[jira] [Commented] (CASSANDRA-7386) JBOD threshold to prevent unbalanced disk utilization

["Robert Stupp (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/CASSANDRA-7386?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14099753#comment-14099753 ] 

Robert Stupp commented on CASSANDRA-7386:
-----------------------------------------

I guess, what [~lyubent] sais is, that he started with C* configured to use 2 disks for data directories. Then added data to it.
After that, he restarted C* with 5 more drives (7 in sum now) and resumed adding data to the node.
The 5 new drives were used for new sstables (because they have a higher "write value" since these 5 new drives have more disk space left).
So it seems to work as expected.

I'm looking forward to see the results with busy disk results, when I/O read/write usage comes into play.

> 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: 2.1.1
>
>         Attachments: 7386-v1.patch, 7386v2.diff, Mappe1.ods, mean-writevalue-7disks.png, patch_2_1_branch_proto.diff, sstable-count-second-run.png
>
>
> 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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