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From "Sean Busbey (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (ACCUMULO-3090) VolumeChooser should be able to decide per-file
Date Fri, 29 Aug 2014 17:03:53 GMT

    [ https://issues.apache.org/jira/browse/ACCUMULO-3090?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14115476#comment-14115476
] 

Sean Busbey commented on ACCUMULO-3090:
---------------------------------------

{quote}
So, when Sean asked how a tablet spread across volumes would affect disaster recovery, I took
it to mean backup and recovery.
{quote}

Correct, I meant recovering a given crashed cluster. Replication is needed for a real disaster
recovery plan, but there will be plenty of folks who need to fix broken systems without having
a hot-swappable backup.

Even those with one would probably prefer to avoid copying as much as possible, esp those
who have large enough clusters to warrant the complications of multiple volumes.

For example, how does [recovering from a lost ZK quorum|http://accumulo.apache.org/1.6/accumulo_user_manual.html#zookeeper_failure]
get impacted if the files for a given tablet are spread across volumes?

> VolumeChooser should be able to decide per-file
> -----------------------------------------------
>
>                 Key: ACCUMULO-3090
>                 URL: https://issues.apache.org/jira/browse/ACCUMULO-3090
>             Project: Accumulo
>          Issue Type: Improvement
>    Affects Versions: 1.6.0
>            Reporter: Christopher Tubbs
>
> Currently, the VolumeChooser decides only once per-tablet which volume to use for that
tablet. The directory is "sticky" after the decision is made. This can cause unexpected behavior
for users, which makes it harder to manage volume usage/capacity.
> One unexpected behavior is that data will still be written to an existing tablet's predetermined
volume, even if the volume is removed from instance.volumes.
> Another unexpected behavior this causes for users is when adding a new volume. One might
expect to compact tablets after adding a new volume, and have the new usage to be balanced
across all the volumes (using the provided RandomVolumeChooser), but due to the stickiness,
that is not the behavior seen. Instead, only new tablets (from new tables, or new splits)
will begin to randomly use the new volume.
> If the sticky behavior is desired, a volume chooser could still do that, by accepting
a "preferredTabletVolume"/"preferredTabletDirectory" in its environment, provided by the caller,
to use to make decisions.



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