dm-era is a target that behaves similar to the linear target. In addition it keeps track of which blocks were written within a user defined period of time called an ‘era’. Each era target instance maintains the current era as a monotonically increasing 32-bit counter.
Use cases include tracking changed blocks for backup software, and partially invalidating the contents of a cache to restore cache coherency after rolling back a vendor snapshot.
era <metadata dev> <origin dev> <block size>
metadata dev fast device holding the persistent metadata origin dev device holding data blocks that may change block size block size of origin data device, granularity that is tracked by the target
None of the dm messages take any arguments.
Possibly move to a new era. You shouldn’t assume the era has incremented. After sending this message, you should check the current era via the status line.
Create a clone of the metadata, to allow a userland process to read it.
Drop the metadata snapshot.
<metadata block size> <#used metadata blocks>/<#total metadata blocks> <current era> <held metadata root | ‘-‘>
|metadata block size||Fixed block size for each metadata block in sectors|
|#used metadata blocks||Number of metadata blocks used|
|#total metadata blocks||Total number of metadata blocks|
|current era||The current era|
|held metadata root||The location, in blocks, of the metadata root that has been ‘held’ for userspace read access. ‘-‘ indicates there is no held root|
Detailed use case¶
The scenario of invalidating a cache when rolling back a vendor snapshot was the primary use case when developing this target:
Taking a vendor snapshot¶
- Send a checkpoint message to the era target
- Make a note of the current era in its status line
- Take vendor snapshot (the era and snapshot should be forever associated now).
Rolling back to an vendor snapshot¶
- Cache enters passthrough mode (see: dm-cache’s docs in cache.txt)
- Rollback vendor storage
- Take metadata snapshot
- Ascertain which blocks have been written since the snapshot was taken by checking each block’s era
- Invalidate those blocks in the caching software
- Cache returns to writeback/writethrough mode
The target uses a bitset to record writes in the current era. It also has a spare bitset ready for switching over to a new era. Other than that it uses a few 4k blocks for updating metadata:
(4 * nr_blocks) bytes + buffers
Metadata is updated on disk before a write to a previously unwritten block is performed. As such dm-era should not be effected by a hard crash such as power failure.
Userland tools are found in the increasingly poorly named thin-provisioning-tools project: