1dm-zero 2======= 3 4Device-Mapper's "zero" target provides a block-device that always returns 5zero'd data on reads and silently drops writes. This is similar behavior to 6/dev/zero, but as a block-device instead of a character-device. 7 8Dm-zero has no target-specific parameters. 9 10One very interesting use of dm-zero is for creating "sparse" devices in 11conjunction with dm-snapshot. A sparse device reports a device-size larger 12than the amount of actual storage space available for that device. A user can 13write data anywhere within the sparse device and read it back like a normal 14device. Reads to previously unwritten areas will return a zero'd buffer. When 15enough data has been written to fill up the actual storage space, the sparse 16device is deactivated. This can be very useful for testing device and 17filesystem limitations. 18 19To create a sparse device, start by creating a dm-zero device that's the 20desired size of the sparse device. For this example, we'll assume a 10TB 21sparse device. 22 23TEN_TERABYTES=`expr 10 \* 1024 \* 1024 \* 1024 \* 2` # 10 TB in sectors 24echo "0 $TEN_TERABYTES zero" | dmsetup create zero1 25 26Then create a snapshot of the zero device, using any available block-device as 27the COW device. The size of the COW device will determine the amount of real 28space available to the sparse device. For this example, we'll assume /dev/sdb1 29is an available 10GB partition. 30 31echo "0 $TEN_TERABYTES snapshot /dev/mapper/zero1 /dev/sdb1 p 128" | \ 32 dmsetup create sparse1 33 34This will create a 10TB sparse device called /dev/mapper/sparse1 that has 3510GB of actual storage space available. If more than 10GB of data is written 36to this device, it will start returning I/O errors. 37 38