file, 58<FILENAME>/usr/include/linux/ext2_fs.h</FILENAME> 59</Para> 60</ListItem> 61<ListItem> 62 63<Para> 64 The contents of the directory <FILENAME>/usr/src/linux/fs/ext2</FILENAME>. 65</Para> 66</ListItem> 67<ListItem> 68 69<Para> 70 The VFS layer sources (only a bit). 71</Para> 72</ListItem> 73 74</ItemizedList> 75 76</Para> 77</ListItem> 78<ListItem> 79 80<Para> 81 The slides: The Second Extended File System, Current State, Future 82Development, by <personname><firstname>Remy</firstname> <surname>Card</surname></personname>. 83</Para> 84</ListItem> 85<ListItem> 86 87<Para> 88 The slides: Optimisation in File Systems, by <personname><firstname>Stephen</firstname> <surname>Tweedie</surname></personname>. 89</Para> 90</ListItem> 91<ListItem> 92 93<Para> 94 The various ext2 utilities. 95</Para> 96</ListItem> 97 98</ItemizedList> 99 100</Para> 101 102</Sect1> 103 104<Sect1> 105<Title>Introduction</Title> 106 107<Para> 108The <Literal remap="tt">Second Extended File System (Ext2fs)</Literal> is very popular among Linux 109users. If you use Linux, chances are that you are using the ext2 filesystem. 110</Para> 111 112<Para> 113Ext2fs was designed by <personname><firstname>Remy</firstname> <surname>Card</surname></personname> and <personname><firstname>Wayne</firstname> <surname>Davison</surname></personname>. It was 114implemented by <personname><firstname>Remy</firstname> <surname>Card</surname></personname> and was further enhanced by <personname><firstname>Stephen</firstname> 115<surname>Tweedie</surname></personname> and <personname><firstname>Theodore</firstname> <surname>Ts'o</surname></personname>. 116</Para> 117 118<Para> 119The ext2 filesystem is still under development. I will document here 120version 0.5a, which is distributed along with Linux 1.2.x. At this time of 121writing, the most recent version of Linux is 1.3.13, and the version of the 122ext2 kernel source is 0.5b. A lot of fancy enhancements are planned for the 123ext2 filesystem in Linux 1.3, so stay tuned. 124</Para> 125 126</Sect1> 127 128<Sect1> 129<Title>A filesystem - Why do we need it?</Title> 130 131<Para> 132I thought that before we dive into the various small details, I'll reserve a 133few minutes for the discussion of filesystems from a general point of view. 134</Para> 135 136<Para> 137A <Literal remap="tt">filesystem</Literal> consists of two word - <Literal remap="tt">file</Literal> and <Literal remap="tt">system</Literal>. 138</Para> 139 140<Para> 141Everyone knows the meaning of the word <Literal remap="tt">file</Literal> - A bunch of data put 142somewhere. where? This is an important question. I, for example, usually 143throw almost everything into a single drawer, and have difficulties finding 144something later. 145</Para> 146 147<Para> 148This is where the <Literal remap="tt">system</Literal> comes in - Instead of just throwing the data 149to the device, we generalize and construct a <Literal remap="tt">system</Literal> which will 150virtualize for us a nice and ordered structure in which we could arrange our 151data in much the same way as books are arranged in a library. The purpose of 152the filesystem, as I understand it, is to make it easy for us to update and 153maintain our data. 154</Para> 155 156<Para> 157Normally, by <Literal remap="tt">mounting</Literal> filesystems, we just use the nice and logical 158virtual structure. However, the disk knows nothing about that - The device 159driver views the disk as a large continuous paper in which we can write notes 160wherever we wish. It is the task of the filesystem management code to store 161bookkeeping information which will serve the kernel for showing us the nice 162and ordered virtual structure. 163</Para> 164 165<Para> 166In this document, we consider one particular administrative structure - The 167Second Extended Filesystem. 168</Para> 169 170</Sect1> 171 172<Sect1> 173<Title>The Linux VFS layer</Title> 174 175<Para> 176When Linux was first developed, it supported only one filesystem - The 177<Literal remap="tt">Minix</Literal> filesystem. Today, Linux has the ability to support several 178filesystems concurrently. This was done by the introduction of another layer 179between the kernel and the filesystem code - The Virtual File System (VFS). 180</Para> 181 182<Para> 183The kernel "speaks" with the VFS layer. The VFS layer passes the kernel's 184request to the proper filesystem management code. I haven't learned much of 185the VFS layer as I didn't need it for the construction of EXT2ED so that I 186can't elaborate on it. Just be aware that it exists. 187</Para> 188 189</Sect1> 190 191<Sect1> 192<Title>About blocks and block groups</Title> 193 194<Para> 195In order to ease management, the ext2 filesystem logically divides the disk 196into small units called <Literal remap="tt">blocks</Literal>. A block is the smallest unit which 197can be allocated. Each block in the filesystem can be <Literal remap="tt">allocated</Literal> or 198<Literal remap="tt">free</Literal>. 199<FOOTNOTE> 200 201<Para> 202The Ext2fs source code refers to the concept of <Literal remap="tt">fragments</Literal>, which I 203believe are supposed to be sub-block allocations. As far as I know, 204fragments are currently unsupported in Ext2fs. 205</Para> 206 207</FOOTNOTE> 208 209The block size can be selected to be 1024, 2048 or 4096 bytes when creating 210the filesystem. 211</Para> 212 213<Para> 214Ext2fs groups together a fixed number of sequential blocks into a <Literal remap="tt">group 215block</Literal>. The resulting situation is that the filesystem is managed as a 216series of group blocks. This is done in order to keep related information 217physically close on the disk and to ease the management task. As a result, 218much of the filesystem management reduces to management of a single blocks 219group. 