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https://issues.apache.org/jira/browse/CASSANDRA-2816?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13057339#comment-13057339
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Terje Marthinussen commented on CASSANDRA-2816:
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This is what heap looks like when GC start slowing things down so much that
even gossip gets delayed long enough for nodes to be down for some seconds.
num #instances #bytes class name
----------------------------------------------
1: 9453188 453753024 java.nio.HeapByteBuffer
2: 10081546 392167064 [B
3: 7616875 243740000 org.apache.cassandra.db.Column
4: 9739914 233757936
java.util.concurrent.ConcurrentSkipListMap$Node
5: 4131938 99166512
java.util.concurrent.ConcurrentSkipListMap$Index
6: 1549230 49575360 org.apache.cassandra.db.DeletedColumn
I guess this really ends up maybe being the mix of everything going on in total
and all the reading and writing that may occur when repair runs (valiadation
compactions, streaming, normal compactions and regular traffic all at the same
time and maybe many CFs at the same time).
However, I have suspected for some time that our young size was a bit on the
small side and after increasing it and giving the heap a few more GB to work
with, it seems like things are behaving quite a bit better.
I mentioned issues with this patch when testing for CASSANDRA-2521. That was a
problem caused by me. Was playing around with git for the first time and I
manage to apply 2816 to a different branch than the one I used for testing....
:(
My appologies.
Initial testing with that corrected looks a lot better for my small scale test
case, but I noticed one time where I deleted an sstable and restarted. It did
not get repaired (repair scanned but did nothing).
Not entirely sure what to make out of that, I then tested to delete another
sstable and repair started running.
I will test more over the next days.
> Repair doesn't synchronize merkle tree creation properly
> --------------------------------------------------------
>
> Key: CASSANDRA-2816
> URL: https://issues.apache.org/jira/browse/CASSANDRA-2816
> Project: Cassandra
> Issue Type: Bug
> Components: Core
> Reporter: Sylvain Lebresne
> Assignee: Sylvain Lebresne
> Labels: repair
> Fix For: 0.8.2
>
> Attachments: 0001-Schedule-merkle-tree-request-one-by-one.patch
>
>
> Being a little slow, I just realized after having opened CASSANDRA-2811 and
> CASSANDRA-2815 that there is a more general problem with repair.
> When a repair is started, it will send a number of merkle tree to its
> neighbor as well as himself and assume for correction that the building of
> those trees will be started on every node roughly at the same time (if not,
> we end up comparing data snapshot at different time and will thus mistakenly
> repair a lot of useless data). This is bogus for many reasons:
> * Because validation compaction runs on the same executor that other
> compaction, the start of the validation on the different node is subject to
> other compactions. 0.8 mitigates this in a way by being multi-threaded (and
> thus there is less change to be blocked a long time by a long running
> compaction), but the compaction executor being bounded, its still a problem)
> * if you run a nodetool repair without arguments, it will repair every CFs.
> As a consequence it will generate lots of merkle tree requests and all of
> those requests will be issued at the same time. Because even in 0.8 the
> compaction executor is bounded, some of those validations will end up being
> queued behind the first ones. Even assuming that the different validation are
> submitted in the same order on each node (which isn't guaranteed either),
> there is no guarantee that on all nodes, the first validation will take the
> same time, hence desynchronizing the queued ones.
> Overall, it is important for the precision of repair that for a given CF and
> range (which is the unit at which trees are computed), we make sure that all
> node will start the validation at the same time (or, since we can't do magic,
> as close as possible).
> One (reasonably simple) proposition to fix this would be to have repair
> schedule validation compactions across nodes one by one (i.e, one CF/range at
> a time), waiting for all nodes to return their tree before submitting the
> next request. Then on each node, we should make sure that the node will start
> the validation compaction as soon as requested. For that, we probably want to
> have a specific executor for validation compaction and:
> * either we fail the whole repair whenever one node is not able to execute
> the validation compaction right away (because no thread are available right
> away).
> * we simply tell the user that if he start too many repairs in parallel, he
> may start seeing some of those repairing more data than it should.
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