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From Boris Yen <yulin...@gmail.com>
Subject Re: Major compaction does not seems to free the disk space a lot if wide rows are used.
Date Sat, 18 May 2013 14:26:13 GMT
Thank you for the reply. It is really helpful.

We will take a look at the patch to see if we could apply it on 1.0 branch
or try to workaround it by changing our application implementation.

Regards,
Boris

On Thu, May 16, 2013 at 10:43 PM, Yuki Morishita <mor.yuki@gmail.com> wrote:

> You are right about the behavior of cassandra compaction.
> It checks if the key exists on the other SSTable files that are not in
> the compaction set.
>
> I think https://issues.apache.org/jira/browse/CASSANDRA-4671 would
> help if you upgrade to latest 1.2,
> but in your version, I think the workaround is to stop write not to
> flush, then compact.
>
> On Thu, May 16, 2013 at 3:07 AM, Boris Yen <yulinyen@gmail.com> wrote:
> > Hi All,
> >
> > Sorry for the wide distribution.
> >
> > Our cassandra is running on 1.0.10. Recently, we are facing a weird
> > situation. We have a column family containing wide rows (each row might
> > have a few million of columns). We delete the columns on a daily basis
> and
> > we also run major compaction on it everyday to free up disk space (the
> > gc_grace is set to 600 seconds).
> >
> > However, every time we run the major compaction, only 1 or 2GB disk space
> > is freed. We tried to delete most of the data before running compaction,
> > however, the result is pretty much the same.
> >
> > So, we tried to check the source code. It seems that the column
> tombstones
> > could only be purged when the row key is not in other sstables. I know
> the
> > major compaction should include all sstables, however, in our use case,
> > columns get inserted rapidly. This will make the cassandra flush the
> > memtables to disk and create new sstables. The newly created sstables
> will
> > have the same keys as the sstables that are being compacted (the
> compaction
> > will take 2 or 3 hours to finish). My question is that will these newly
> > created sstables be the cause of why most of the column-tombstone not
> being
> > purged?
> >
> > p.s. We also did some other tests. We inserted data to the same CF with
> the
> > same wide-row pattern and deleted most of the data. This time we stopped
> > all the writes to cassandra and did the compaction. The disk usage
> > decreased dramatically.
> >
> > Any suggestions or is this a know issue.
> >
> > Thanks and Regards,
> > Boris
>
>
>
> --
> Yuki Morishita
>  t:yukim (http://twitter.com/yukim)
>

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