[Dovecot] Fwd: Re: Dotlock dovecot-uidlist errors / NFS / High Load

[list at airstreamcomm.net]

Stan,

Thanks for the reply.  In our case we have actually already done most of
the work you suggested to no avail.  We had rebuilt two new ntp servers
that sync against two stratum 1 sources, and all our nfs clients,
regardless of using dovecot, sync to those two machines.  You bring up the
difference between bare metal and hypervisor, and we are running these
machines on vmware 4.0.  All the vmware knowledge base articles tend to
push us towards ntp, and since we are using centos 5.5 there are no kernel
modifications that need to be made regarding timing from what we can find. 
I will give the ntpdate option a try and see what happens.

I was also hoping to understand why the uidlist file is the only file that
uses dotlock, or if there was plans to give it the option to use other
locking mechanisms in the future.

Thanks again!

Michael

-------- Original Message --------
Subject: Re: [Dovecot] Dotlock dovecot-uidlist errors / NFS / High Load
Date: Thu, 20 Jan 2011 10:57:24 -0600
From: Stan Hoeppner <stan at hardwarefreak.com>
To: dovecot at dovecot.org

list at airstreamcomm.net put forth on 1/20/2011 8:32 AM:

> Secondly we thought the issues were due to NTP as the time stamps vary
so
> widely, so we rebuilt our NTP servers and found closer stratum 1 source
> clocks to synchronize to hoping it would alleviate the problem but the
> dotlock errors returned after about 12 hours.  We have fcntl locking set
in
> our configuration file, but it is our understanding from look at the
source
> code that this file is locked with dotlock.  
> 
> Any help troubleshooting is appreciated.

>From your description it sounds as if you're ntpd syncing each of the 4
servers
against an external time source, first stratum 2/3 sources, then stratum 1
sources in an attempt to cure this problem.

In a clustered server environment, _always_ run a local physical
box/router ntpd
server (preferably two) that queries a set of external sources, and
services
your internal machine queries.  With RTTs all on your LAN, and using the
same
internal time sources for every query, this clock drift issue should be
eliminated.  Obviously, when you first set this up, stop ntpd and run
ntpdate to
get an initial time sync for each cluster host.

If after setting this up, and we're dealing with bare metal cluster member
servers, then I'd guess you've got a failed/defective clock chip on one
host.
If this is Linux, you can work around that by changing the local time
source.
There are something like 5 options.  Google for "Linux time" or similar. 
Or,
simply replace the hardware--RTC chip, mobo, etc.

If any of these cluster members are virtual machines, regardless of
hypervisor,
I'd recommend disabling using ntpd, and cron'ing ntpdate to run once every
5
minutes, or once a a minute, whatever it takes to get the times to remain
synced, against your local ntpd server mentioned above.  I got to the
point with
VMWare ESX that I could make any Linux distro VM of 2.4 or 2.6 stay within
one
minute a month before needing a manual ntdate against our local time
source.
The time required to get to that point is a total waste.  Cron'ing ntpdate
as I
mentioned is the quick, reliable way to solve this issue, if you're using
VMs.

-- 
Stan