linux/Documentation/device-mapper/zero.txt
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   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
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