[Dovecot] Providing shared folders with multiple backend servers

[Sven Hartge]

Stan Hoeppner <stan at hardwarefreak.com> wrote:

> It's highly likely your problems can be solved without the drastic
> architecture change, and new problems it will introduce, that you
> describe below.

The main reason is I need to replace the hardware as its service
contract ends this year and I am not able to extend it further.

The box so far is fine, there are normally no problems during normal
operations with speed or responsiveness towards the end-user.

Sometimes, higher peak loads tend to strain the system a bit and this is
starting to occur more often.

First thought was to move this setup into our VMware cluster (yeah, I
know, spare me the screams), since the hardware used there is way more
powerfull than the hardware used now and I wouldn't have to buy new
servers for my mail system (which is kind of painful to do in an
universitary environment, especially in Germany, if you want to invest
an amount of money above a certain amount).

But then I thought about the problems with VMs this size and got to the
idea with the distributed setup, splitting the one server into 4 or 6
backend servers.

As I said: "idea". Other ideas making my life easier are more than
welcome.

>> Ideas? Suggestions? Nudges in the right direction?

> Yes.  We need more real information.  Please provide:

> 1.  Mailbox count, total maildir file count and size

about 10,000 Maildir++ boxes

900GB for 1300GB used, "df -i" says 11 million inodes used

I know, this is very _tiny_ compared to the systems ISPs are using.

> 2.  Average/peak concurrent user connections

IMAP: Average 800 concurrent user connections, peaking at about 1400.
POP3: Average 300 concurrent user connections, peaking at about 600.

> 3.  CPU type/speed/total core count, total RAM, free RAM (incl buffers)

Currently dual-core AMD Opteron 2210, 1.8GHz.

Right now, in the middle of the night (2:30 AM here) on a Sunday, thus a
low point in the usage pattern:

             total       used       free     shared    buffers     cached
Mem:      12335820    9720252    2615568          0      53112     680424
-/+ buffers/cache:    8986716    3349104
Swap:      5855676      10916    5844760


System reaches its 7 year this summer which is the end of its service
contract.

> 4.  Storage configuration--total spindles, RAID level, hard or soft RAID

RAID 6 with 12 SATA1.5 disks, external 4Gbit FC 

Back in 2005, a SAS enclosure was way to expensive for us to afford.

> 5.  Filesystem type

XFS in a LVM to allow snapshots for backup

I of course aligned the partions on the RAID correctly and of course
created a filesystem with the correct parameters wrt. spindels, chunk
size, etc.

> 6.  Backup software/method

Full backup with Bacula, taking about 24 hours right now. Because of
this, I switched to virtual full backups, only ever doing incremental
and differental backups off of the real system and creating synthetic
full backups inside Bacula. Works fine though, incremental taking 2
hours, differential about 4 hours.

The main problem of the backup time is Maildir++. During a test, I
copied the mail storage to a spare box, converted it to mdbox (50MB
file size) and the backup was lightning fast compared to the Maildir++
format.

Additonally compressing the mails inside the mdbox and not having Bacula
compress them for me reduce the backup time further (and speeding up the
access through IMAP and POP3).

So this is the way to go, I think, regardless of which way I implement
the backend mail server.

> 7.  Operating system

Debian Linux Lenny, currently with kernel 2.6.39

> Instead of telling us what you think the solution to your unidentified
> bottleneck is and then asking "yeah or nay", tell us what the problem is
> and allow us to recommend solutions.

I am not asking for "yay or nay", I just pointed out my idea, but I am
open to other suggestions.

If the general idea is to buy a new big single storage system, I am more
than happy to do just this, because this will prevent any problems I might
have with a distributed one before they even can occur.

Maybe two HP DL180s (one for production and one as test/standby-system)
with an SAS attached enclosure for storage?

Keeping in mind the new system has to work for some time (again 5 to 7
years) I have to be able to extend the storage space without to much
hassle.

Grüße,
S°

-- 
Sigmentation fault. Core dumped.