1 ======= 1 =======
2 dm-zero 2 dm-zero
3 ======= 3 =======
4 4
5 Device-Mapper's "zero" target provides a block 5 Device-Mapper's "zero" target provides a block-device that always returns
6 zero'd data on reads and silently drops writes 6 zero'd data on reads and silently drops writes. This is similar behavior to
7 /dev/zero, but as a block-device instead of a 7 /dev/zero, but as a block-device instead of a character-device.
8 8
9 Dm-zero has no target-specific parameters. 9 Dm-zero has no target-specific parameters.
10 10
11 One very interesting use of dm-zero is for cre 11 One very interesting use of dm-zero is for creating "sparse" devices in
12 conjunction with dm-snapshot. A sparse device 12 conjunction with dm-snapshot. A sparse device reports a device-size larger
13 than the amount of actual storage space availa 13 than the amount of actual storage space available for that device. A user can
14 write data anywhere within the sparse device a 14 write data anywhere within the sparse device and read it back like a normal
15 device. Reads to previously unwritten areas wi 15 device. Reads to previously unwritten areas will return a zero'd buffer. When
16 enough data has been written to fill up the ac 16 enough data has been written to fill up the actual storage space, the sparse
17 device is deactivated. This can be very useful 17 device is deactivated. This can be very useful for testing device and
18 filesystem limitations. 18 filesystem limitations.
19 19
20 To create a sparse device, start by creating a 20 To create a sparse device, start by creating a dm-zero device that's the
21 desired size of the sparse device. For this ex 21 desired size of the sparse device. For this example, we'll assume a 10TB
22 sparse device:: 22 sparse device::
23 23
24 TEN_TERABYTES=`expr 10 \* 1024 \* 1024 \* 10 24 TEN_TERABYTES=`expr 10 \* 1024 \* 1024 \* 1024 \* 2` # 10 TB in sectors
25 echo "0 $TEN_TERABYTES zero" | dmsetup creat 25 echo "0 $TEN_TERABYTES zero" | dmsetup create zero1
26 26
27 Then create a snapshot of the zero device, usi 27 Then create a snapshot of the zero device, using any available block-device as
28 the COW device. The size of the COW device wil 28 the COW device. The size of the COW device will determine the amount of real
29 space available to the sparse device. For this 29 space available to the sparse device. For this example, we'll assume /dev/sdb1
30 is an available 10GB partition:: 30 is an available 10GB partition::
31 31
32 echo "0 $TEN_TERABYTES snapshot /dev/mapper/ 32 echo "0 $TEN_TERABYTES snapshot /dev/mapper/zero1 /dev/sdb1 p 128" | \
33 dmsetup create sparse1 33 dmsetup create sparse1
34 34
35 This will create a 10TB sparse device called / 35 This will create a 10TB sparse device called /dev/mapper/sparse1 that has
36 10GB of actual storage space available. If mor 36 10GB of actual storage space available. If more than 10GB of data is written
37 to this device, it will start returning I/O er 37 to this device, it will start returning I/O errors.
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