Commit | Line | Data |
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1da177e4 LT |
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 |