220</Para> 221 222</Sect1> 223 224<Sect1> 225<Title>The view of inodes from the point of view of a blocks group</Title> 226 227<Para> 228Each file in the filesystem is reserved a special <Literal remap="tt">inode</Literal>. I don't want 229to explain inodes now. Rather, I would like to treat it as another resource, 230much like a <Literal remap="tt">block</Literal> - Each blocks group contains a limited number of 231inode, while any specific inode can be <Literal remap="tt">allocated</Literal> or 232<Literal remap="tt">unallocated</Literal>. 233</Para> 234 235</Sect1> 236 237<Sect1> 238<Title>The group descriptors</Title> 239 240<Para> 241Each blocks group is accompanied by a <Literal remap="tt">group descriptor</Literal>. The group 242descriptor summarizes some necessary information about the specific group 243block. Follows the definition of the group descriptor, as defined in 244<FILENAME>/usr/include/linux/ext2_fs.h</FILENAME>: 245</Para> 246 247<Para> 248 249<ProgramListing> 250struct ext2_group_desc 251{ 252 __u32 bg_block_bitmap; /* Blocks bitmap block */ 253 __u32 bg_inode_bitmap; /* Inodes bitmap block */ 254 __u32 bg_inode_table; /* Inodes table block */ 255 __u16 bg_free_blocks_count; /* Free blocks count */ 256 __u16 bg_free_inodes_count; /* Free inodes count */ 257 __u16 bg_used_dirs_count; /* Directories count */ 258 __u16 bg_pad; 259 __u32 bg_reserved[3]; 260}; 261</ProgramListing> 262 263</Para> 264 265<Para> 266The last three variables: <Literal remap="tt">bg_free_blocks_count, bg_free_inodes_count and bg_used_dirs_count</Literal> provide statistics about the use of the three 267resources in a blocks group - The <Literal remap="tt">blocks</Literal>, the <Literal remap="tt">inodes</Literal> and the 268<Literal remap="tt">directories</Literal>. I believe that they are used by the kernel for balancing 269the load between the various blocks groups. 270</Para> 271 272<Para> 273<Literal remap="tt">bg_block_bitmap</Literal> contains the block number of the <Literal remap="tt">block allocation 274bitmap block</Literal>. This is used to allocate / deallocate each block in the 275specific blocks group. 276</Para> 277 278<Para> 279<Literal remap="tt">bg_inode_bitmap</Literal> is fully analogous to the previous variable - It 280contains the block number of the <Literal remap="tt">inode allocation bitmap block</Literal>, which 281is used to allocate / deallocate each specific inode in the filesystem. 282</Para> 283 284<Para> 285<Literal remap="tt">bg_inode_table</Literal> contains the block number of the start of the 286<Literal remap="tt">inode table of the current blocks group</Literal>. The <Literal remap="tt">inode table</Literal> is 287just the actual inodes which are reserved for the current block. 288</Para> 289 290<Para> 291The block bitmap block, inode bitmap block and the inode table are created 292when the filesystem is created. 293</Para> 294 295<Para> 296The group descriptors are placed one after the other. Together they make the 297<Literal remap="tt">group descriptors table</Literal>. 298</Para> 299 300<Para> 301Each blocks group contains the entire table of group descriptors in its 302second block, right after the superblock. However, only the first copy (in 303group 0) is actually used by the kernel. The other copies are there for 304backup purposes and can be of use if the main copy gets corrupted. 305</Para> 306 307</Sect1> 308 309<Sect1> 310<Title>The block bitmap allocation block</Title> 311 312<Para> 313Each blocks group contains one special block which is actually a map of the 314entire blocks in the group, with respect to their allocation status. Each 315<Literal remap="tt">bit</Literal> in the block bitmap indicated whether a specific block in the 316group is used or free. 317</Para> 318 319<Para> 320The format is actually quite simple - Just view the entire block as a series 321of bits. For example, 322</Para> 323 324<Para> 325Suppose the block size is 1024 bytes. As such, there is a place for 3261024*8=8192 blocks in a group block. This number is one of the fields in the 327filesystem's <Literal remap="tt">superblock</Literal>, which will be explained later. 328</Para> 329 330<Para> 331 332<ItemizedList> 333<ListItem> 334 335<Para> 336 Block 0 in the blocks group is managed by bit 0 of byte 0 in the bitmap 337block. 338</Para> 339</ListItem> 340<ListItem> 341 342<Para> 343 Block 7 in the blocks group is managed by bit 7 of byte 0 in the bitmap 344block. 345</Para> 346</ListItem> 347<ListItem> 348 349<Para> 350 Block 8 in the blocks group is managed by bit 0 of byte 1 in the bitmap 351block. 352</Para> 353</ListItem> 354<ListItem> 355 356<Para> 357 Block 8191 in the blocks group is managed by bit 7 of byte 1023 in the 358bitmap block. 359</Para> 360</ListItem> 361 362</ItemizedList> 363 364</Para> 365 366<Para> 367A value of "<Literal remap="tt">1</Literal>" in the appropriate bit signals that the block is 368allocated, while a value of "<Literal remap="tt">0</Literal>" signals that the block is 369unallocated. 370</Para> 371 372<Para> 373You will probably notice that typically, all the bits in a byte contain the 374same value, making the byte's value <Literal remap="tt">0</Literal> or <Literal remap="tt">0ffh</Literal>. This is done by 375the kernel on purpose in order to group related data in physically close 376blocks, since the physical device is usually optimized to handle such a close 377relationship. 378</Para> 379 380</Sect1> 381 382<Sect1> 383<Title>The inode allocation bitmap</Title> 384 385<Para> 386The format of the inode allocation bitmap block is exactly like the format of 387the block allocation bitmap block. The explanation above is valid here, with 388the work <Literal remap="tt">block</Literal> replaced by <Literal remap="tt">inode</Literal>. Typically, there are much less 389inodes then blocks in a blocks group and thus only part of the inode bitmap 390block is used. The number of inodes in a blocks group is another variable 391which is listed in the <Literal remap="tt">superblock</Literal>. 392</Para> 393 394</Sect1> 395 396<Sect1> 397<Title>On the inode and the inode tables</Title> 398 399<Para> 400An inode is a main resource in the ext2 filesystem. It is used for various 401purposes, but the main two are: 402 403<ItemizedList> 404<ListItem> 405 406<Para> 407 Support of files 408</Para> 409</ListItem> 410<ListItem> 411 412<Para> 413 Support of directories 414</Para> 415</ListItem> 416 417</ItemizedList> 418 419</Para> 420 421<Para> 422Each file, for example, will allocate one inode from the filesystem 423resources. 