[Dovecot] Real-time sync using dsync

[list at airstreamcomm.net]

On 2/27/13 3:10 PM, Timo Sirainen wrote:
> On 27.2.2013, at 21.19, Nikolaos Milas <nmilas at noa.gr> wrote:
>
>> Any suggestions?
>>
>> I am looking for a solution that would work in creating a failover cluster with two nodes, utilizing (two) CentOS 6 VMs, each on a different data center; this requirement makes technologies like drbd unusable (due to the inherent lack of complete link reliability between the two nodes).
>>
>> I was thinking that dsync might be a good foundation for such scenarios.
> dsync was meant exactly for that kind of replication. For a relatively few number of users this should work well (minus the initial bugs until they get all fixed). It's a little bit heavy operation to run dsync for each small change though, so I wouldn't necessarily use it for large systems. Then again it's mainly CPU usage, and Dovecot uses normally about 0% CPU, so maybe it's not so bad.
>
> The other possibility that is more efficient and easier to scale to large systems is to use one of the scalable object storage backends and Dovecot's object storage plugin (commercial-only, available soon).
>
> The idea behind both of these ways is to make it easy, cheap and reliable to do multi-datacenter replication for IMAP servers. None of the cluster filesystems can do that.
>
>
Timo, has this been tested on large systems yet?  I plan on hammering a 
two node dsync cluster running 2.2rc2 (each node is 100 miles apart in a 
different data center connected via 10gb ring) with a SMTP/IMAP/POP 
generating bot cluster we have in our test network to see how well it 
scales.  I will update with my findings next week when I get a chance to 
work on it.  I have to +1 Nikolaos' sentiment for a geographically 
distributed mail cluster, we have been hoping for a Dovecot solution to 
this problem for the last few years.