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LCS: Fix terrible write amplification when reshaping level 0

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LCS reshape is basically 'major compacting' level 0 until it contains less than
N sstables.

That produces terrible write amplification, because any given byte will be
compacted (initial # of sstables / max_threshold (32)) times. So if L0 initially
contained 256 ssts, there would be a WA of about 8.

This terrible write amplification can be reduced by performing STCS instead on
L0, which will leave L0 in a good shape without hurting WA as it happens
now.

Fixes #8345.

Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
Message-Id: <20210322150655.27011-1-raphaelsc@scylladb.com>
This commit is contained in:
Raphael S. Carvalho
2021-03-22 12:06:55 -03:00
committed by Avi Kivity
parent 24a43681b4
commit bcbb39999b
1 changed files with 2 additions and 4 deletions
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6
sstables/leveled_compaction_strategy.cc
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@@ -189,10 +189,8 @@ leveled_compaction_strategy::get_reshaping_job(std::vector<shared_sstable> input
};
if (level_info[0].size() > offstrategy_threshold) {
level_info[0].resize(std::min(level_info[0].size(), max_sstables));
compaction_descriptor desc(std::move(level_info[0]), std::optional<sstables::sstable_set>(), iop);
desc.options = compaction_options::make_reshape();
return desc;
size_tiered_compaction_strategy stcs(_stcs_options);
return stcs.get_reshaping_job(std::move(level_info[0]), schema, iop, mode);
}
for (unsigned level = leveled_manifest::MAX_LEVELS - 1; level > 0; --level) {