[Dovecot] Dovecot never release preallocated space in mdbox

[Stéphane BERTHELOT]

Hello everyone,

I am currently evaluating dovecot for our new email production servers 
(20k+ mailboxes) and found out something strange.

I'm using those settings on Dovecot 2.2.4 (x86_64 / Slackware / compiled 
from sources)

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.
 From what I understand those files will never be appended again because 
of the mdbox_rotate_interval. Then doveadm purge creates new files so 
old ones would never grow again.

Here is an example of a mdbox storage using ls -ls (which shows 
allocated VS used space)
total 4065176
   1884 -rw------- 1 mail mail   1926656 Jul 29 10:55 dovecot.map.index
      4 -rw------- 1 mail mail       460 Jul 29 11:26 dovecot.map.index.log
     48 -rw------- 1 mail mail     44304 Jul 29 10:55 
dovecot.map.index.log.2
131072 -rw------- 1 mail mail 133165066 Jul 19 15:31 m.10
131072 -rw------- 1 mail mail 133507393 Jul 19 15:32 m.13
131072 -rw------- 1 mail mail 134155182 Jul 19 15:33 m.14
131072 -rw------- 1 mail mail 134213403 Jul 19 15:30 m.2
131072 -rw------- 1 mail mail     46464 Jul 21 04:30 m.21
131072 -rw------- 1 mail mail 134215030 Jul 19 15:30 m.3
131072 -rw------- 1 mail mail     25852 Jul 25 01:54 m.32
131072 -rw------- 1 mail mail      2360 Jul 26 00:05 m.34
131072 -rw------- 1 mail mail    169073 Jul 27 23:18 m.35
131072 -rw------- 1 mail mail     31624 Jul 27 01:55 m.36
131072 -rw------- 1 mail mail 134216982 Jul 28 04:30 m.37
131076 -rw------- 1 mail mail 134217804 Jul 28 04:30 m.38
131072 -rw------- 1 mail mail 134217341 Jul 28 04:30 m.39
131072 -rw------- 1 mail mail 134213719 Jul 19 15:30 m.4
131072 -rw------- 1 mail mail  29740970 Jul 28 04:30 m.40
131072 -rw------- 1 mail mail 129175917 Jul 28 04:30 m.41
131072 -rw------- 1 mail mail 133174937 Jul 28 04:30 m.42
131072 -rw------- 1 mail mail    633436 Jul 28 04:30 m.43
131072 -rw------- 1 mail mail   3154623 Jul 28 04:30 m.44
131072 -rw------- 1 mail mail   3676879 Jul 28 04:30 m.45
131072 -rw------- 1 mail mail    468158 Jul 28 04:30 m.46
131072 -rw------- 1 mail mail     26964 Jul 28 04:30 m.47
131072 -rw------- 1 mail mail   3574599 Jul 28 04:30 m.48
131072 -rw------- 1 mail mail   3789133 Jul 28 04:30 m.49
131072 -rw------- 1 mail mail 134215016 Jul 19 15:30 m.5
131072 -rw------- 1 mail mail   1280074 Jul 28 04:30 m.50
131076 -rw------- 1 mail mail    635459 Jul 28 22:47 m.51
131072 -rw------- 1 mail mail   1459418 Jul 29 10:55 m.52
131072 -rw------- 1 mail mail 132941013 Jul 29 11:26 m.53
131072 -rw------- 1 mail mail 134213475 Jul 19 15:30 m.7
131072 -rw------- 1 mail mail 132240074 Jul 19 15:31 m.9


There's a lot of "lost" space since preallocated space would only be 
reclaimed when *all* emails in m.X file have refcount=0 and after a 
doveadm purge call, if I read well the dovecot docs.

On mailboxes patterns with low incoming mail (< 100kb / day) this would 
waste much space. Of course I can decrease rotate size a lot but it 
would then produce a lot of files and would certainly become similar 
performance-wise to sdbox/maildir/...

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.

I will disable space preallocation for our next tests since it wastes 
much storage for us ; did you have any feedback on how much it may 
affect performance ? I found in this ML archives some messages about the 
implementation but didn't see anyone clearly stating how much better 
preallocation is.

Thanks, best regards,
Stephane Berthelot.