zero.rst 1.7 KB

12345678910111213141516171819202122232425262728293031323334353637
  1. =======
  2. dm-zero
  3. =======
  4. Device-Mapper's "zero" target provides a block-device that always returns
  5. zero'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. Dm-zero has no target-specific parameters.
  8. One very interesting use of dm-zero is for creating "sparse" devices in
  9. conjunction with dm-snapshot. A sparse device reports a device-size larger
  10. than the amount of actual storage space available for that device. A user can
  11. write data anywhere within the sparse device and read it back like a normal
  12. device. Reads to previously unwritten areas will return a zero'd buffer. When
  13. enough data has been written to fill up the actual storage space, the sparse
  14. device is deactivated. This can be very useful for testing device and
  15. filesystem limitations.
  16. To create a sparse device, start by creating a dm-zero device that's the
  17. desired size of the sparse device. For this example, we'll assume a 10TB
  18. sparse device::
  19. TEN_TERABYTES=`expr 10 \* 1024 \* 1024 \* 1024 \* 2` # 10 TB in sectors
  20. echo "0 $TEN_TERABYTES zero" | dmsetup create zero1
  21. Then create a snapshot of the zero device, using any available block-device as
  22. the COW device. The size of the COW device will determine the amount of real
  23. space available to the sparse device. For this example, we'll assume /dev/sdb1
  24. is an available 10GB partition::
  25. echo "0 $TEN_TERABYTES snapshot /dev/mapper/zero1 /dev/sdb1 p 128" | \
  26. dmsetup create sparse1
  27. This will create a 10TB sparse device called /dev/mapper/sparse1 that has
  28. 10GB of actual storage space available. If more than 10GB of data is written
  29. to this device, it will start returning I/O errors.