dovecot replication (active-active) - server specs

Urban Loesch bind at
Thu Oct 9 18:41:06 UTC 2014


Am 09.10.2014 12:35, schrieb Martin Schmidt:

> Our MX server is delivering ca. 30 GB new mails per day.
> Two IMAP proxy server get the connections from the users. Atm. without dovecot director.
> We've got around 700k connections per day (imap 200k / pop3 500k)

Are this the hole connections per day? How many concurrend connections 
do you have at the same time on each server?

> So we want to make a new system.
> We desire the new system to use mdbox format ( bigger files, less I/O)
> and replication through dovecot replication (active/active) instead of drbd.

I have no experience with dovecot replication (Still on our roadmap). We 
are currently using drbd on a 10Gbit dedicated link. Works very well for us.

> Each fileserver should know every mailbox/user and for the time being 2 dovecot proxies for the user connections (IMAP/POP).
> (later after the migration from the old system to the new, dovecot director instead of proxies, for caching reasons).

As Florian said, enable zlib. This also decreases I/O, but needs a bit 
more of CPU. But not that much.

> we've got 2 new fileservers, they have each SSD HDDs for "new-storage"
> and 7200rpm SATA HDDs on RAID 5 with 10 TB for "alt-storage"
> 32 GB RAM per Server

You also could move the INDEX files from mdbox to different SSDs. We are 
doing so with 40k accounts and 2TB user data. Index partition has only 
22GB used and is increasing not very fast.

> Do you have some tips for the system?
> Do you believe 32 GB RAM are enough for one fileserver each and have you experience with the I/O Waiting problem with huge amounts of Data on the alt-storage?
> Could there be issues with the RAM, if one fileserver has a downtime, so the second one has to take over all mailboxes for a short amount of time?

I think memory is not the problem. On IMAP/POP3 servers the main 
problem is I/O. But with dovecot mdbox and index files on SSD's we have 
no problem at the moment.

> In general are only 2 new fileserver enough or should we think in bigger dimensions, like 4 fileserver
> Storage expansion in the new servers should not be a problem (bigger HDDs and a few slots free, so we can expand the raid 5).
We are using raid 10 hardware raid controller with cache and sata 
7200rpm disks. OK, raid 10 needs more disks, but is much faster than 
raid 5. Raid 5 is not very fast in my eyes.

> thank you
> kind regards
> Martin Schmidt


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