[Dovecot] Dovecot very slow on a mailbox with > 700 IMAP Folders

[Becker, Ronny]

Am 05.05.2011 22:24, schrieb Scott Silva:
> on 5/5/2011 1:04 AM Becker, Ronny spake the following:
>> Hello,
>> I am using Dovecot 1.2.9 on a 32bit on Ubuntu 10.04.2 LTS.
>>
>> We got one user who uses folders in an "exessive" way. He has got 704
>> subscribed folders. That means about nearly 3000 folders on the filesystem.
>> Since ~ a week the user couldn't work because of timeouts when connecting to
>> Dovecot (IMAP). Thunderbird doesn't show any folders - timeout - the webmail
>> System (Groupoffice) only presents the INBOX.
>>
>> I did some debugging and found that a lsub "" "*" that is used by Thunderbird
>> takes>  5 minutes. Thunderbird disconnects after ~1 Minute showing the
>> timeout. I entered the command directly via port communication. So this is no
>> clientside problem. But there is no error message or something else in any logs.
>>
>> I did some debugging with strace, too. It's about 22MB only for running the
>> lsub command !? The Mailbox size is about ~2.2GB, so this should be no
>> problem. The maximum subfolder level is about 13 - could this be a problem?
>> You can find the structure (simply shown) attached.
>>
>>
>> Thanks for any help!!
>>
>>
>> Best regards
>> Ronny Becker
>>
>>
> What is the underlying filesystem, and if its ext3, is dir_index set on?
>
The underlying Filesystem is ext3 with dir_index enabled. As Timo 
Sirainen explained the problem seems to be the ACL plugin. With ACL 
enabled every folder is scanned for a dovecot-acl file and that needs a 
lot of time. In this case there are >700 subscribed folders using >3600 
folders on the filesystem.
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