mirror of
https://github.com/edk2-porting/linux-next.git
synced 2024-12-16 01:04:08 +08:00
1da177e4c3
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!
38 lines
1.6 KiB
Plaintext
38 lines
1.6 KiB
Plaintext
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.
|
|
|