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