[Dovecot] Best Cluster Storage

Stan Hoeppner stan at hardwarefreak.com
Fri Jan 21 20:13:54 EET 2011

Henrique Fernandes put forth on 1/21/2011 9:50 AM:

> Let me try explain better.
> We have 3 virtual machines with this set up:
> /dev/sda1             3.6T  2.4T  1.3T  66% /A
> /dev/sdb1             1.0T   36G  989G   4% /B
> /dev/sdc1             1.0T  3.3G 1021G   1% /C
> /dev/sda1 on /A type ocfs2 (rw,_netdev,heartbeat=local)
> /dev/sdb1 on /B type ocfs2 (rw,_netdev,heartbeat=local)
> /dev/sdc1 on /C type ocfs2 (rw,_netdev,heartbeat=local)
> My question is, what is faster ? Configuring just one big disk with ocfs2 (
> sda1) or using more and smaller disks sdb1 and sdc1 and more ?
> It is ok now ?
> All our emails are in sda1  and we are having many many performance
> problens. So we are migrating some of email to sdb1 and eventualy to sdc1.
> Right now, seens to be much better performance in sdb1 than in sda1. But we
> are not sure if it is because have so much less emails and concurrency or
> because is acctualy better.

None of this means much in absence of an accurate ESX host hardware and iSCSI
network layout description.  You haven't stated how /dev/sd[abc]1 are physically
connected to the ESX hosts.  You haven't given a _physical hardware description_
of /dev/sd[abc]1 or the connections to the EMC CX4.

For instance, if /dev/sda1 in a 10 disk RAID5 group in the CX4, but /dev/sdb1 is
a 24 disk RAID10 group in the CX4, *AND*

/dev/sda1 is LUN mapped out of an iSCSI port on the CX4 along with many many
other LUNS which are under constant heavy use, *AND* /dev/sdb1 is LUN mapped out
of an iSCSI port that shares no other LUNs, *then*

I would say the reason /dev/sdb1 is much faster is due to:

A.  24 drive RAID10 vs 10 drive RAID6 will yield ~10x increase in random IOPS
B.  Zero congestion on the /dev/sdb1 iSCSI port will decrease latency

We need to know the physical characteristics of the hardware.  SAN performance
issues are not going to be related (most of the time) to how you have Dovecot setup.

Do you have any iostat data to share with us?  Any data/graphs from the EMC
controller showing utilization per port and per array?

If you're unable to gather such performance metric data it will be difficult to
assist you.


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