dovecot - Nov 2012 - upgrade 1.0.15 -> 2.1.7: MBOX index compatibility and performance

Hi,

we just upgraded our mailserver from Dovecot 1.0.15 to Dovecot 2.1.7.

We use MBOX format (due to legacy compatibility), system users, 
PAM+GSSAPI auth, filesystem quotas, and indexes located on a separate 
filesystem:

mail_location = 
mbox:~/mail:INBOX=~/mail/INBOX:INDEX=/var/cache/dovecot/indexes/%16Hu/%u

The 2.1.7 configuration files have been rewritten based on default 
templates instead of converting it from 1.0.15.

The server has ~8000 mailboxes and about ~2000 simultaneous IMAP/POP 
active connexions.

The problem is:

- indexes seem to be rebuilt: first IMAP/POP connexion for each user 
thows lots of error messages in the log, and the global index size 
decreases.

Example error log:
Nov 19 08:56:38 myhost dovecot: imap(myuser): Error: Cached message size 
larger than expected (27884 > 27855)
Nov 19 08:56:38 myhost dovecot: imap(myuser): Error: Corrupted index 
cache file 
/var/cache/dovecot/indexes/4/myuser/.imap/INBOX/dovecot.index.cache: 
Broken physical size for mail UID 4414
Nov 19 08:56:38 myhost dovecot: imap(myuser): Error: Cached message size 
larger than expected (27884 > 27855)
Nov 19 08:56:38 myhost dovecot: imap(myuser): Error: Corrupted index 
cache file 
/var/cache/dovecot/indexes/4/myuser/.imap/INBOX/dovecot.index.cache: 
Broken physical size for mail UID 4414
Nov 19 08:56:38 myhost dovecot: imap(myuser): Error: Cached message size 
larger than expected (27884 > 27855)
Nov 19 08:56:38 myhost dovecot: imap(myuser): Error: Corrupted index 
cache file 
/var/cache/dovecot/indexes/4/myuser/.imap/INBOX/dovecot.index.cache: 
Broken physical size for mail UID 4414
Nov 19 08:56:38 myhost dovecot: imap(myuser): Error: Cached message size 
larger than expected (27884 > 27855)
Nov 19 08:56:38 myhost dovecot: imap(myuser): Error: Corrupted index 
cache file 
/var/cache/dovecot/indexes/4/myuser/.imap/INBOX/dovecot.index.cache: 
Broken physical size for mail UID 4414
Nov 19 08:56:38 myhost dovecot: imap(myuser): Error: copy: 
i_stream_read() failed: Input/output error
Nov 19 08:56:38 myhost dovecot: imap(myuser): Error: Cached message size 
larger than expected (27884 > 27855)
Nov 19 08:56:38 myhost dovecot: imap(myuser): Error: Corrupted index 
cache file 
/var/cache/dovecot/indexes/4/myuser/.imap/INBOX/dovecot.index.cache: 
Broken physical size for mail UID 4414


- load average is extremely high (more than 10x the usual one), 
resulting from an significant increase of disk I/O, and for now (4h 
after the "monday rush") this doesn't seem to decrease.


Questions:

- are 1.05 indexes supposed to be backward compatible with Dovecot 2.1.7 ?
- are there some technical reasons which could explain the increase of 
disk I/O, apart from index rebuild ?


-- 
Benoit BRANCIARD
Service InfraStructures (SIS) - Direction du Syst?me d'Information (DSI)
Universit? Paris 1 Panth?on-Sorbonne
Centre Pierre Mend?s France
B 406 - 90, rue de Tolbiac - 75634 Paris cedex 13 - France
T?l : +33 1 44 07 89 68 - Fax : +33 1 44 07 89 66
Accueil t?l. : +33 1 44 07 89 65
Assistance : assistance-dsi at univ-paris1.fr
Web : http://dsi.univ-paris1.fr

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