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From "Paul Joseph Davis (JIRA)" <j...@apache.org>
Subject [jira] Commented: (COUCHDB-568) When delayed_commits = true, keep updated btree nodes in memory until the commit
Date Wed, 11 Nov 2009 20:24:39 GMT

    [ https://issues.apache.org/jira/browse/COUCHDB-568?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12776635#action_12776635
] 

Paul Joseph Davis commented on COUCHDB-568:
-------------------------------------------

My current idea is to have each b~tree use a pid between btree actions and the disk. This
would allow writes to be collected and flushed simultaneously thus saving rewrites of the
upper nodes, as well as having transparent "memory-only" edits to the tree that will be flushed
some time in the future.

Then for parallel writers, we allow them to traverse the tree and 'prepare writes' that can
be merged into the current memory state and eventually flushed. This has the benefit of releasing
all of the key collation from being bound to a single core in a fairly intuitive way.

This would also allow a mapping onto the current database delayed_commits with a write causing
the nodes in RAM to all be flushed or not as well as saving on the upper node duplications.

Or something to that effect.


> When delayed_commits = true, keep updated btree nodes in memory until the commit
> --------------------------------------------------------------------------------
>
>                 Key: COUCHDB-568
>                 URL: https://issues.apache.org/jira/browse/COUCHDB-568
>             Project: CouchDB
>          Issue Type: Improvement
>    Affects Versions: 0.10
>            Reporter: Adam Kocoloski
>
> rnewson reported on IRC that the new batch=ok implementation results in significantly
larger overhead in the .couch files.  This makes sense; the old batch mode waited 1 second
before saving, but the new implementation just updates the doc asynchronously.  With fast
hardware and moderate write rates it's likely that each document is being written separately.
> The overhead presumably arises from frequently updated btree inner nodes being written
to disk many times over.  I'm interested in exploring a modification of the delayed_commits
mode whereby the updated btree nodes are not actually written to disk immediately, but are
instead held in memory until the commit.  I'd like to think that this will result in more
compact files without any decrease in durability.  New read requests would still be able to
access these in-memory nodes.
> I realize the notion that updates go directly to disk is baked pretty deeply into couch_btree,
but I still thought this was worth bringing up to a wider audience.

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