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Documentation / device-mapper / zero.txt




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Based on kernel version 3.13. Page generated on 2014-01-20 22:00 EST.

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.
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