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From graham sanderson <>
Subject Disaster recovery question
Date Sat, 16 Nov 2013 13:48:16 GMT
We are currently looking to deploy on the 2.0 line of cassandra, but obviously are watching
for bugs (we are currently on 2.0.2) - we are aware of a couple of interesting known bugs
to be fixed in 2.0.3 and one in 2.1, but none have been observed (in production use cases)
or are likely to affect our current proposed deployment.

I have a few general questions:

The first particular test we tried was to physically remove the SSD commit drive for one of
the nodes whilst under HEAVY write load (maybe a few hundred MB/s of data to be replicated
3 times - 6 node single local data center) and also while running read performance tests..
We currently have both node (CQL3) and Astyanax (Thrift) clients.

Frankly everything was pretty good (no read/write failures or indeed (observed) latency issues)
except, and maybe people can comment on any of these:

1) There were NO errors in the log on the node where we removed the commit log SSD drive -
this surprised us (of course our ops monitoring would detect the downed disk too, but we hope
to be able to look for ERROR level logging in system.log to cause alerts also)
2) The node with no commit log disk just kept writing to memtables, but:
3) This was causing major CMS GC issues which eventually caused the node to appear down (nodetool
status) to all other nodes, and indeed it itself saw all other nodes as down. That said dynamic
snitch and latency detection in clients seemed to prevent this being much of a problem other
than it seems potentially undesirable from a server side standpoint.
4) nodetool gossipinfo didn’t report anything abnormal for any nodes when run from any node.

Sadly because of an Astyanax issue (we were using the thrift code path that does a (now unnecessary)
describe cluster to check for schema disagreement before schema changes) we weren’t able
to create a new CF with a node marked down, and thus couldn’t immediately add more data
to see what would have happened: EOM or failure (we have since fixed this to go thru CQL3
code path but not yet re-run the tests because of other application level testing going on)…
that said maybe someone knows off the top of their head if there is a config setting that
would start failing writes (due to memtable size) before GC became an issue, and we just have
this misconfigured.

Secondly, our test was perhaps unrealistic in that when we brought the node back up, we did
so with the partial commit log on the replaced disk intact (but the memory data lost), but
we did get the following sorts of errors:

At level 1, SSTableReader(path='/data/2/cassandra/searchapi_dsp_approved_feed_beta/20131113151746_20131113_140712_1384348032/searchapi_dsp_approved_feed_beta-20131113151746_20131113_140712_1384348032-jb-12-Data.db')
[DecoratedKey(3508309769529441563, 2d37363730383735353837333637383432323934), DecoratedKey(9158434231083901894,
343934353436393734343637393130393335)] overlaps SSTableReader(path='/data/5/cassandra/searchapi_dsp_approved_feed_beta/20131113151746_20131113_140712_1384348032/searchapi_dsp_approved_feed_beta-20131113151746_20131113_140712_1384348032-jb-6-Data.db')
[DecoratedKey(7446234284568345539, 33393230303730373632303838373837373436), DecoratedKey(9158426253052616687,
2d313430303837343831393637343030313136)].  This could be caused by a bug in Cassandra 1.1.0
.. 1.1.3 or due to the fact that you have dropped sstables from another node into the data
directory. Sending back to L0.  If you didn't drop in sstables, and have not yet run scrub,
you should do so since you may also have rows out-of-order within an sstable

5) I guess the question is what is the best way to bring up a failed node 
	a) delete all data first? 
	b) clear data but restore from previous sstable from backup to miminise subsequent data transfer
	c) other suggestions

6) Our experience is that taking nodes down that have problems, then deleting data (subsets
if we can see partial corruption) and re-adding is much safer (but our cluster is VERY fast).
That said can we re-sync data before re-enabling gossip, or at least before serving read requests
from those nodes (not a huge issue but it would mitigate consistency issues with partially
recovered data in the case that multiple quorum read members were recovering) - note we fallback
from (LOCAL_)QUORUM to (LOCAL_ONE) on UnavaibleException, so have less guarantee compared
with both writing and reading at LOCAL_QUORUM (note that if our LOCAL_QUORUM writes fail we
will just retry when the cluster is fixed - stale data is not ideal but OK for a while)

That said given that the commit log on disk pre-dated any uncommitted lost memtable data,
it seems that we shouldn’t have seen exceptions because this is kind of like 5)b) in that
it should have gotten us closer to the correct state before the rest of the data was repaired
rather than causing any weirdness (unless it was a missed fsync problem), but maybe I’m
being naive.

Sorry for the long post, any thoughts would be appreciated.


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