[Dovecot] Best Cluster Storage

[Stan Hoeppner]

Henrique Fernandes put forth on 1/22/2011 2:59 PM:

> About change the EMC to raid 10, can not do it because other people are
> using it. So we can not edit anything one strage. Those other luns i talk
> about, they are ment to be for WEB but as we are testing we are allowed to
> use it.

You need to look at this:  http://www.hardwarefreak.com/thin-provisioning.jpg

The exported LUNS need to be configured as independent of the underlying
physical disk RAID level.  You can reconfigure up to 16 disks of the CX4 as RAID
10, and export as many LUNS of any size as you like.  I.e. you can keep the same
exported LUNS you have now, although you may have to adjust their sizes a bit.
What you gain by doing this is a doubling of random IOPS.

You will need to backup any real/live data on the current LUNS.  You only have
16 disks in the CX4 correct?  So reconfigure with 2 spares and 14 disks in a
RAID 10.  Then create the same LUNS you had before.  I don't use EMC products so
I don't know their terminology.  But, this is usually called something like
"virtual disks" or "virtual LUNS".  The industry calls this "thin provisioning".
 You *need* to do this if you're going to support hundreds or more concurrent
users.  An effective stripe width of only 4 spindles, which is what you
currently have with an 8 disk RAID10, isn't enough.  That gives you only
600-1200 IOPS depending on the RPM of the disks in the array:  600 for 7.2k
disks, and 1200 for 15k disks.  With a 14 disk RAID10 you'll have IOPS of 1050
to 2100.

> Much that you said, we are trying to do, but we don't have the hardware.
> 
> Our XEN just have 2  1gbE enterfaces, and we are using one for get external
> and another one to use the storage ( ours vms is also in ocfs2 to be able to
> migrate and etc to any host )

In that case you _really really_ need more IOPS and throughput from the CX4.

You didn't answer my previous question regarding the physical connections from
the CX4 to your ethernet switch.  Do you have both ports connected, and do you
have them link aggregated?  If not, this is probably the single most important
change you could make at this point.  I seriously doubt that 100 MB/s is
sufficient for the load you're putting on the CX4.

> Anyway, we are considering very much the idea of making DLM on a dedicated
> network. We gonna study some way to do it with or hardware.

As I said a single decent quality 24 port GbE managed switch will carry the DLM
and iSCSI traffic just fine, especially if all the devices can support jumbo
frames.  Cheap desktop/soho switches such as DLink are going to cripple your
operation.

> Apreciate your help, i guess i lernad a lot. ( i did forward this emails for
> some of my bosses, hope will change anything about hardware, but who knows.

Glad to be of assistance.  Hope you get all the problems worked out.  The only
money you really need to spend is on a decent GbE managed switch with 16-24 ports.

> Anotehr thing to say, the email is not very tunning yet, but as we gonna
> improving we start to get more money to buy more stuff to the service. Thats
> is we we try to make it better with poor hardware. AS i said, before it was
> everything on fisica desktop with 500gb of disk. So right now we make a
> really big improvement.

It's still better than before, even with all the current performance problems?
Well at least you're making some progress. :)

> Thanks a lot to all, i still apreciate any help. I am reading it all and
> trying to take the best of it!

Are you running OCFS in both the Xen guests and the Xen hosts?  If so that may
also be part of the performance problem.  You need to look at ways to optimize
(i.e. decrease) the OCFS metadata load.  Can Xen export a filesystem up to the
guest via some virtual mechanism, such as ESX presents virtual disks to a guest?
 If so you should do that.

-- 
Stan