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From Edward Capriolo <edlinuxg...@gmail.com>
Subject Re: Major compaction does not seems to free the disk space a lot if wide rows are used.
Date Thu, 16 May 2013 20:30:21 GMT
This makes sense. Unless you are running major compaction a delete could
only happen if the bloom filters confirmed the row was not in the sstables
not being compacted. If your rows are wide the odds are that they are in
most/all sstables and then finally removing them would be tricky.


On Thu, May 16, 2013 at 12:00 PM, Louvet, Jacques <
Jacques_Louvet@cable.comcast.com> wrote:

>  Boris,
>
>  We hit exactly the same issue, and you are correct the newly created
> SSTables are the cause of why most of the column-tombstone not being purged.
>
>  There is an improvement in 1.2 train where both the minimum and maximum
> timestamp for a row is now stored and used during the compaction to
> determine if the portion of the row can be purged.
> However, this only appears to help Major compaction has the other
> restriction where all the files encompassing the deleted rows must be part
> of the compaction for the row to be purged still remains.
>
>  We have switched to column delete rather that row delete wherever
> practical. A little more work on the app, but a big improvement in reads
> due to much more efficient compaction.
>
>  Regards,
> Jacques
>
>   From: Boris Yen <yulinyen@gmail.com>
> Reply-To: "user@cassandra.apache.org" <user@cassandra.apache.org>
> Date: Thursday, May 16, 2013 04:07
> To: "user@cassandra.apache.org" <user@cassandra.apache.org>, "
> dev@cassandra.apache.org" <dev@cassandra.apache.org>
> Subject: Major compaction does not seems to free the disk space a lot if
> wide rows are used.
>
>      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
>

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