424</Para> 425 426<Para> 427An ext2 filesystem has a total number of available inodes which is determined 428while creating the filesystem. When all the inodes are used, for example, you 429will not be able to create an additional file even though there will still 430be free blocks on the filesystem. 431</Para> 432 433<Para> 434Each inode takes up 128 bytes in the filesystem. By default, <Literal remap="tt">mke2fs</Literal> 435reserves an inode for each 4096 bytes of the filesystem space. 436</Para> 437 438<Para> 439The inodes are placed in several tables, each of which contains the same 440number of inodes and is placed at a different blocks group. The goal is to 441place inodes and their related files in the same blocks group because of 442locality arguments. 443</Para> 444 445<Para> 446The number of inodes in a blocks group is available in the superblock variable 447<Literal remap="tt">s_inodes_per_group</Literal>. For example, if there are 2000 inodes per group, 448group 0 will contain the inodes 1-2000, group 2 will contain the inodes 4492001-4000, and so on. 450</Para> 451 452<Para> 453Each inode table is accessed from the group descriptor of the specific 454blocks group which contains the table. 455</Para> 456 457<Para> 458Follows the structure of an inode in Ext2fs: 459</Para> 460 461<Para> 462 463<ProgramListing> 464struct ext2_inode { 465 __u16 i_mode; /* File mode */ 466 __u16 i_uid; /* Owner Uid */ 467 __u32 i_size; /* Size in bytes */ 468 __u32 i_atime; /* Access time */ 469 __u32 i_ctime; /* Creation time */ 470 __u32 i_mtime; /* Modification time */ 471 __u32 i_dtime; /* Deletion Time */ 472 __u16 i_gid; /* Group Id */ 473 __u16 i_links_count; /* Links count */ 474 __u32 i_blocks; /* Blocks count */ 475 __u32 i_flags; /* File flags */ 476 union { 477 struct { 478 __u32 l_i_reserved1; 479 } linux1; 480 struct { 481 __u32 h_i_translator; 482 } hurd1; 483 struct { 484 __u32 m_i_reserved1; 485 } masix1; 486 } osd1; /* OS dependent 1 */ 487 __u32 i_block[EXT2_N_BLOCKS];/* Pointers to blocks */ 488 __u32 i_version; /* File version (for NFS) */ 489 __u32 i_file_acl; /* File ACL */ 490 __u32 i_size_high; /* High 32bits of size */ 491 __u32 i_faddr; /* Fragment address */ 492 union { 493 struct { 494 __u8 l_i_frag; /* Fragment number */ 495 __u8 l_i_fsize; /* Fragment size */ 496 __u16 i_pad1; 497 __u32 l_i_reserved2[2]; 498 } linux2; 499 struct { 500 __u8 h_i_frag; /* Fragment number */ 501 __u8 h_i_fsize; /* Fragment size */ 502 __u16 h_i_mode_high; 503 __u16 h_i_uid_high; 504 __u16 h_i_gid_high; 505 __u32 h_i_author; 506 } hurd2; 507 struct { 508 __u8 m_i_frag; /* Fragment number */ 509 __u8 m_i_fsize; /* Fragment size */ 510 __u16 m_pad1; 511 __u32 m_i_reserved2[2]; 512 } masix2; 513 } osd2; /* OS dependent 2 */ 514}; 515</ProgramListing> 516 517</Para> 518 519<Sect2> 520<Title>The allocated blocks</Title> 521 522<Para> 523The basic functionality of an inode is to group together a series of 524allocated blocks. There is no limitation on the allocated blocks - Each 525block can be allocated to each inode. Nevertheless, block allocation will 526usually be done in series to take advantage of the locality principle. 527</Para> 528 529<Para> 530The inode is not always used in that way. I will now explain the allocation 531of blocks, assuming that the current inode type indeed refers to a list of 532allocated blocks. 533</Para> 534 535<Para> 536It was found experimentally that many of the files in the filesystem are 537actually quite small. To take advantage of this effect, the kernel provides 538storage of up to 12 block numbers in the inode itself. Those blocks are 539called <Literal remap="tt">direct blocks</Literal>. The advantage is that once the kernel has the 540inode, it can directly access the file's blocks, without an additional disk 541access. Those 12 blocks are directly specified in the variables 542<Literal remap="tt">i_block[0] to i_block[11]</Literal>. 543</Para> 544 545<Para> 546<Literal remap="tt">i_block[12]</Literal> is the <Literal remap="tt">indirect block</Literal> - The block pointed by 547i_block[12] will <Literal remap="tt">not</Literal> be a data block. Rather, it will just contain a 548list of direct blocks. For example, if the block size is 1024 bytes, since 549each block number is 4 bytes long, there will be place for 256 indirect 550blocks. That is, block 13 till block 268 in the file will be accessed by the 551<Literal remap="tt">indirect block</Literal> method. The penalty in this case, compared to the 552direct blocks case, is that an additional access to the device is needed - 553We need <Literal remap="tt">two</Literal> accesses to reach the required data block. 554</Para> 555 556<Para> 557In much the same way, <Literal remap="tt">i_block[13]</Literal> is the <Literal remap="tt">double indirect block</Literal> 558and <Literal remap="tt">i_block[14]</Literal> is the <Literal remap="tt">triple indirect block</Literal>. 559</Para> 560 561<Para> 562<Literal remap="tt">i_block[13]</Literal> points to a block which contains pointers to indirect 563blocks. Each one of them is handled in the way described above. 564</Para> 565 566<Para> 567In much the same way, the triple indirect block is just an additional level 568of indirection - It will point to a list of double indirect blocks. 569</Para> 570 571</Sect2> 572 573<Sect2> 574<Title>The i_mode variable</Title> 575 576<Para> 577The i_mode variable is used to determine the <Literal remap="tt">inode type</Literal> and the 578associated <Literal remap="tt">permissions</Literal>. It is best described by representing it as an 579octal number. Since it is a 16 bit variable, there will be 6 octal digits. 580Those are divided into two parts - The rightmost 4 digits and the leftmost 2 581digits. 