[Dovecot] Please help to make decision

[Stan Hoeppner]

On 3/28/2013 3:34 PM, Ed W wrote:
> I believe a variation on that theme is also to "double" each machine
> using DRBD so that machines are arranged in pairs. One can fail and the
> other will take over the load. ie each pair of machines mirrors the
> storage for the other. With this arrangement only warm failover is
> usually required and hence DRBD can run in async mode and performance
> impact is low

This is an active/passive setup, and doubles your hardware costs across
the board, with no parallel performance gain.  This is not financially
feasible for 1M users.  Going active/active would be better as you can
cut in half the number of server nodes required.  But here you must use
a cluster filesystem, and you're still buying double the quantity of
disks that are needed.

At this scale it is much more cost effective to acquire 4 midrange
FC/iSCSI SAN heads with 120x 15K 600GB SAS drives each, 480 total.  With
RAID10 you get 144TB net capacity.  An active/active DRBD solution would
require 960 drives instead of 480 for the same net storage and IOPS.
These drives run about $400 USD in such a bulk purchase depending on
vendor.  That's an extra ~$192,0000 wasted on drives.  Not to mention
all the extra JBOD chassis required, and more importantly the extra
power/cooling cost.  You can obtain 4 low frills high performance
midrange SAN heads for quite a bit less than that $192,000.  The Nexsan
E60 comes to mind.  Four FC SAN heads each with dual active/active
controllers and four 8Gb FC ports plus four expansion chassis, w/480x
600GB 15K drives in 32U, leaving 8U at the bottom of the rack for the
10KVA UPS needed to power them.

-- 
Stan