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Cleanup compaction works by rewriting all sstables that need clean up, one at
a time.
This approach can cause bad write amplification because the output data is
being made incrementally available for regular compaction.
Cleanup is a long operation on large data sets, and while it's happening,
new data can be written to buckets, triggering regular compaction.
Cleanup fighting for resources with regular compaction is a known problem.
With cleanup adding one file at a time to buckets, regular may require multiple
rounds to compact the data in a given bucket B, producing bad writeamp.
To fix this problem, cleanup will be made bucket aware. As each compaction
strategy has its own definition of bucket, strategies will implement their
own method to retrieve cleanup jobs. The method will be implemented such that
all files in a bucket B will be cleaned up together, and on completion,
they'll be made available for regular at once.
For STCS / ICS, a bucket is a size tier.
For TWCS, a bucket is a window.
For LCS, a bucket is a level.
In this way, writeamp problem is fixed as regular won't have to perform
multiple rounds to compact the data in a given bucket. Additionally, cleanup
will now be able to deduplicate data and will become way more efficient at
garbage collecting expired data.
The space requirement shouldn't be an issue, as compacting an entire bucket
happens during regular compaction anyway.
With leveled strategy, compacting an entire level is also not a problem because
files in a level L don't overlap and therefore incremental compaction is
employed to limit the space requirement.
By the time being, only STCS cleanup was made bucket aware. The others will be
using a default method, where one file is cleaned up at a time. Making cleanup
of other strategies bucket aware is relatively easy now and will be done soon.
Refs #10097.
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* 'cleanup-compaction-revamp/v3' of https://github.com/raphaelsc/scylla:
test: sstable_compaction_test: Add test for strategy cleanup method
compaction: STCS: Implement cleanup strategy
compaction_manager: Wire cleanup task into the strategy cleanup method
compaction_strategy: Allow strategies to define their own cleanup strategy
compaction: Introduce compaction_descriptor::sstables_size
compaction: Move decision of garbage collection from strategy to task type
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