1 ======= 2 dm-zero 3 ======= 4 5 Device-Mapper's "zero" target provides a block-device that always returns 6 zero'd data on reads and silently drops writes. This is similar behavior to 7 /dev/zero, but as a block-device instead of a character-device. 8 9 Dm-zero has no target-specific parameters. 10 11 One very interesting use of dm-zero is for creating "sparse" devices in 12 conjunction with dm-snapshot. A sparse device reports a device-size larger 13 than the amount of actual storage space available for that device. A user can 14 write data anywhere within the sparse device and read it back like a normal 15 device. Reads to previously unwritten areas will return a zero'd buffer. When 16 enough data has been written to fill up the actual storage space, the sparse 17 device is deactivated. This can be very useful for testing device and 18 filesystem limitations. 19 20 To create a sparse device, start by creating a dm-zero device that's the 21 desired size of the sparse device. For this example, we'll assume a 10TB 22 sparse device:: 23 24 TEN_TERABYTES=`expr 10 \* 1024 \* 1024 \* 1024 \* 2` # 10 TB in sectors 25 echo "0 $TEN_TERABYTES zero" | dmsetup create zero1 26 27 Then create a snapshot of the zero device, using any available block-device as 28 the COW device. The size of the COW device will determine the amount of real 29 space available to the sparse device. For this example, we'll assume /dev/sdb1 30 is an available 10GB partition:: 31 32 echo "0 $TEN_TERABYTES snapshot /dev/mapper/zero1 /dev/sdb1 p 128" | \ 33 dmsetup create sparse1 34 35 This will create a 10TB sparse device called /dev/mapper/sparse1 that has 36 10GB of actual storage space available. If more than 10GB of data is written 37 to this device, it will start returning I/O errors.
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