|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
|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
|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.