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