1================= 2Queue sysfs files 3================= 4 5This text file will detail the queue files that are located in the sysfs tree 6for each block device. Note that stacked devices typically do not export 7any settings, since their queue merely functions are a remapping target. 8These files are the ones found in the /sys/block/xxx/queue/ directory. 9 10Files denoted with a RO postfix are readonly and the RW postfix means 11read-write. 12 13add_random (RW) 14--------------- 15This file allows to turn off the disk entropy contribution. Default 16value of this file is '1'(on). 17 18chunk_sectors (RO) 19------------------ 20This has different meaning depending on the type of the block device. 21For a RAID device (dm-raid), chunk_sectors indicates the size in 512B sectors 22of the RAID volume stripe segment. For a zoned block device, either host-aware 23or host-managed, chunk_sectors indicates the size in 512B sectors of the zones 24of the device, with the eventual exception of the last zone of the device which 25may be smaller. 26 27dax (RO) 28-------- 29This file indicates whether the device supports Direct Access (DAX), 30used by CPU-addressable storage to bypass the pagecache. It shows '1' 31if true, '0' if not. 32 33discard_granularity (RO) 34------------------------ 35This shows the size of internal allocation of the device in bytes, if 36reported by the device. A value of '0' means device does not support 37the discard functionality. 38 39discard_max_hw_bytes (RO) 40------------------------- 41Devices that support discard functionality may have internal limits on 42the number of bytes that can be trimmed or unmapped in a single operation. 43The discard_max_bytes parameter is set by the device driver to the maximum 44number of bytes that can be discarded in a single operation. Discard 45requests issued to the device must not exceed this limit. A discard_max_bytes 46value of 0 means that the device does not support discard functionality. 47 48discard_max_bytes (RW) 49---------------------- 50While discard_max_hw_bytes is the hardware limit for the device, this 51setting is the software limit. Some devices exhibit large latencies when 52large discards are issued, setting this value lower will make Linux issue 53smaller discards and potentially help reduce latencies induced by large 54discard operations. 55 56discard_zeroes_data (RO) 57------------------------ 58Obsolete. Always zero. 59 60fua (RO) 61-------- 62Whether or not the block driver supports the FUA flag for write requests. 63FUA stands for Force Unit Access. If the FUA flag is set that means that 64write requests must bypass the volatile cache of the storage device. 65 66hw_sector_size (RO) 67------------------- 68This is the hardware sector size of the device, in bytes. 69 70io_poll (RW) 71------------ 72When read, this file shows whether polling is enabled (1) or disabled 73(0). Writing '0' to this file will disable polling for this device. 74Writing any non-zero value will enable this feature. 75 76io_poll_delay (RW) 77------------------ 78If polling is enabled, this controls what kind of polling will be 79performed. It defaults to -1, which is classic polling. In this mode, 80the CPU will repeatedly ask for completions without giving up any time. 81If set to 0, a hybrid polling mode is used, where the kernel will attempt 82to make an educated guess at when the IO will complete. Based on this 83guess, the kernel will put the process issuing IO to sleep for an amount 84of time, before entering a classic poll loop. This mode might be a 85little slower than pure classic polling, but it will be more efficient. 86If set to a value larger than 0, the kernel will put the process issuing 87IO to sleep for this amount of microseconds before entering classic 88polling. 89 90io_timeout (RW) 91--------------- 92io_timeout is the request timeout in milliseconds. If a request does not 93complete in this time then the block driver timeout handler is invoked. 94That timeout handler can decide to retry the request, to fail it or to start 95a device recovery strategy. 96 97iostats (RW) 98------------- 99This file is used to control (on/off) the iostats accounting of the 100disk. 101 102logical_block_size (RO) 103----------------------- 104This is the logical block size of the device, in bytes. 105 106max_discard_segments (RO) 107------------------------- 108The maximum number of DMA scatter/gather entries in a discard request. 109 110max_hw_sectors_kb (RO) 111---------------------- 112This is the maximum number of kilobytes supported in a single data transfer. 113 114max_integrity_segments (RO) 115--------------------------- 116Maximum number of elements in a DMA scatter/gather list with integrity 117data that will be submitted by the block layer core to the associated 118block driver. 119 120max_active_zones (RO) 121--------------------- 122For zoned block devices (zoned attribute indicating "host-managed" or 123"host-aware"), the sum of zones belonging to any of the zone states: 124EXPLICIT OPEN, IMPLICIT OPEN or CLOSED, is limited by this value. 125If this value is 0, there is no limit. 126 127If the host attempts to exceed this limit, the driver should report this error 128with BLK_STS_ZONE_ACTIVE_RESOURCE, which user space may see as the EOVERFLOW 129errno. 130 131max_open_zones (RO) 132------------------- 133For zoned block devices (zoned attribute indicating "host-managed" or 134"host-aware"), the sum of zones belonging to any of the zone states: 135EXPLICIT OPEN or IMPLICIT OPEN, is limited by this value. 136If this value is 0, there is no limit. 137 138If the host attempts to exceed this limit, the driver should report this error 139with BLK_STS_ZONE_OPEN_RESOURCE, which user space may see as the ETOOMANYREFS 140errno. 