zero.txt 1.6 KB

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