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From "Benedict (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-7040) Replace read/write stage with per-disk access coordination
Date Tue, 15 Apr 2014 13:02:25 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-7040?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13969514#comment-13969514
] 

Benedict commented on CASSANDRA-7040:
-------------------------------------

Further, once we have this, we can experiment with periodically locking access to the disks
(for short, say 20-50ms periods) in order to let compactions/flushes catch up with any outstanding
work, if they appear to be getting behind.

> Replace read/write stage with per-disk access coordination
> ----------------------------------------------------------
>
>                 Key: CASSANDRA-7040
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-7040
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: Benedict
>              Labels: performance
>             Fix For: 3.0
>
>
> As discussed in CASSANDRA-6995, current coordination of access to disk is suboptimal:
instead of ensuring disk accesses alone are coordinated, we instead coordinate at the level
of operations that may touch the disks, ensuring only so many are proceeding at once. As such,
tuning is difficult, and we incur unnecessary delays for operations that would not touch the
disk(s).
> Ideally we would instead simply use a shared coordination primitive to gate access to
the disk when we perform a rebuffer. This work would dovetail very nicely with any work in
CASSANDRA-5863, as we could prevent any blocking or context switching for data that we know
to be cached. It also, as far as I can tell, obviates the need for CASSANDRA-5239.



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