582</Para> 583 584<Sect3> 585<Title>The rightmost 4 octal digits</Title> 586 587<Para> 588The rightmost 4 digits are <Literal remap="tt">bit options</Literal> - Each bit has its own 589purpose. 590</Para> 591 592<Para> 593The last 3 digits (Octal digits 0,1 and 2) are just the usual permissions, 594in the known form <Literal remap="tt">rwxrwxrwx</Literal>. Digit 2 refers to the user, digit 1 to 595the group and digit 2 to everyone else. They are used by the kernel to grant 596or deny access to the object presented by this inode. 597<FOOTNOTE> 598 599<Para> 600A <Literal remap="tt">smarter</Literal> permissions control is one of the enhancements planned for 601Linux 1.3 - The ACL (Access Control Lists). Actually, from browsing of the 602kernel source, some of the ACL handling is already done. 603</Para> 604 605</FOOTNOTE> 606 607</Para> 608 609<Para> 610Bit number 9 signals that the file (I'll refer to the object presented by 611the inode as file even though it can be a special device, for example) is 612<Literal remap="tt">set VTX</Literal>. I still don't know what is the meaning of "VTX". 613</Para> 614 615<Para> 616Bit number 10 signals that the file is <Literal remap="tt">set group id</Literal> - I don't know 617exactly the meaning of the above either. 618</Para> 619 620<Para> 621Bit number 11 signals that the file is <Literal remap="tt">set user id</Literal>, which means that 622the file will run with an effective user id root. 623</Para> 624 625</Sect3> 626 627<Sect3> 628<Title>The leftmost two octal digits</Title> 629 630<Para> 631Note the the leftmost octal digit can only be 0 or 1, since the total number 632of bits is 16. 633</Para> 634 635<Para> 636Those digits, as opposed to the rightmost 4 digits, are not bit mapped 637options. They determine the type of the "file" to which the inode belongs: 638 639<ItemizedList> 640<ListItem> 641 642<Para> 643 <Literal remap="tt">01</Literal> - The file is a <Literal remap="tt">FIFO</Literal>. 644</Para> 645</ListItem> 646<ListItem> 647 648<Para> 649 <Literal remap="tt">02</Literal> - The file is a <Literal remap="tt">character device</Literal>. 650</Para> 651</ListItem> 652<ListItem> 653 654<Para> 655 <Literal remap="tt">04</Literal> - The file is a <Literal remap="tt">directory</Literal>. 656</Para> 657</ListItem> 658<ListItem> 659 660<Para> 661 <Literal remap="tt">06</Literal> - The file is a <Literal remap="tt">block device</Literal>. 662</Para> 663</ListItem> 664<ListItem> 665 666<Para> 667 <Literal remap="tt">10</Literal> - The file is a <Literal remap="tt">regular file</Literal>. 668</Para> 669</ListItem> 670<ListItem> 671 672<Para> 673 <Literal remap="tt">12</Literal> - The file is a <Literal remap="tt">symbolic link</Literal>. 674</Para> 675</ListItem> 676<ListItem> 677 678<Para> 679 <Literal remap="tt">14</Literal> - The file is a <Literal remap="tt">socket</Literal>. 680</Para> 681</ListItem> 682 683</ItemizedList> 684 685</Para> 686 687</Sect3> 688 689</Sect2> 690 691<Sect2> 692<Title>Time and date</Title> 693 694<Para> 695Linux records the last time in which various operations occurred with the 696file. The time and date are saved in the standard C library format - The 697number of seconds which passed since 00:00:00 GMT, January 1, 1970. The 698following times are recorded: 699 700<ItemizedList> 701<ListItem> 702 703<Para> 704 <Literal remap="tt">i_ctime</Literal> - The time in which the inode was last allocated. In 705other words, the time in which the file was created. 706</Para> 707</ListItem> 708<ListItem> 709 710<Para> 711 <Literal remap="tt">i_mtime</Literal> - The time in which the file was last modified. 712</Para> 713</ListItem> 714<ListItem> 715 716<Para> 717 <Literal remap="tt">i_atime</Literal> - The time in which the file was last accessed. 718</Para> 719</ListItem> 720<ListItem> 721 722<Para> 723 <Literal remap="tt">i_dtime</Literal> - The time in which the inode was deallocated. In 724other words, the time in which the file was deleted. 725</Para> 726</ListItem> 727 728</ItemizedList> 729 730</Para> 731 732</Sect2> 733 734<Sect2> 735<Title>i_size</Title> 736 737<Para> 738<Literal remap="tt">i_size</Literal> contains information about the size of the object presented by 739the inode. If the inode corresponds to a regular file, this is just the size 740of the file in bytes. In other cases, the interpretation of the variable is 741different. 742</Para> 743 744</Sect2> 745 746<Sect2> 747<Title>User and group id</Title> 748 749<Para> 750The user and group id of the file are just saved in the variables 751<Literal remap="tt">i_uid</Literal> and <Literal remap="tt">i_gid</Literal>. 752</Para> 753 754</Sect2> 755 756<Sect2> 757<Title>Hard links</Title> 758 759<Para> 760Later, when we'll discuss the implementation of directories, it will be 761explained that each <Literal remap="tt">directory entry</Literal> points to an inode. It is quite 762possible that a <Literal remap="tt">single inode</Literal> will be pointed to from <Literal remap="tt">several</Literal> 763directories. In that case, we say that there exist <Literal remap="tt">hard links</Literal> to the 764file - The file can be accessed from each of the directories. 765</Para> 766 767<Para> 768The kernel keeps track of the number of hard links in the variable 769<Literal remap="tt">i_links_count</Literal>. The variable is set to "1" when first allocating the 770inode, and is incremented with each additional link. Deletion of a file will 771delete the current directory entry and will decrement the number of links. 772Only when this number reaches zero, the inode will be actually deallocated. 773</Para> 774 775<Para> 776The name <Literal remap="tt">hard link</Literal> is used to distinguish between the alias method 777described above, to another alias method called <Literal remap="tt">symbolic linking</Literal>, 778which will be described later. 