path: root/Documentation/device-mapper/zero.txt
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authorLinus Torvalds <>2005-04-16 15:20:36 -0700
committerLinus Torvalds <>2005-04-16 15:20:36 -0700
commit1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 (patch)
tree0bba044c4ce775e45a88a51686b5d9f90697ea9d /Documentation/device-mapper/zero.txt
Initial git repository build. I'm not bothering with the full history, even though we have it. We can create a separate "historical" git archive of that later if we want to, and in the meantime it's about 3.2GB when imported into git - space that would just make the early git days unnecessarily complicated, when we don't have a lot of good infrastructure for it. Let it rip!
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+Device-Mapper's "zero" target provides a block-device that always returns
+zero'd data on reads and silently drops writes. This is similar behavior to
+/dev/zero, but as a block-device instead of a character-device.
+Dm-zero has no target-specific parameters.
+One very interesting use of dm-zero is for creating "sparse" devices in
+conjunction with dm-snapshot. A sparse device reports a device-size larger
+than the amount of actual storage space available for that device. A user can
+write data anywhere within the sparse device and read it back like a normal
+device. Reads to previously unwritten areas will return a zero'd buffer. When
+enough data has been written to fill up the actual storage space, the sparse
+device is deactivated. This can be very useful for testing device and
+filesystem limitations.
+To create a sparse device, start by creating a dm-zero device that's the
+desired size of the sparse device. For this example, we'll assume a 10TB
+sparse device.
+TEN_TERABYTES=`expr 10 \* 1024 \* 1024 \* 1024 \* 2` # 10 TB in sectors
+echo "0 $TEN_TERABYTES zero" | dmsetup create zero1
+Then create a snapshot of the zero device, using any available block-device as
+the COW device. The size of the COW device will determine the amount of real
+space available to the sparse device. For this example, we'll assume /dev/sdb1
+is an available 10GB partition.
+echo "0 $TEN_TERABYTES snapshot /dev/mapper/zero1 /dev/sdb1 p 128" | \
+ dmsetup create sparse1
+This will create a 10TB sparse device called /dev/mapper/sparse1 that has
+10GB of actual storage space available. If more than 10GB of data is written
+to this device, it will start returning I/O errors.