141 142max_sectors_kb (RW) 143------------------- 144This is the maximum number of kilobytes that the block layer will allow 145for a filesystem request. Must be smaller than or equal to the maximum 146size allowed by the hardware. 147 148max_segments (RO) 149----------------- 150Maximum number of elements in a DMA scatter/gather list that is submitted 151to the associated block driver. 152 153max_segment_size (RO) 154--------------------- 155Maximum size in bytes of a single element in a DMA scatter/gather list. 156 157minimum_io_size (RO) 158-------------------- 159This is the smallest preferred IO size reported by the device. 160 161nomerges (RW) 162------------- 163This enables the user to disable the lookup logic involved with IO 164merging requests in the block layer. By default (0) all merges are 165enabled. When set to 1 only simple one-hit merges will be tried. When 166set to 2 no merge algorithms will be tried (including one-hit or more 167complex tree/hash lookups). 168 169nr_requests (RW) 170---------------- 171This controls how many requests may be allocated in the block layer for 172read or write requests. Note that the total allocated number may be twice 173this amount, since it applies only to reads or writes (not the accumulated 174sum). 175 176To avoid priority inversion through request starvation, a request 177queue maintains a separate request pool per each cgroup when 178CONFIG_BLK_CGROUP is enabled, and this parameter applies to each such 179per-block-cgroup request pool. IOW, if there are N block cgroups, 180each request queue may have up to N request pools, each independently 181regulated by nr_requests. 182 183nr_zones (RO) 184------------- 185For zoned block devices (zoned attribute indicating "host-managed" or 186"host-aware"), this indicates the total number of zones of the device. 187This is always 0 for regular block devices. 188 189optimal_io_size (RO) 190-------------------- 191This is the optimal IO size reported by the device. 192 193physical_block_size (RO) 194------------------------ 195This is the physical block size of device, in bytes. 196 197read_ahead_kb (RW) 198------------------ 199Maximum number of kilobytes to read-ahead for filesystems on this block 200device. 201 202rotational (RW) 203--------------- 204This file is used to stat if the device is of rotational type or 205non-rotational type. 206 207rq_affinity (RW) 208---------------- 209If this option is '1', the block layer will migrate request completions to the 210cpu "group" that originally submitted the request. For some workloads this 211provides a significant reduction in CPU cycles due to caching effects. 212 213For storage configurations that need to maximize distribution of completion 214processing setting this option to '2' forces the completion to run on the 215requesting cpu (bypassing the "group" aggregation logic). 216 217scheduler (RW) 218-------------- 219When read, this file will display the current and available IO schedulers 220for this block device. The currently active IO scheduler will be enclosed 221in [] brackets. Writing an IO scheduler name to this file will switch 222control of this block device to that new IO scheduler. Note that writing 223an IO scheduler name to this file will attempt to load that IO scheduler 224module, if it isn't already present in the system. 225 226write_cache (RW) 227---------------- 228When read, this file will display whether the device has write back 229caching enabled or not. It will return "write back" for the former 230case, and "write through" for the latter. Writing to this file can 231change the kernels view of the device, but it doesn't alter the 232device state. This means that it might not be safe to toggle the 233setting from "write back" to "write through", since that will also 234eliminate cache flushes issued by the kernel. 235 236write_same_max_bytes (RO) 237------------------------- 238This is the number of bytes the device can write in a single write-same 239command. A value of '0' means write-same is not supported by this 240device. 241 242wbt_lat_usec (RW) 243----------------- 244If the device is registered for writeback throttling, then this file shows 245the target minimum read latency. If this latency is exceeded in a given 246window of time (see wb_window_usec), then the writeback throttling will start 247scaling back writes. Writing a value of '0' to this file disables the 248feature. Writing a value of '-1' to this file resets the value to the 249default setting. 250 251throttle_sample_time (RW) 252------------------------- 253This is the time window that blk-throttle samples data, in millisecond. 254blk-throttle makes decision based on the samplings. Lower time means cgroups 255have more smooth throughput, but higher CPU overhead. This exists only when 256CONFIG_BLK_DEV_THROTTLING_LOW is enabled. 257 258write_zeroes_max_bytes (RO) 259--------------------------- 260For block drivers that support REQ_OP_WRITE_ZEROES, the maximum number of 261bytes that can be zeroed at once. The value 0 means that REQ_OP_WRITE_ZEROES 262is not supported. 263 264zoned (RO) 265---------- 266This indicates if the device is a zoned block device and the zone model of the 267device if it is indeed zoned. The possible values indicated by zoned are 268"none" for regular block devices and "host-aware" or "host-managed" for zoned 269block devices. The characteristics of host-aware and host-managed zoned block 270devices are described in the ZBC (Zoned Block Commands) and ZAC 271(Zoned Device ATA Command Set) standards. These standards also define the 272"drive-managed" zone model. However, since drive-managed zoned block devices 273do not support zone commands, they will be treated as regular block devices 274and zoned will report "none". 275 276Jens Axboe <jens.axboe@oracle.com>, February 2009 277