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https://issues.apache.org/jira/browse/ACCUMULO-2268?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Eric Newton updated ACCUMULO-2268:
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Description:
For correctness Accumulo requires that only one tablet server at a time serve a
tablet. In order to enforce this constraint, Accumulo uses zookeeper locks.
It's assumed when a tablet server lock disappears that the tablet server will
kill itself. Therefore a tablet that's assigned to a dead tablet server can be
safely reassigned. However sometimes tablet servers continue to operate for a
period of time after losing their locks. Sometimes this is caused by bugs in
Accumulo, sometimes it's the Java GC or swapping (and the tserver does die),
sometimes it's problems with zookeeper (like the zk thread that reports lock
lost dies).
In Accumulo 1.6 conditional mutations were added. Making all tablet metadata
updates use conditional mutations could make multiply-assigned tablets less
able to do damage.
For example if after a minor compaction, the metadata update mutation could
require the tablet location to be the current tserver: it would prevent a
zombie tserver from adding an extraneous file to the metadata table for a
tablet.
[~ctubbsii] has discussed refactoring all metadata code so that its more
modular and works with zookeeper (for root tablet) and metadata table using
same API. This solution could depend on that. It may also be useful to make
the root tablet operate more like a regular tablet and store its list of files
in zookeeper. Then the root tablet could benefit from these changes with the
right abstraction layer.
was:
For correctness Accumulo requires that only one tablet server at a time serve a
tablet. Inorder to enforce this Accumulo uses zookeeper locks. Its assumed
when a tablet server lock disappears that the tablet server will kill itself.
Therefore a tablet thats assigned to a dead tablet server can be safely
reassigned. However sometimes tablet servers continue to operate for a period
of time after losing their locks. Sometimes this is caused by bugs in
Accumulo, sometimes its the Java GC or swapping (and the tserver does die),
sometimes its problems w/ zookeeper (like the zk thread that reports lock lost
dies).
In Accumulo 1.6 contditional mutations were added. Making all tablet metadata
updates use conditional mutations could make multiply assigned tablets less
able to do damage.
For example if after a minor compaction, the metadata update mutation required
the tablet location to be the current tserver that would prevent a zombie
tserver from adding an extraneuous file to the metadata table for a tablet.
[~ctubbsii] has discussed refactoring all metadata code so that its more
modular and works w/ zookeeper (for root tablet) and metadata table using same
API. This solution could depend on that. It may also be useful to make the
root tablet operate more like a regular tablet and store its list of files in
zookeeper. Then the root tablet could benefit from these changes w/ the right
abstraction layer.
> Use conditional mutations to update metadata table
> --------------------------------------------------
>
> Key: ACCUMULO-2268
> URL: https://issues.apache.org/jira/browse/ACCUMULO-2268
> Project: Accumulo
> Issue Type: Improvement
> Reporter: Keith Turner
> Fix For: 1.7.0
>
>
> For correctness Accumulo requires that only one tablet server at a time serve
> a tablet. In order to enforce this constraint, Accumulo uses zookeeper
> locks. It's assumed when a tablet server lock disappears that the tablet
> server will kill itself. Therefore a tablet that's assigned to a dead tablet
> server can be safely reassigned. However sometimes tablet servers continue
> to operate for a period of time after losing their locks. Sometimes this is
> caused by bugs in Accumulo, sometimes it's the Java GC or swapping (and the
> tserver does die), sometimes it's problems with zookeeper (like the zk thread
> that reports lock lost dies).
> In Accumulo 1.6 conditional mutations were added. Making all tablet metadata
> updates use conditional mutations could make multiply-assigned tablets less
> able to do damage.
> For example if after a minor compaction, the metadata update mutation could
> require the tablet location to be the current tserver: it would prevent a
> zombie tserver from adding an extraneous file to the metadata table for a
> tablet.
> [~ctubbsii] has discussed refactoring all metadata code so that its more
> modular and works with zookeeper (for root tablet) and metadata table using
> same API. This solution could depend on that. It may also be useful to make
> the root tablet operate more like a regular tablet and store its list of
> files in zookeeper. Then the root tablet could benefit from these changes
> with the right abstraction layer.
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