[Dovecot] Inotify max_user_instances

[Steffen Kaiser]

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On Thu, 7 Mar 2013, Daniel L. Miller wrote:

> Maybe I have multiple problems - dunno.
>
> I've started seeing the following log lines:
> Mar  7 07:46:22 bubba dovecot: imap(dmiller at amfes.com): Warning: Inotify 
> instance limit for user 5000 (UID vmail) exceeded, disabling. Increase 
> /proc/sys/fs/inotify/max_user_instances
>
> max_user_instances is currently 128.
>
> I've tried stopping and restarting dovecot - the message immediately returns. 
> I could just increase max_user_instances - but I'd like to understand what 
> the number SHOULD be and why simply restarting Dovecot doesn't fix it.  If 
> this issue is for user "vmail" this is used by mail services only - and I've 
> only got a few users on my system.

see
http://stackoverflow.com/questions/11110245/inotify-fd-why-is-the-limit-per-user-id-and-not-per-process

Dovecot needs one inotify entity per IDLE, maybe more for internal 
purposes. So in theory, you will need: <max number of simultaneous users> 
* <number of watched mailboxes per users> + <fixed amount>, because you 
use one _system_ user for all IMAP-users.

> I'm also fighting a netfilter issue - my connection tracking counters keep 
> climbing.  Don't know if this is in any way related.

Do you use NAT on the same machine? Or some intrusion detection system? 
When you get many short connections, esp. UDP ones, the connection tracker 
fills up easily.

- -- 
Steffen Kaiser
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