[Dovecot] dsync is SLOW compared to rsync

[Ed W]

On 24/03/2012 13:21, Maarten Bezemer wrote:
>
> On Fri, 23 Mar 2012, Jeff Gustafson wrote:
>
>>     That didn't seem to make much of a difference. On a 3.1GB backup it
>> shaved off 5 seconds. dsync's time was over 6 minutes with or without
>> the mail_fsync=never. rsync copied the same 3.1GB mailbox in 15 seconds.
>>     It seems to me that dsync *should* be able to be just as fast, 
>> but it
>> currently is spending way too much time doing something. What is it?
>
> Syncing 3.1GB in 15 seconds would require a speed of more than 200MB 
> per second. Depending on the harddisks used, that would be quite a 
> challenge.

rsync is only going to transfer files it believes has changed, so the 
transfer bandwidth will likely be lower

> If you use rsync to only transfer the files that changed (based on 
> file modification time) you may or may not miss files that have 
> changed but still have the same time stamp. I assume you didn't use 
> the --checksum parameter to rsync, right?

Dovecot is not very resiliant to files changing under it, but without 
the filename changing.  I have no idea if it's supposed to work at all, 
but you might at least expect to see problems if you start doing this?

> dsync does so much more than simply copy some files...

Quite probably, but I don't think your expose above illustrates this?

Regards

Ed W