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From "Adam Kocoloski (JIRA)" <>
Subject [jira] Commented: (COUCHDB-738) more efficient DB compaction (fewer seeks)
Date Tue, 04 May 2010 02:22:55 GMT


Adam Kocoloski commented on COUCHDB-738:

Any objections if I go ahead and commit this?

It would still be good to investigate DB size and view indexing performance for DBs with frequently-edited
documents, but I think maybe it's best to get this patch into trunk before it rots.

> more efficient DB compaction (fewer seeks)
> ------------------------------------------
>                 Key: COUCHDB-738
>                 URL:
>             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
> 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
> The patch can be applied directly to trunk@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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