779</Para> 780 781</Sect2> 782 783<Sect2> 784<Title>The Ext2fs extended flags</Title> 785 786<Para> 787The ext2 filesystem associates additional flags with an inode. The extended 788attributes are stored in the variable <Literal remap="tt">i_flags</Literal>. <Literal remap="tt">i_flags</Literal> is a 32 789bit variable. Only the 7 rightmost bits are defined. Of them, only 5 bits 790are used in version 0.5a of the filesystem. Specifically, the 791<Literal remap="tt">undelete</Literal> and the <Literal remap="tt">compress</Literal> features are not implemented, and 792are to be introduced in Linux 1.3 development. 793</Para> 794 795<Para> 796The currently available flags are: 797 798<ItemizedList> 799<ListItem> 800 801<Para> 802 bit 0 - Secure deletion. 803 804When this bit is on, the file's blocks are zeroed when the file is 805deleted. With this bit off, they will just be left with their 806original data when the inode is deallocated. 807</Para> 808</ListItem> 809<ListItem> 810 811<Para> 812 bit 1 - Undelete. 813 814This bit is not supported yet. It will be used to provide an 815<Literal remap="tt">undelete</Literal> feature in future Ext2fs developments. 816</Para> 817</ListItem> 818<ListItem> 819 820<Para> 821 bit 2 - Compress file. 822 823This bit is also not supported. The plan is to offer "compression on 824the fly" in future releases. 825</Para> 826</ListItem> 827<ListItem> 828 829<Para> 830 bit 3 - Synchronous updates. 831 832With this bit on, the meta-data will be written synchronously to the 833disk, as if the filesystem was mounted with the "sync" mount option. 834</Para> 835</ListItem> 836<ListItem> 837 838<Para> 839 bit 4 - Immutable file. 840 841When this bit is on, the file will stay as it is - Can not be 842changed, deleted, renamed, no hard links, etc, before the bit is 843cleared. 844</Para> 845</ListItem> 846<ListItem> 847 848<Para> 849 bit 5 - Append only file. 850 851With this option active, data will only be appended to the file. 852</Para> 853</ListItem> 854<ListItem> 855 856<Para> 857 bit 6 - Do not dump this file. 858 859I think that this bit is used by the port of dump to linux (ported by 860<Literal remap="tt">Remy Card</Literal>) to check if the file should not be dumped. 861</Para> 862</ListItem> 863 864</ItemizedList> 865 866</Para> 867 868</Sect2> 869 870<Sect2> 871<Title>Symbolic links</Title> 872 873<Para> 874The <Literal remap="tt">hard links</Literal> presented above are just another pointers to the same 875inode. The important aspect is that the inode number is <Literal remap="tt">fixed</Literal> when 876the link is created. This means that the implementation details of the 877filesystem are visible to the user - In a pure abstract usage of the 878filesystem, the user should not care about inodes. 879</Para> 880 881<Para> 882The above causes several limitations: 883 884<ItemizedList> 885<ListItem> 886 887<Para> 888 Hard links can be done only in the same filesystem. This is obvious, 889since a hard link is just an inode number in some directory entry, 890and the above elements are filesystem specific. 891</Para> 892</ListItem> 893<ListItem> 894 895<Para> 896 You can not "replace" the file which is pointed to by the hard link 897after the link creation. "Replacing" the file in one directory will 898still leave the original file in the other directory - The 899"replacement" will not deallocate the original inode, but rather 900allocate another inode for the new version, and the directory entry 901at the other place will just point to the old inode number. 902</Para> 903</ListItem> 904 905</ItemizedList> 906 907</Para> 908 909<Para> 910<Literal remap="tt">Symbolic link</Literal>, on the other hand, is analyzed at <Literal remap="tt">run time</Literal>. A 911symbolic link is just a <Literal remap="tt">pathname</Literal> which is accessible from an inode. 912As such, it "speaks" in the language of the abstract filesystem. When the 913kernel reaches a symbolic link, it will <Literal remap="tt">follow it in run time</Literal> using 914its normal way of reaching directories. 915</Para> 916 917<Para> 918As such, symbolic link can be made <Literal remap="tt">across different filesystems</Literal> and a 919replacement of a file with a new version will automatically be active on all 920its symbolic links. 921</Para> 922 923<Para> 924The disadvantage is that hard link doesn't consume space except to a small 925directory entry. Symbolic link, on the other hand, consumes at least an 926inode, and can also consume one block. 927</Para> 928 929<Para> 930When the inode is identified as a symbolic link, the kernel needs to find 931the path to which it points. 932</Para> 933 934<Sect3> 935<Title>Fast symbolic links</Title> 936 937<Para> 938When the pathname contains up to 64 bytes, it can be saved directly in the 939inode, on the <Literal remap="tt">i_block[0] - i_block[15]</Literal> variables, since those are not 940needed in that case. This is called <Literal remap="tt">fast</Literal> symbolic link. It is fast 941because the pathname resolution can be done using the inode itself, without 942accessing additional blocks. It is also economical, since it allocates only 943an inode. The length of the pathname is stored in the <Literal remap="tt">i_size</Literal> 944variable. 945</Para> 946 947</Sect3> 948 949<Sect3> 950<Title>Slow symbolic links</Title> 951 952<Para> 953Starting from 65 bytes, additional block is allocated (by the use of 954<Literal remap="tt">i_block[0]</Literal>) and the pathname is stored in it. It is called slow 955because the kernel needs to read additional block to resolve the pathname. 956The length is again saved in <Literal remap="tt">i_size</Literal>. 957</Para> 958 959</Sect3> 960 961</Sect2> 962 963<Sect2> 964<Title>i_version</Title> 965 966<Para> 967<Literal remap="tt">i_version</Literal> is used with regard to Network File System. I don't know 968its exact use. 969</Para> 970 971</Sect2> 972 973<Sect2> 974<Title>Reserved variables</Title> 975 976<Para> 977As far as I know, the variables which are connected to ACL and fragments 978are not currently used. They will be supported in future versions. 