linux-next/Documentation/device-mapper/zero.txt

dm-zero
=======

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.