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https://issues.apache.org/jira/browse/COUCHDB-738?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12863670#action_12863670
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Adam Kocoloski commented on COUCHDB-738:
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An updated eprof:
http://friendpaste.com/44ZPZ4dq88oIUBkOHfWfWh
The big difference is the disappearance of couch_key_tree:get_full_key_paths/4
> more efficient DB compaction (fewer seeks)
> ------------------------------------------
>
> Key: COUCHDB-738
> URL: https://issues.apache.org/jira/browse/COUCHDB-738
> Project: CouchDB
> Issue Type: Improvement
> Components: Database Core
> Affects Versions: 0.9.2, 0.10.1, 0.11
> Reporter: Adam Kocoloski
> Assignee: Adam Kocoloski
> Fix For: 1.1
>
> Attachments: 738-efficient-compaction-v1.patch,
> 738-efficient-compaction-v2.patch
>
>
> CouchDB's database compaction algorithm walks the by_seq btree, then does a
> lookup in the by_id btree for every document in the database. It does this
> because the #full_doc_info{} record with the full revision tree is only
> stored in the by_id tree. I'm proposing instead to store duplicate copies of
> #full_doc_info{} in both trees, and to have the compactor use the by_seq tree
> exclusively. The net effect is significantly fewer calls to pread(), and an
> compaction IO pattern where reads tend to be clustered close to each other in
> the file.
> If the by_id tree is fully cached, or if the id tree nodes are located near
> the seq tree nodes, the performance improvement is small but noticeable (~10%
> in some simple tests). On the other hand, in the worst-case scenario of
> randomly-generated docids and a database much larger than main memory the
> improvement is huge. Joe Williams did some simple benchmarks with a 50k
> document, 600 MB database on a 256MB VPS. The compaction time for that DB
> dropped from 15m to 2m20s, so more than 6x faster.
> Storing the #full_doc_info{} in the seq tree also allows for some similar
> optimizations in the replicator.
> This patch might have downsides when documents have a large number of edits.
> These include an increase in the size of the database and slower view
> indexing. I expect both to be small effects.
> The patch can be applied directly to tr...@934272. Existing DBs are still
> readable, new updates will be written in the new format, and databases can be
> fully upgraded by compacting.
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