979</Para> 980 981<Para> 982Ext2fs is being ported to other operating systems. As far as I know, 983at least in linux, the os dependent variables are also not used. 984</Para> 985 986</Sect2> 987 988<Sect2> 989<Title>Special reserved inodes</Title> 990 991<Para> 992The first ten inodes on the filesystem are special inodes: 993 994<ItemizedList> 995<ListItem> 996 997<Para> 998 Inode 1 is the <Literal remap="tt">bad blocks inode</Literal> - I believe that its data 999blocks contain a list of the bad blocks in the filesystem, which 1000should not be allocated. 1001</Para> 1002</ListItem> 1003<ListItem> 1004 1005<Para> 1006 Inode 2 is the <Literal remap="tt">root inode</Literal> - The inode of the root directory. 1007It is the starting point for reaching a known path in the filesystem. 1008</Para> 1009</ListItem> 1010<ListItem> 1011 1012<Para> 1013 Inode 3 is the <Literal remap="tt">acl index inode</Literal>. Access control lists are 1014currently not supported by the ext2 filesystem, so I believe this 1015inode is not used. 1016</Para> 1017</ListItem> 1018<ListItem> 1019 1020<Para> 1021 Inode 4 is the <Literal remap="tt">acl data inode</Literal>. Of course, the above applies 1022here too. 1023</Para> 1024</ListItem> 1025<ListItem> 1026 1027<Para> 1028 Inode 5 is the <Literal remap="tt">boot loader inode</Literal>. I don't know its 1029usage. 1030</Para> 1031</ListItem> 1032<ListItem> 1033 1034<Para> 1035 Inode 6 is the <Literal remap="tt">undelete directory inode</Literal>. It is also a 1036foundation for future enhancements, and is currently not used. 1037</Para> 1038</ListItem> 1039<ListItem> 1040 1041<Para> 1042 Inodes 7-10 are <Literal remap="tt">reserved</Literal> and currently not used. 1043</Para> 1044</ListItem> 1045 1046</ItemizedList> 1047 1048</Para> 1049 1050</Sect2> 1051 1052</Sect1> 1053 1054<Sect1> 1055<Title>Directories</Title> 1056 1057<Para> 1058A directory is implemented in the same way as files are implemented (with 1059the direct blocks, indirect blocks, etc) - It is just a file which is 1060formatted with a special format - A list of directory entries. 1061</Para> 1062 1063<Para> 1064Follows the definition of a directory entry: 1065</Para> 1066 1067<Para> 1068 1069<ProgramListing> 1070struct ext2_dir_entry { 1071 __u32 inode; /* Inode number */ 1072 __u16 rec_len; /* Directory entry length */ 1073 __u16 name_len; /* Name length */ 1074 char name[EXT2_NAME_LEN]; /* File name */ 1075}; 1076</ProgramListing> 1077 1078</Para> 1079 1080<Para> 1081Ext2fs supports file names of varying lengths, up to 255 bytes. The 1082<Literal remap="tt">name</Literal> field above just contains the file name. Note that it is 1083<Literal remap="tt">not zero terminated</Literal>; Instead, the variable <Literal remap="tt">name_len</Literal> contains 1084the length of the file name. 1085</Para> 1086 1087<Para> 1088The variable <Literal remap="tt">rec_len</Literal> is provided because the directory entries are 1089padded with zeroes so that the next entry will be in an offset which is 1090a multiplication of 4. The resulting directory entry size is stored in 1091<Literal remap="tt">rec_len</Literal>. If the directory entry is the last in the block, it is 1092padded with zeroes till the end of the block, and rec_len is updated 1093accordingly. 1094</Para> 1095 1096<Para> 1097The <Literal remap="tt">inode</Literal> variable points to the inode of the above file. 1098</Para> 1099 1100<Para> 1101Deletion of directory entries is done by appending of the deleted entry 1102space to the previous (or next, I am not sure) entry. 1103</Para> 1104 1105</Sect1> 1106 1107<Sect1> 1108<Title>The superblock</Title> 1109 1110<Para> 1111The <Literal remap="tt">superblock</Literal> is a block which contains information which describes 1112the state of the internal filesystem. 1113</Para> 1114 1115<Para> 1116The superblock is located at the <Literal remap="tt">fixed offset 1024</Literal> in the device. Its 1117length is 1024 bytes also. 1118</Para> 1119 1120<Para> 1121The superblock, like the group descriptors, is copied on each blocks group 1122boundary for backup purposes. However, only the main copy is used by the 1123kernel. 1124</Para> 1125 1126<Para> 1127The superblock contain three types of information: 1128 1129<ItemizedList> 1130<ListItem> 1131 1132<Para> 1133 Filesystem parameters which are fixed and which were determined when 1134this specific filesystem was created. Some of those parameters can 1135be different in different installations of the ext2 filesystem, but 1136can not be changed once the filesystem was created. 1137</Para> 1138</ListItem> 1139<ListItem> 1140 1141<Para> 1142 Filesystem parameters which are tunable - Can always be changed. 1143</Para> 1144</ListItem> 1145<ListItem> 1146 1147<Para> 1148 Information about the current filesystem state. 1149</Para> 1150</ListItem> 1151 1152</ItemizedList> 1153 1154</Para> 1155 1156<Para> 1157Follows the superblock definition: 1158</Para> 1159 1160<Para> 1161 1162<ProgramListing> 1163struct ext2_super_block { 1164 __u32 s_inodes_count; /* Inodes count */ 1165 __u32 s_blocks_count; /* Blocks count */ 1166 __u32 s_r_blocks_count; /* Reserved blocks count */ 1167 __u32 s_free_blocks_count; /* Free blocks count */ 1168 __u32 s_free_inodes_count; /* Free inodes count */ 1169 __u32 s_first_data_block; /* First Data Block */ 1170 __u32 s_log_block_size; /* Block size */ 1171 __s32 s_log_frag_size; /* Fragment size */ 1172 __u32 s_blocks_per_group; /* # Blocks per group */ 1173 __u32 s_frags_per_group; /* # Fragments per group */ 1174 __u32 s_inodes_per_group; /* # Inodes per group */ 1175 __u32 s_mtime; /* Mount time */ 1176 __u32 s_wtime; /* Write time */ 1177 __u16 s_mnt_count; /* Mount count */ 1178 __s16 s_max_mnt_count; /* Maximal mount count */ 1179 __u16 s_magic; /* Magic signature */ 1180 __u16 s_state; /* File system state */ 1181 __u16 s_errors; /* Behaviour when detecting errors */ 1182 __u16 s_pad; 1183 __u32 s_lastcheck; /* time of last check */ 1184 __u32 s_checkinterval; /* max. time between checks */ 1185 __u32 s_creator_os; /* OS */ 1186 __u32 s_rev_level; /* Revision level */ 1187 __u16 s_def_resuid; /* Default uid for reserved blocks */ 1188 __u16 s_def_resgid; /* Default gid for reserved blocks */ 1189 __u32 s_reserved[235]; /* Padding to the end of the block */ 1190}; 1191</ProgramListing> 1192 1193</Para> 1194 1195<Sect2> 1196<Title>superblock identification</Title> 1197 1198<Para> 1199The ext2 filesystem's superblock is identified by the <Literal remap="tt">s_magic</Literal> field. 