[Dovecot] Providing shared folders with multiple backend servers

[Sven Hartge]

Stan Hoeppner <stan at hardwarefreak.com> wrote:
> On 1/8/2012 2:15 PM, Sven Hartge wrote:

>> Wouldn't such a setup be the "Best of Both Worlds"? Having the main
>> traffic going to local disks (being RDMs) and also being able to provide
>> shared folders to every user who needs them without the need to move
>> those users onto one server?

> The only problems I can see at this time are:

> 1.  Some users will have much larger mailboxes than others.
>    Each year ~1/4 of your student population rotates, so if you
>    manually place existing mailboxes now based on current size
>    you have no idea who the big users are in the next freshman
>    class, or the next.  So you may have to do manual re-balancing
>    of mailboxes, maybe frequently.

The quota for students is 1GiB here. If I provide each of my 4 nodes
with 500GiB of storage space, this gives me 2TiB now, which should be
sufficient. If a nodes fills, I increase its storage space. Only if it
fills too fast, I may have to rebalance users.

And I never wanted to place the users based on their current size. I
knew this was not going to work because of the reasons you mentioned.

I just want to hash their username and use this as a function to
distribute the users, keeping it simple and stupid.

> 2.  If you lose a Dovecot VM guest due to image file or other
>    corruption, or some other rare cause, you can't restart that guest,
>    but will have to build a new image from a template.  This could
>    cause either minor or significant downtime for ~1/4 of your mail
>    users w/4 nodes.  This is likely rare enough it's not worth
>    consideration.

Yes, I know. But right now, if I lose my one and only mail storage
servers, all users mailboxes will be offline, until I am either a) able
to repair the server, b) move the disks to my identical backup system (or
the backup system to the location of the failed one) or c) start the
backup system and lose all mails not rsynced since the last rsync-run.

It is not easy designing a mail system without a SPoF which still
performs under load.

For example, once a time I had a DRDB (active/passive( setup between the
two storage systems. This would allow me to start my standby system
without losing (nearly) any mail. But this was awful slow and sluggish.

> 3.  You will consume more SAN volumes and LUNs.  Most arrays have a
>    fixed number of each.  May or may not be an issue.

Not really an issue here. The SAN is exclusive for the VMware cluster,
so most LUNs are quite big (1TiB to 2TiB) but there are not many of
them.

Grüße,
Sven.

-- 
Sigmentation fault. Core dumped.