1 =======
2 dm-zero
3 =======
4
5 Device-Mapper's "zero" target provides a block
6 zero'd data on reads and silently drops writes
7 /dev/zero, but as a block-device instead of a
8
9 Dm-zero has no target-specific parameters.
10
11 One very interesting use of dm-zero is for cre
12 conjunction with dm-snapshot. A sparse device
13 than the amount of actual storage space availa
14 write data anywhere within the sparse device a
15 device. Reads to previously unwritten areas wi
16 enough data has been written to fill up the ac
17 device is deactivated. This can be very useful
18 filesystem limitations.
19
20 To create a sparse device, start by creating a
21 desired size of the sparse device. For this ex
22 sparse device::
23
24 TEN_TERABYTES=`expr 10 \* 1024 \* 1024 \* 10
25 echo "0 $TEN_TERABYTES zero" | dmsetup creat
26
27 Then create a snapshot of the zero device, usi
28 the COW device. The size of the COW device wil
29 space available to the sparse device. For this
30 is an available 10GB partition::
31
32 echo "0 $TEN_TERABYTES snapshot /dev/mapper/
33 dmsetup create sparse1
34
35 This will create a 10TB sparse device called /
36 10GB of actual storage space available. If mor
37 to this device, it will start returning I/O er
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