| ======= |
| 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. |
