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From Jonathan Ellis <jbel...@gmail.com>
Subject Re: do I need to add more nodes? minor compaction eat all IO
Date Sun, 24 Jul 2011 20:57:54 GMT
It's sequential per-sstable.  If you are compacting a lot of sstables
how closely this approximates "completely sequential" will
deteriorate.

On Sun, Jul 24, 2011 at 1:18 PM, Francois Richard <frichard@xobni.com> wrote:
> Jonathan,
>
> Are you sure that the reads done for compaction are sequential with Cassandra 0.6.13?
 This is not what I am observing right now.  During a minor compaction I usually observe
~ 1500 to 1900 r/s while rMB/s is barely around 30 to 35MB/s.
>
> Just asking out of curiosity.
>
>
> FR
>
> -----Original Message-----
> From: Jonathan Ellis [mailto:jbellis@gmail.com]
> Sent: Saturday, July 23, 2011 5:05 PM
> To: user@cassandra.apache.org
> Subject: Re: do I need to add more nodes? minor compaction eat all IO
>
> On Sat, Jul 23, 2011 at 4:16 PM, Francois Richard <frichard@xobni.com> wrote:
>> My understanding is that during compaction cassandra does a lot of non sequential
readsa then dumps the results with a big sequential write.
>
> Compaction reads and writes are both sequential, and 0.8 allows setting a MB/s to cap
compaction at.
>
> As to the original question "do I need to add more machines" I'd say that depends more
on whether your application's SLA is met, than what % io util spikes to.
>
> --
> Jonathan Ellis
> Project Chair, Apache Cassandra
> co-founder of DataStax, the source for professional Cassandra support http://www.datastax.com
>



-- 
Jonathan Ellis
Project Chair, Apache Cassandra
co-founder of DataStax, the source for professional Cassandra support
http://www.datastax.com

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