Introduction ============ 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. Constructor =========== 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 Messages ======== None of the dm messages take any arguments. checkpoint ---------- 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. take_metadata_snap ------------------ Create a clone of the metadata, to allow a userland process to read it. drop_metadata_snap ------------------ Drop the metadata snapshot. Status ====== <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 Memory usage ============ 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 Resilience ========== 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 ============== Userland tools are found in the increasingly poorly named thin-provisioning-tools project: https://github.com/jthornber/thin-provisioning-tools