1200The current ext2 magic number is 0xEF53. I presume that "EF" means "Extended 1201Filesystem". In versions of the ext2 filesystem prior to 0.2B, the magic 1202number was 0xEF51. Those filesystems are not compatible with the current 1203versions; Specifically, the group descriptors definition is different. I 1204doubt if there still exists such a installation. 1205</Para> 1206 1207</Sect2> 1208 1209<Sect2> 1210<Title>Filesystem fixed parameters</Title> 1211 1212<Para> 1213By using the word <Literal remap="tt">fixed</Literal>, I mean fixed with respect to a particular 1214installation. Those variables are usually not fixed with respect to 1215different installations. 1216</Para> 1217 1218<Para> 1219The <Literal remap="tt">block size</Literal> is determined by using the <Literal remap="tt">s_log_block_size</Literal> 1220variable. The block size is 1024*pow (2,s_log_block_size) and should be 1221between 1024 and 4096. The available options are 1024, 2048 and 4096. 1222</Para> 1223 1224<Para> 1225<Literal remap="tt">s_inodes_count</Literal> contains the total number of available inodes. 1226</Para> 1227 1228<Para> 1229<Literal remap="tt">s_blocks_count</Literal> contains the total number of available blocks. 1230</Para> 1231 1232<Para> 1233<Literal remap="tt">s_first_data_block</Literal> specifies in which of the <Literal remap="tt">device block</Literal> the 1234<Literal remap="tt">superblock</Literal> is present. The superblock is always present at the fixed 1235offset 1024, but the device block numbering can differ. For example, if the 1236block size is 1024, the superblock will be at <Literal remap="tt">block 1</Literal> with respect to 1237the device. However, if the block size is 4096, offset 1024 is included in 1238<Literal remap="tt">block 0</Literal> of the device, and in that case <Literal remap="tt">s_first_data_block</Literal> 1239will contain 0. At least this is how I understood this variable. 1240</Para> 1241 1242<Para> 1243<Literal remap="tt">s_blocks_per_group</Literal> contains the number of blocks which are grouped 1244together as a blocks group. 1245</Para> 1246 1247<Para> 1248<Literal remap="tt">s_inodes_per_group</Literal> contains the number of inodes available in a group 1249block. I think that this is always the total number of inodes divided by the 1250number of blocks groups. 1251</Para> 1252 1253<Para> 1254<Literal remap="tt">s_creator_os</Literal> contains a code number which specifies the operating 1255system which created this specific filesystem: 1256 1257<ItemizedList> 1258<ListItem> 1259 1260<Para> 1261 <Literal remap="tt">Linux</Literal> :-) is specified by the value <Literal remap="tt">0</Literal>. 1262</Para> 1263</ListItem> 1264<ListItem> 1265 1266<Para> 1267 <Literal remap="tt">Hurd</Literal> is specified by the value <Literal remap="tt">1</Literal>. 1268</Para> 1269</ListItem> 1270<ListItem> 1271 1272<Para> 1273 <Literal remap="tt">Masix</Literal> is specified by the value <Literal remap="tt">2</Literal>. 1274</Para> 1275</ListItem> 1276 1277</ItemizedList> 1278 1279</Para> 1280 1281<Para> 1282<Literal remap="tt">s_rev_level</Literal> contains the major version of the ext2 filesystem. 1283Currently this is always <Literal remap="tt">0</Literal>, as the most recent version is 0.5B. It 1284will probably take some time until we reach version 1.0. 1285</Para> 1286 1287<Para> 1288As far as I know, fragments (sub-block allocations) are currently not 1289supported and hence a block is equal to a fragment. As a result, 1290<Literal remap="tt">s_log_frag_size</Literal> and <Literal remap="tt">s_frags_per_group</Literal> are always equal to 1291<Literal remap="tt">s_log_block_size</Literal> and <Literal remap="tt">s_blocks_per_group</Literal>, respectively. 1292</Para> 1293 1294</Sect2> 1295 1296<Sect2> 1297<Title>Ext2fs error handling</Title> 1298 1299<Para> 1300The ext2 filesystem error handling is based on the following philosophy: 1301 1302<OrderedList> 1303<ListItem> 1304 1305<Para> 1306 Identification of problems is done by the kernel code. 1307</Para> 1308</ListItem> 1309<ListItem> 1310 1311<Para> 1312 The correction task is left to an external utility, such as 1313<Literal remap="tt">e2fsck by Theodore Ts'o</Literal> for <Literal remap="tt">automatic</Literal> analysis and 1314correction, or perhaps <Literal remap="tt">debugfs by Theodore Ts'o</Literal> and 1315<Literal remap="tt">EXT2ED by myself</Literal>, for <Literal remap="tt">hand</Literal> analysis and correction. 1316</Para> 1317</ListItem> 1318 1319</OrderedList> 1320 1321</Para> 1322 1323<Para> 1324The <Literal remap="tt">s_state</Literal> variable is used by the kernel to pass the identification 1325result to third party utilities: 1326 1327<ItemizedList> 1328<ListItem> 1329 1330<Para> 1331 <Literal remap="tt">bit 0</Literal> of s_state is reset when the partition is mounted and 1332set when the partition is unmounted. Thus, a value of 0 on an 1333unmounted filesystem means that the filesystem was not unmounted 1334properly - The filesystem is not "clean" and probably contains 1335errors. 1336</Para> 1337</ListItem> 1338<ListItem> 1339 1340<Para> 1341 <Literal remap="tt">bit 1</Literal> of s_state is set by the kernel when it detects an 1342error in the filesystem. A value of 0 doesn't mean that there isn't 1343an error in the filesystem, just that the kernel didn't find any. 1344</Para> 1345</ListItem> 1346 1347</ItemizedList> 1348 1349</Para> 1350 1351<Para> 1352The kernel behavior when an error is found is determined by the user tunable 1353parameter <Literal remap="tt">s_errors</Literal>: 1354 1355<ItemizedList> 1356<ListItem> 1357 1358<Para> 1359 The kernel will ignore the error and continue if <Literal remap="tt">s_errors=1</Literal>. 