1=======
2dm-zero
3=======
4
5Device-Mapper's "zero" target provides a block-device that always returns
6zero'd data on reads and silently drops writes. This is similar behavior to
7/dev/zero, but as a block-device instead of a character-device.
8
9Dm-zero has no target-specific parameters.
10
11One very interesting use of dm-zero is for creating "sparse" devices in
12conjunction with dm-snapshot. A sparse device reports a device-size larger
13than the amount of actual storage space available for that device. A user can
14write data anywhere within the sparse device and read it back like a normal
15device. Reads to previously unwritten areas will return a zero'd buffer. When
16enough data has been written to fill up the actual storage space, the sparse
17device is deactivated. This can be very useful for testing device and
18filesystem limitations.
19
20To create a sparse device, start by creating a dm-zero device that's the
21desired size of the sparse device. For this example, we'll assume a 10TB
22sparse device::
23
24  TEN_TERABYTES=`expr 10 \* 1024 \* 1024 \* 1024 \* 2`   # 10 TB in sectors
25  echo "0 $TEN_TERABYTES zero" | dmsetup create zero1
26
27Then create a snapshot of the zero device, using any available block-device as
28the COW device. The size of the COW device will determine the amount of real
29space available to the sparse device. For this example, we'll assume /dev/sdb1
30is an available 10GB partition::
31
32  echo "0 $TEN_TERABYTES snapshot /dev/mapper/zero1 /dev/sdb1 p 128" | \
33     dmsetup create sparse1
34
35This will create a 10TB sparse device called /dev/mapper/sparse1 that has
3610GB of actual storage space available. If more than 10GB of data is written
37to this device, it will start returning I/O errors.
38