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From Jonathan Ellis <>
Subject Re: Cassandra on iSCSI?
Date Fri, 21 Jan 2011 03:50:53 GMT
On Thu, Jan 20, 2011 at 2:13 PM, Mick Semb Wever <> wrote:
> To go with raid-5 disks our hosting provider requires proof that iSCSI
> won't work. I tried various things (eg `nodetool cleanup` on 12Gb load
> giving 5k IOPS) but iSCSI seems to keep up to the performance of the
> local raid-5 disks...
> Should i be worried about using iSCSI?

It should work fine; the main reason to go with local storage is the
huge cost advantage.

Of course with a SAN you'd want RF=1 since it's replicating internally.

> Are there better tests i should be running?

I would test write scalability going from 1 machine, to half your
planned cluster size, to your full cluster size, or as close as is
feasible, using enough client machines running contrib/stress* (much
faster than contrib/py_stress) that you saturate it.

Writes should be CPU bound, so you expect those to scale roughly
linearly as you add Cassandra nodes.

Reads (once your data set can't be cached in RAM) will be i/o bound,
so I imagine with a SAN you'll be able to max that out at some number
of machines and adding more Cassandra nodes won't help.  What that
limit is depends on your SAN iops and how much of it is being consumed
by other applications.

*I just committed a README for contrib/stress to the 0.7 svn branch

Jonathan Ellis
Project Chair, Apache Cassandra
co-founder of Riptano, the source for professional Cassandra support

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