1dm-zero
2=======
3
4Device-Mapper's "zero" target provides a block-device that always returns
5zero'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
8Dm-zero has no target-specific parameters.
9
10One very interesting use of dm-zero is for creating "sparse" devices in
11conjunction with dm-snapshot. A sparse device reports a device-size larger
12than the amount of actual storage space available for that device. A user can
13write data anywhere within the sparse device and read it back like a normal
14device. Reads to previously unwritten areas will return a zero'd buffer. When
15enough data has been written to fill up the actual storage space, the sparse
16device is deactivated. This can be very useful for testing device and
17filesystem limitations.
18
19To create a sparse device, start by creating a dm-zero device that's the
20desired size of the sparse device. For this example, we'll assume a 10TB
21sparse device.
22
23TEN_TERABYTES=`expr 10 \* 1024 \* 1024 \* 1024 \* 2`   # 10 TB in sectors
24echo "0 $TEN_TERABYTES zero" | dmsetup create zero1
25
26Then create a snapshot of the zero device, using any available block-device as
27the COW device. The size of the COW device will determine the amount of real
28space available to the sparse device. For this example, we'll assume /dev/sdb1
29is an available 10GB partition.
30
31echo "0 $TEN_TERABYTES snapshot /dev/mapper/zero1 /dev/sdb1 p 128" | \
32   dmsetup create sparse1
33
34This will create a 10TB sparse device called /dev/mapper/sparse1 that has
3510GB of actual storage space available. If more than 10GB of data is written
36to this device, it will start returning I/O errors.
37
38