[Dovecot] dsync is SLOW compared to rsync

[Jeff Gustafson]

On Sat, 2012-03-24 at 14:21 +0100, 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. 
> 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?

	The destination directory was empty. I was doing a full backup.

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

	I realize that. I am hoping that the extra data that dsync has
available to it would improve the speed of syncing backups. My baseline
testing of simply backing up a mailbox to an empty directory shows that
dsync is takes way too long to backup a single mailbox. I have over a
terabyte of data to backup.  
	I'm currently using rsync and it must traverse tens of thousands of
files and check the time information. It works, but I was hoping dsync
would be a better solution. dsync should be able to sync faster, by
gulping in the index information for each mailbox. I haven't even moved
to the point of sync'ing since the baseline test of simply exporting a
mailbox is so slow.

				...Jeff