1360</Para> 1361</ListItem> 1362<ListItem> 1363 1364<Para> 1365 The kernel will remount the filesystem in read-only mode if 1366<Literal remap="tt">s_errors=2</Literal>. 1367</Para> 1368</ListItem> 1369<ListItem> 1370 1371<Para> 1372 A kernel panic will be issued if <Literal remap="tt">s_errors=3</Literal>. 1373</Para> 1374</ListItem> 1375 1376</ItemizedList> 1377 1378</Para> 1379 1380<Para> 1381The default behavior is to ignore the error. 1382</Para> 1383 1384</Sect2> 1385 1386<Sect2> 1387<Title>Additional parameters used by e2fsck</Title> 1388 1389<Para> 1390Of-course, <Literal remap="tt">e2fsck</Literal> will check the filesystem if errors were detected 1391or if the filesystem is not clean. 1392</Para> 1393 1394<Para> 1395In addition, each time the filesystem is mounted, <Literal remap="tt">s_mnt_count</Literal> is 1396incremented. When s_mnt_count reaches <Literal remap="tt">s_max_mnt_count</Literal>, <Literal remap="tt">e2fsck</Literal> 1397will force a check on the filesystem even though it may be clean. It will 1398then zero s_mnt_count. <Literal remap="tt">s_max_mnt_count</Literal> is a tunable parameter. 1399</Para> 1400 1401<Para> 1402E2fsck also records the last time in which the file system was checked in 1403the <Literal remap="tt">s_lastcheck</Literal> variable. The user tunable parameter 1404<Literal remap="tt">s_checkinterval</Literal> will contain the number of seconds which are allowed 1405to pass since <Literal remap="tt">s_lastcheck</Literal> until a check is forced. A value of 1406<Literal remap="tt">0</Literal> disables time-based check. 1407</Para> 1408 1409</Sect2> 1410 1411<Sect2> 1412<Title>Additional user tunable parameters</Title> 1413 1414<Para> 1415<Literal remap="tt">s_r_blocks_count</Literal> contains the number of disk blocks which are 1416reserved for root, the user whose id number is <Literal remap="tt">s_def_resuid</Literal> and the 1417group whose id number is <Literal remap="tt">s_deg_resgid</Literal>. The kernel will refuse to 1418allocate those last <Literal remap="tt">s_r_blocks_count</Literal> if the user is not one of the 1419above. This is done so that the filesystem will usually not be 100% full, 1420since 100% full filesystems can affect various aspects of operation. 1421</Para> 1422 1423<Para> 1424<Literal remap="tt">s_def_resuid</Literal> and <Literal remap="tt">s_def_resgid</Literal> contain the id of the user and 1425of the group who can use the reserved blocks in addition to root. 1426</Para> 1427 1428</Sect2> 1429 1430<Sect2> 1431<Title>Filesystem current state</Title> 1432 1433<Para> 1434<Literal remap="tt">s_free_blocks_count</Literal> contains the current number of free blocks 1435in the filesystem. 1436</Para> 1437 1438<Para> 1439<Literal remap="tt">s_free_inodes_count</Literal> contains the current number of free inodes in the 1440filesystem. 1441</Para> 1442 1443<Para> 1444<Literal remap="tt">s_mtime</Literal> contains the time at which the system was last mounted. 1445</Para> 1446 1447<Para> 1448<Literal remap="tt">s_wtime</Literal> contains the last time at which something was changed in the 1449filesystem. 1450</Para> 1451 1452</Sect2> 1453 1454</Sect1> 1455 1456<Sect1> 1457<Title>Copyright</Title> 1458 1459<Para> 1460This document contains source code which was taken from the Linux ext2 1461kernel source code, mainly from <FILENAME>/usr/include/linux/ext2_fs.h</FILENAME>. Follows 1462the original copyright: 1463</Para> 1464 1465<Para> 1466 1467<ProgramListing> 1468/* 1469 * linux/include/linux/ext2_fs.h 1470 * 1471 * Copyright (C) 1992, 1993, 1994, 1995 1472 * Remy Card (card@masi.ibp.fr) 1473 * Laboratoire MASI - Institut Blaise Pascal 1474 * Universite Pierre et Marie Curie (Paris VI) 1475 * 1476 * from 1477 * 1478 * linux/include/linux/minix_fs.h 1479 * 1480 * Copyright (C) 1991, 1992 Linus Torvalds 1481 */ 1482 1483</ProgramListing> 1484 1485</Para> 1486 1487</Sect1> 1488 1489<Sect1> 1490<Title>Acknowledgments</Title> 1491 1492<Para> 1493I would like to thank the following people, who were involved in the 1494design and implementation of the ext2 filesystem kernel code and support 1495utilities: 1496 1497<ItemizedList> 1498<ListItem> 1499 1500<Para> 1501 <Literal remap="tt">Remy Card</Literal> 1502 1503Who designed, implemented and maintains the ext2 filesystem kernel 1504code, and some of the ext2 utilities. <Literal remap="tt">Remy Card</Literal> is also the 1505author of several helpful slides concerning the ext2 filesystem. 1506Specifically, he is the author of <Literal remap="tt">File Management in the Linux 1507Kernel</Literal> and of <Literal remap="tt">The Second Extended File System - Current 1508State, Future Development</Literal>. 1509 1510</Para> 1511</ListItem> 1512<ListItem> 1513 1514<Para> 1515 <Literal remap="tt">Wayne Davison</Literal> 1516 1517Who designed the ext2 filesystem. 1518</Para> 1519</ListItem> 1520<ListItem> 1521 1522<Para> 1523 <Literal remap="tt">Stephen Tweedie</Literal> 1524 1525Who helped designing the ext2 filesystem kernel code and wrote the 1526slides <Literal remap="tt">Optimizations in File Systems</Literal>. 1527</Para> 1528</ListItem> 1529<ListItem> 1530 1531<Para> 1532 <Literal remap="tt">Theodore Ts'o</Literal> 1533 1534Who is the author of several ext2 utilities and of the ext2 library 1535<Literal remap="tt">libext2fs</Literal> (which I didn't use, simply because I didn't know 1536it exists when I started to work on my project). 1537</Para> 1538</ListItem> 1539 1540</ItemizedList> 1541 1542</Para> 1543 1544<Para> 1545Lastly, I would like to thank, of-course, <Literal remap="tt">Linus Torvalds</Literal> and the 1546<Literal remap="tt">Linux community</Literal> for providing all of us with such a great operating 1547system. 1548</Para> 1549 1550<Para> 1551Please contact me in a case of an error report, suggestions, or just about 1552anything concerning this document. 1553</Para> 1554 1555<Para> 1556Enjoy, 1557</Para> 1558 1559<Para> 1560Gadi Oxman <tgud@tochnapc2.technion.ac.il> 1561</Para> 1562 1563<Para> 1564Haifa, August 95 1565</Para> 1566 1567</Sect1> 1568 1569</Article> 1570