Le 02/08/2013 14:30, Timo Sirainen a écrit :
On Mon, 2013-07-29 at 11:48 +0200, Stéphane BERTHELOT wrote:
mdbox_rotate_size = 128M
mdbox_rotate_interval = 1d
mdbox_preallocate_space = yes
with virtual users and location like :
mail_location = mdbox:~/mdbox
I don't think the remaining config is relevant but ask me if you need
some other parts.
Using test accounts for 2 weeks now I've figured that the 128M
preallocated space is never 'hole punched" (to use a similar term than
"man fallocate" on Linux), even when rotating m.* files.
Yeah, those settings weren't really intended to be used together.
I am not sure to understand. Preallocation is based on rotate size
actually, so you mean I shouldn't use a rotate interval with this kind
of settings ?
It would effectively "lose" less space since all files would be around
the preallocated size except the last one (current).
There would certainly be smart to use something similar to
"FALLOC_FL_PUNCH_HOLE" on rotation (when doing close() ?) so that when
we're sure there won't be anymore data appended to file that the
allocated space == used space.
The problem is that there is no "rotation time". In normal operation
Dovecot would be wasting time (=disk IO) looking at old files and
figuring out if they would need hole punching. I guess the doveadm purge
job could do that, but I'm not sure if that's always the best idea
either. I remember some people using different mdbox settings for normal
operations and for doveadm purge runs, so this could also
unintentionally break things..
Ok, I'm sorry I forgot to add that I'm using a cronjob to doveadm purge
weekly. That effectively may be a good time to remove unneeded
preallocated sectors.
I didn't have a look at the internals of the devecot process but I meant
that this could happen when it chooses to switch to another file
(because of rotation time, not purge). Effectively it may be at a time
that we would want to avoid adding more IO...
When reading linux docs lately the more appropriate term may be
"truncation" rather than "hole punching". That's what I did on command
line (truncate with proper args) and it resetted the allocated size to
"real" size.
I am pretty sure my understanding on those options was wrong so I
already changed to 2M/no preallocation (since I didn't find any evidence
that preallocation is making dovecot perform so much better ; there
seemed to be a thread once on this list but noone concluded on that I
think).
My objective was to get a good compromise on performance (since I'm
using ext4 anyways) and backup time (otherwise I wouldn't bother with
rotation to "compensate" the potentially full rebuild of doveadm purge)
Maybe a documentation update should do the trick or a configuration
warning stating that preallocation+rotate time would leave "wasted"
space on disk.
