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