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.
Thanks,
Graham.
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