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Diffstat (limited to 'Documentation/device-mapper/era.txt')
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diff --git a/Documentation/device-mapper/era.txt b/Documentation/device-mapper/era.txt deleted file mode 100644 index 3c6d01be3560..000000000000 --- a/Documentation/device-mapper/era.txt +++ /dev/null @@ -1,108 +0,0 @@ -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 |