Dovecot Replication - Architecture Endianness?

[Timo Sirainen]

On 04 May 2015, at 17:11, Reuben Farrelly <reuben-dovecot at reub.net> wrote:
> 
> On 4/05/2015 11:06 PM, Teemu Huovila wrote:
>> On 05/03/2015 01:48 PM, Reuben Farrelly wrote:
>>> Hi all,
>>> 
>>> I've had an interesting use case come up which - to cut the story short - one way to solve the problem I am looking at may be to
>>> replicate a small number of mailboxes to a third remote server.
>>> 
>>> I've currently had replication running between my main dovecot machine and another remote VM for some time and working well (so
>>> I'm not new to replication and I've got a good working config), but I've a need to add a third to the mix for a select number of
>>> mailboxes.  The arch on both of those is Gentoo x86_64 and with latest 2.1.16 -hg.
>>> 
>>> I have attempted this so far by rsync'ing the initial Maildirs and then once the bulk of the data has been transferred rely on
>>> dovecot's replication to keep things in sync.  I figure that this should in theory mean that the subsequent updates in both
>>> directions are incremental and the bulk of the data gets moved while the device is here on my desk using rsync.
>>> 
>>> I've attempted to do this using a Raspberry Pi as a remote device, but when I set it up the dovecot replication process seems to
>>> need to start the replication over from scratch even after the rsync is done.  I know this is happening as the disk utilisation
>>> on the Pi skyrockets once the replication starts and I end up with thousands of double ups of all the mails ...  which defeats
>>> the entire point of the process.
>>> 
>>> If I do an identical configuration but on a third Gentoo x86_64 VM locally it all works as expected.  No double ups of mails and
>>> the "catchup" between the two devices is practically instant.  Same filesystem even.  The only difference appears to be the
>>> system architecture.
>>> 
>>> So main my question is this.  Is there a known architecture/endian limitation on replication?   I guess cross-arch replication
>>> is not something many people try but is it supposed to work anyway?
>> I think you are bumping against Dovecot index endianess restrictions. I dont think cross-arch dsync can currently work very
>> efficiently.
>> http://wiki2.dovecot.org/Design/Indexes/MainIndex?highlight=%28endian%29
> 
> Ok.  That explains why the rsync won't work.  But if I kick off a dovecot to dovecot replication (without doing the rsync first) will this work any better once the system catches up?  This assumes (possibly incorrectly - please correct me if I am wrong) that the index files themselves aren't dsync'd byte-by-byte but instead the metadata/content from them is sent instead, and then the indexes are written to remote disk by the remote dovecot in the right arch and format that the remote machine can read and understand.  Because if that's the case then I can probably make that work - just taking a hit on the initial sync which could take longer.
> 
> Even if this doesn't end up working I figure I'll get to learn a little more about the indexes themselves in the process.

dsyncing between servers (or in general using dsync-server) transfers all data using a portable protocol. So dsync source and destination can then have different endianess and it doesn't matter.