NFSv4 delegation

Tue Aug 18 15:25:37 UTC 2015

On 8/18/2015 10:46 AM, Alessio Cecchi wrote:
> Hi,
>
> in this tipical setup (Dovecot/Director thate share Maildir via NFS)  on
> your NFS Server you have (about) 90% of read operations and only 10% of
> write operations.
>
> If you see detailed stats for NFS operations you have 40-50% of GETATTR,
> this means that NFS/Dovecot clients are caching data (mainly dovecot
> index files) but they have to revalidate the cache frequently asking to
> the NFS Server if the file is changed (via GETATTR) but the file never
> change because only this client opens it.
>
> So the NFS Server is wasting operations with (unnecessary) GETATTR request.
>
> With NFSv4 and delegation you can practically eliminate these (GETATTR)
> requests and speed up your NFS server (instead of buy SSD disk).
>
> This because with delegation a client open file and since is the only
> client to open these file (that is true with Director) the NFS server
> delegate the client to manage the file without check if was changed (via
> GETATTR) on the NFS share.

Enabling delegations on Maildir is going to scale very badly. The NFS
client will end up requesting one on every message open, and because an
NFS server can only support a limited number of active delegations, it
will be forced to constantly recall them, only to issue new short-lived
ones to the next message open.

While it's a decent idea for indexes, the client has no way to request
delegations selectively for them. Perhaps if you used mdbox where the
file count is lower, but even then, there will be many users and many
mailboxes so it is important to be sure there are enough delegations
available at the NFS server. That's a NetApp question in your case,
and not a Dovecot one.

>
> You can find more information (on NFSv4 and delegation) here:
> http://www.fsl.cs.stonybrook.edu/docs/nfs4perf/nfs4perf-login.pdf
>
> It would be very useful if NFSv4 Delegation if it worked fine with dovecot.
>
> Please talks about this with Timo.
>
> Il 18/08/2015 16:20, Sami Ketola ha scritto:
>> Hi,
>>
>> Just out of curiosity what is in nfsv4 delegation that you think would
>> give a benefit on your configuration?
>>
>> If I read back the thread you seem to have dovecot configured with
>> director ring in front of the backends. In that case Dovecot already
>> manages storage in a way that only one of the backends is accessing
>> each users data at a time. So I can’t see anything but problems form
>> enabling delegations.
>>
>> Sorry but I have zero experience in running dovegot with nfsv4
>> delegations since in general it is not needed.
>>
>> Sami
>>
>>> On 18 Aug 2015, at 15:42, Alessio Cecchi <alessio at skye.it> wrote:
>>>
>>> Hi,
>>>
>>> last day I tried to enable "nfsv4 delegation" on my cluster (enable
>>> nfsv4.0 and read delegation on Netapp and remount the volume on Linux
>>> Centos 6.7 clients with Dovecot+Director).
>>>
>>> It was a failure, after some minutes the load on clients was high,
>>> and in dovecot.log I found these errors (repeated continuously):
>>>
>>> Aug 17 20:28:17 pop01eeh dovecot: imap(info at domain.com): Error:
>>> mail_index_wait_lock_fd() failed with file
>>> /home/domains/domain.com/info/Maildir/dovecot.index.log: Input/output
>>> error
>>> Aug 17 20:28:21 pop01eeh dovecot: imap(info at domain.com): Error:
>>> fcntl(/home/domains/domain.com/info/Maildir/dovecot.index.cache,
>>> write-lock, F_SETLKW) locking failed: Input/output error
>>>
>>> Is really impossible to run NFSv4 with delegation with dovecot? If it
>>> were possible the number of NFS operations would drop so much with
>>> many benefits on performance.
>>>
>>> Il 09/07/2015 17:08, brd ha scritto:
>>>> just a quick update:
>>>>
>>>> no issues with NFSv4.0 (load is slowly growing, currently ~7k
>>>> mailboxes)
>>>>
>>>> instead, bad news from delegation front, we enabled it for a couple of
>>>> days but we ran in ugly issues: processes went in "uninterruptible
>>>> sleep" state, load average gets huge, reboot was the only escape :-(
>>>>
>>>> -brd
>>>>
>>>> Alessio Cecchi wrote:
>>>>> Il 12/06/2015 13:02, brd ha scritto:
>>>>>> Alessio Cecchi wrote:
>>>>>> [...]
>>>>>>> Have you already try to run NFSv4?
>>>>>> it's in place on a (very) small sample of mailboxes in dbox
>>>>>> format, no issues
>>>>>> up to now (Debian Wheezy mainline kernel)
>>>>>
>>>>> Good to know, let me know what will be when the load will grow.
>>>>>
>>>>>>> When we switch to netapp and nfsv4 we had many problems (lock
>>>>>>> problems
>>>>>>> and instability) and we had to go immediately to NFSv3. I don't
>>>>>>> know if
>>>>>>> was a netapp problem or nfs client (Debian with 2.6 kernel), now
>>>>>>> we are
>>>>>>> using Centos 6 as NFS client and we should re-try to mount
>>>>>>> mailbox as nfsv4.
>>>>>>
>>>>>> when you've had problems, the "delegation feature" was active on
>>>>>> Netapp
>>>>>> filers? (AFAIK it is disabled by default)
>>>>>
>>>>> Never enabled "delegation feature" on my netapp.
>>>>>
>>>>> Probably my issue was on client side. I had to switch immediately
>>>>> to NFSv3
>>>>> without being able to investigate the problem.
>>>>>
>>>>> Ciao
>>>>> --
>>>>> Alessio Cecchi
>>>>> http://www.linkedin.com/in/alessice
>>>
>>> --
>>> Alessio Cecchi
>>> http://www.linkedin.com/in/alessice
>>
>