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From koji Lin <koji....@gmail.com>
Subject Using EC2 ephemeral 4disk raid0 cause high iowait trouble
Date Wed, 16 May 2012 02:13:10 GMT
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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