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From Nicolas Peeters <nicoli...@gmail.com>
Subject CouchDB compaction not catching up.
Date Thu, 07 Mar 2013 07:19:22 GMT
Hi CouchDB Users,

*Disclaimer: I'm very aware that the use case is definitely not the best
for CouchDB, but for now, we have to deal with it.*

*Scenario:*

We have a fairly large (~750Gb) CouchDB (1.2.0) database that is being used
for transactional logs (very write heavy) (bad idea/design, I know, but
that's besides the point of this question - we're looking at alternative
designs). Once in a while, we delete some of the records in large batches
and we have scheduled auto compaction, checking every 2 hours.

This is the compaction config:

[image: Inline image 1]

>From what I can see, the DB is being hammered significantly every 12 hours
and the compaction is taking (sometimes 24 hours (with a size of 100GB of
log data, sometimes much more (up to 500GB)).

We run on EC2. Large instances with EBS. No striping (yet), no IOPS. We
tried fatter machines, but the improvement was really minimal.

**

*The problem:*

The problem is that compaction takes a very long time (e.g. 12h+) and
reduces the performance of the entire stack. The main issue seems to be
that it's hard for the compaction process to "keep up" with the insertions,
hence why it takes so long. Also, the compaction of the view takes long
time (sometimes the view is 100GB). During the re-compaction of the view,
clients don't get a response, which is blocking the processes.

[image: Inline image 2]

The view compaction takes approx. 8 hours and the indexing for the view are
therefore slower and during the time that view indexes, another 300k
insertions have been done (and it doesn't catch up). The only way to solve
the problem was to throttle the number of inserts from the app itself and
then eventually the view compaction resolved. If we would have continued to
insert at the same rate, it would not have finished (and ultimately, we
would have run out of disk space).

Any recommendations to set it up on EC2 is welcome. Also configuration
settings for the compaction would be helpful.

Thanks.

Nicolas

PS: We are happily using CouchDB for other (more traditional) use case
where it does go very well.

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