[Dovecot] NFS random redirects

[Timo Sirainen]

On Oct 19, 2009, at 9:35 AM, Thomas Hummel wrote:

> Dovecot documentation states that the "random redirects to multiple  
> servers"
> NFS solution is to be avoided and I'm investigation the actual risks  
> of it and
> a way to put it to test.

If you do it, you'll most likely see some random index related errors.

>  1. Documentation says : "Dovecot locks the maildir while doing  
> modifications
>     to it or while looking for new messages in it" and then mention  
> the
>     "dovecot-uidlist.lock" dotlock file.
>
>      a) if that file fcntl'ed in addition (i.e. dotlocked + fcntled  
> or just dotlocked) ?

No fcntl.

>      b) is that file THE way to lock the Maildir mentionned above or  
> is it
>        just something else used only for updating that particular  
> file (i.e. is
>        Maildir locked in some way + dovecot-uidlist.lock created or  
> just
>        dovecot-uidlist.lock created) ?

Maildir is then locked for Dovecot against doing any modifications.  
Procmail will ignore it, and that's fine.

>  2. Documentation says : "NFS caching is a big problem when multiple  
> computers
>     are accessing the same mailbox simultaneously"
>
>     I guess it's because of the dotlocks files and not fcntl locking ?

No, it's because of NFS caching.. Like if Dovecot writes to  
dovecot.index.log first and then dovecot.index, it expects that it's  
not possible that a later read sees the dovecot.index change, but not  
the dovecot.index.log change. But because of NFS caching this is  
possible, and "corrupted index files" errors will occur. There are  
many other similar issues.

>  3. Documentation says : "Dovecot v1.1 flushes NFS caches when  
> needed if you set mail_nfs_storage=yes"
>
>     How can a program flush the NFS caches ? By which (system) call ?

That's the main problem. There is no reliable way to flush NFS  
caches.. So I've added some highly OS-specific code that works most of  
the time, but not perfectly. It works best with Linux.

>  4. Documentation says : "Besides the NFS cache problems described  
> above,
>     mailbox contents can't be cached as well in the memory either."
>
>    Is it about in-memory indexes or part of indexes loaded into  
> memory ?

I was thinking about OS's filesystem caching in general, so all mail  
files and such.

>  5. How can I torture-test concurrent access to the same mailbox  
> through 2
>     dovecot servers ? I don't see imaptest.c anymore on http://dovecot.org/tools
>     but I see 2 nfs_test ? Is there some command line options I  
> should use ?

imaptest exists now in http://imapwiki.org/ImapTest

>  6. when and why can a random redirect to multiple server can cause  
> mailbox
>     corruption ?

Pretty much randomly..

> On flags only or on content as well ?

Actual mail content should be safe. Unless you just happen to get such  
a cache file corruption that Dovecot doesn't notice it and sends some  
broken headers to IMAP client.

The long term fix for this is http://www.dovecot.org/list/dovecot/2009-August/041983.html