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From Philippe <watche...@gmail.com>
Subject Re: Unable to repair a node
Date Tue, 16 Aug 2011 21:11:31 GMT
Even more interesting behavior : a repair on a CF has consequences on other
CFs. I didn't expect that.

There are no writes being issued to the cluster yet the logs indicate that

   - SSTableReader has opened dozens and dozens of files, most of them
   unrelated to the CF being repaired
   - compactions are taking place continuously on CFs other than the one
   being repaired, even CFs in other keyspaces
   - I see "Sending AEService tree" messages for CF not being repaired.


After a very long time, I got some AES messages indicating that streaming
from node C had finished and then many minutes after that node B. And yet
the pending stream count on node B hasn't changed.

The *-data.db files for the CF being repaired are about 70MB on-disk.

Maybe when a stream is fully received on node B, netstats indicates that no
streams are pending but since they are not acknowledged, node A doesn't ?


2011/8/16 Philippe <watcherfr@gmail.com>

> I'm still trying different stuff. Here are my latest findings, maybe
> someone will find them useful:
>
>    - I have been able to repair some small column families by issuing a
>    repair [KS] [CF]. When testing on the ring with no writes at all, it still
>    takes about 2 repairs to get "consistent" logs for all AES requests.
>    - Launching a repair one the smallest CF of the biggest KS has
>    triggered a flurry of compactions and streams. Some of those streams are for
>    other CF in that keyspace !?
>    - During repairs (one at a time cluster-wide), I get 25-50% io waits &
>    35%-50% cpu usage on a 6 core SATA-disk setup
>
> What is surprising to me (bug?) is that netstats shows me streams going
> from node A to node B at 0% progress. But netstats on node B doesn't show me
> any streams coming in. I'm thinking that repairs may be never ending and
> that may be messing up my compactions hence the huge pile up of compactions
> until the disk fulls.
> I know there's an issue related to failed streams & repairs, could I be
> hitting it ?
>
> Thanks
>
> 2011/8/14 Philippe <watcherfr@gmail.com>
>
>> @Teijo : thanks for the procedure, I hope I won't have to do that
>>
>> Peter, I'll answer inline. Thanks for the detailed answer.
>>
>>
>>>  > the number of SSTables for some keyspaces goes dramatically up (from 3
>>> or 4
>>> > to several dozens).
>>>
>>> Typically with a long running compaction, such as that triggered by
>>> repair, that's what happens as flushed memtables accumulate. In
>>> particular for memtables with frequent flushes.
>>>
>>> Are you running with concurrent compaction enabled?
>>>
>> Yes, it is enabled. On my 0.8 cluster, cassandra.yaml has this (it's
>> commented). BTW, I have 6 cores on each server.
>>
>> #concurrent_compactors: 1
>>
>> > the commit log keeps increasing in size, I'm at 4.3G now, it went up to
>>> 40G
>>> > when the compaction was throttled at 16MB/s. On the other nodes it's
>>> around
>>> > 1GB at most
>>> Hmmmm. The Commit Log should not be retained longer than what is
>>> required for memtables to be flushed. Is it possible you have had an
>>> out-of-disk condition and flushing has stalled? Are you seeing flushes
>>> happening in the log?
>>>
>> No I don't believe there was ever an out of disk.  Yes it is flushing for
>> the first couple of hours.
>> Then, when repair seems locked up, my log is mostly filled with lines such
>> as this
>> INFO [ScheduledTasks:1] 2011-08-14 23:15:47,267 StatusLogger.java (line
>> 88) [My_Keyspace].[My_Columnfamily]           45,105541               50/50
>>               20/20
>>  Why is that ?
>>
>> > the data directory is bigger than on the other nodes. I've seen it go up
>>> to
>>> > 480GB when the compaction was throttled at 16MB/s
>>> How much data are you writing? Is it at all plausible that the huge
>>> spike is a reflection of lots of overwriting writes that aren't being
>>> compacted?
>>>
>> No, there's no bulk loading going on at the moment and I'm pretty sure
>> there wasn't when it spiked up to that load.
>> I've never measured the load because it's a mix of counter increments and
>> new counters all the time. It's not that much though.
>>
>>
>>> Normally when disk space spikes with repair it's due to other nodes
>>> streaming huge amounts (maybe all of their data) to the node, leading
>>> to a temporary spike. But if your "real" size is expected to be 60,
>>> 480 sounds excessive. Are you sure other nodes aren't running repairs
>>> at the same time and magnifying each other's data load spikes?
>>>
>> Yes, the two other nodes were running repairs. I had them scheduled at 8
>> hour intervals but they must have started.
>> When data is streamed from one to another, does that data go into the
>> commit log as a regular write ?
>>  How much of a negative impact can that have on the repair going on on
>> this node ?
>>
>> > What's even weirder is that currently I have 9 compactions running but
>>> CPU
>>> > is throttled at 1/number of cores half the time (while > 80% the rest
>>> of the
>>> > time). Could this be because other repairs are happening in the ring ?
>>> You mean compaction is taking less CPU than it "should"?
>>>
>> Yes
>>
>>
>>> No, this should not be due to other nodes repairing. However it sounds
>>> to me like you are bottlenecking on I/O and the repairs and
>>>
>> Yes, I/O is really high on the node right now. Around 50% I/O waits.
>>
>>
>>> compactions are probably proceeding extremely slowly, probably being
>>> completely drowned out by live traffic (which is probably having an
>>> abnormally high performance impact due to data size spike).
>>>
>> Yes, the live traffic is 3 to 10x times slower during repair. Ouch... I
>> hope I won't to do this too often while in production !
>>
>>
>>>
>>> What's your read concurrency configured on the node? What does "iostat
>>> -x -k 1" show in the average queue size column?
>>
>> Average queue size on the disk (RAID-1 + separate LVM volumes for data,
>> commit log, caches, logs)) varies between 2 and 90. I'd say the average is
>> around 30-40. Very high variation.
>>
>>
>>> Is "nodetool -h
>>> localhost tpstats" showing that ReadStage is usually "full" (@ your
>>> limit)?
>>>
>> No backlog at all in tpstats
>>
>> I've figured out how AES is logging its actions and it looks like it
>> really is going through every CF in every keyspace and doing a tree request
>> for every token range
>> So it really looks like it's just taking forever to compact stuff as it's
>> repairing.
>> I saw in another email that repairing was taking 2-3mn/ GB... it looks
>> like a lot more for my ring. Anybody else have numbers ?
>>
>> Thanks
>>
>
>

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