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From aaron morton <>
Subject Re: Using EC2 ephemeral 4disk raid0 cause high iowait trouble
Date Wed, 16 May 2012 09:59:43 GMT
On Ubuntu ? Sounds like


Aaron Morton
Freelance Developer

On 16/05/2012, at 2:13 PM, koji Lin wrote:

> Hi
> Our service already run cassandra 1.0 on 1x ec2 instances(with ebs), and we saw lots
of discussion talk about using  ephemeral raid for better performance and consistent performance.
> So we want to create new instance using 4 ephemeral raid0, and copy the data from ebs
to finally replace the old instance and reduce some . 
> we create the xlarge instance with -b '/dev/sdb=ephemeral0' -b '/dev/sdc=ephemeral1'
-b '/dev/sdd=ephemeral2' -b '/dev/sde=ephemeral3',
> and use mdadm command like this  mdadm --create /dev/md0 --level=0 -c256 --raid-devices=4
/dev/sdb /dev/sdc /dev/sdd /dev/sde
> after copying file and start the cassandra(same token as old instance it replaced).
> we saw the read is really fast always keep 2xxm/sec, but system load exceed 40, with
high iowait, and lots of client get timeout result. We guess maybe it's the problem of ec2
instance, so we create another one with same setting to replace other machine ,but the result
is same . Then we rollback to ebs with single disk ,read speed keeps at 1xmb/sec but system
becomes well .(using ebs with 2 disks raid0 will keep at 2xmb/sec and higher iowait then single
disk ,but still works)
> Is there anyone meet the same problem too ? or do we forget something to configure?
> thank you
> koji

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