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From "Robert Stupp (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (CASSANDRA-7386) JBOD threshold to prevent unbalanced disk utilization
Date Tue, 18 Nov 2014 23:38:37 GMT

     [ https://issues.apache.org/jira/browse/CASSANDRA-7386?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Robert Stupp updated CASSANDRA-7386:
------------------------------------
    Attachment: 7386-2.1-v4.txt
                7386-2.0-v4.txt

Patch v4 comments:
* Made {{Directories.getWriteableLocation}} return {{null}} if no writable directory present
(to let the calling code react on that).
* Let all callers pass a valid {{writeSize}} which means near-to-full directories can not
be returned as targets for large compactions, streams, etc
* Removed {{StorageService.requestGC}} which was referenced in {{Directories.getDirectoryForNewSSTables}}
but never called (since {{getWriteableLocation}} never returned null).

[~yukim] can you review v4 ?

> JBOD threshold to prevent unbalanced disk utilization
> -----------------------------------------------------
>
>                 Key: CASSANDRA-7386
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-7386
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: Chris Lohfink
>            Assignee: Robert Stupp
>            Priority: Minor
>             Fix For: 2.1.3
>
>         Attachments: 7386-2.0-v3.txt, 7386-2.0-v4.txt, 7386-2.1-v3.txt, 7386-2.1-v4.txt,
7386-v1.patch, 7386v2.diff, Mappe1.ods, mean-writevalue-7disks.png, patch_2_1_branch_proto.diff,
sstable-count-second-run.png, test1_no_patch.jpg, test1_with_patch.jpg, test2_no_patch.jpg,
test2_with_patch.jpg, test3_no_patch.jpg, test3_with_patch.jpg
>
>
> Currently the pick the disks are picked first by number of current tasks, then by free
space.  This helps with performance but can lead to large differences in utilization in some
(unlikely but possible) scenarios.  Ive seen 55% to 10% and heard reports of 90% to 10% on
IRC.  With both LCS and STCS (although my suspicion is that STCS makes it worse since harder
to be balanced).
> I purpose the algorithm change a little to have some maximum range of utilization where
it will pick by free space over load (acknowledging it can be slower).  So if a disk A is
30% full and disk B is 5% full it will never pick A over B until it balances out.



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