Coincidentally, my organization is looking into purchasing a SAN as a replacement for DASD. The big question is whether to plop an enterprise level NAS gateway in front of the SAN with NFS or purchase some implementation of a clustered (read distributed?) file system. The two cluster file systems I've looked into are QFS and Veritas CFS. Both add reasonable levels of complexity with metadata servers, etc. Also, I don't think CFS handles fcntl checks at all and we're definitely Solaris.
We aren't seeing any problems with our current setup; two large Solaris servers, one with DASDs and the other using NFS. We have a network load balancing device that retains destination servers based on source IP for a 30 minute session. It balances great and each user session will stick to the same back-end server if it's within the time-out. With this in mind, I didn't see any locking issues with our mbx's. However, recent replies to this thread have made me rethink the overall added workload involved in IMAP sessions over NFS and have me looking closer at a clustered file system again. We have about 1200 concurrent sessions, be it client or webmail. I realize if we stick with NFS we will be limiting ourselves in IMAP mailbox format and agreeing that their might be locking problems.
With all that said, I'm hoping for responses from people running QFS, CFS or some other clustered file system with IMAP. If it is being done with, or without headaches, I would rather that be the path we choose.
I apologize for the lack of technical contribution to the thread. The chance to ask for others' experience was too much of an opportunity for me to pass up.
-Ken
| Mark Crispin <[EMAIL PROTECTED]>
Sent by: [EMAIL PROTECTED] 11/08/2006 10:36 AM |
|
On Wed, 8 Nov 2006, Brian Thompson wrote:
> True, but the bigger picture is that in our environment there are many
> other uses for the user's home directory besides email. So, I don't foresee
> NFS going away anytime soon. One option might be to patch c-client such
> that the mix folders can use something like /var/mix/username/INBOX/
> instead of ~/INBOX/ .
Alternatively, if the IMAP servers have their own filesystem, then it
doesn't matter if the mailboxes are in the users' home directories
on the IMAP servers.
> That way the mix folders would be local without
> having to worry about creating a separate secondary set of local/non-NFS
> home directories for all of the users just for use on the email server.
I fail to see the savings. You have the same number of directories either
way, only now you have to have multiple algorithms to identify a user's
space.
The way that you get into trouble is when you try to get one box to assume
multiple roles: an IMAP server doing tasks that are not IMAP-related, an
NFS server providing both file and IMAP backend services, etc.
Separate your services onto separate hardware! That's how you eliminate
single points of failure.
> Another option would be to move imapd to the NFS server itself, but
> in my opinion that's worse than moving the mail folders over to what's
> currently the single NFS client serving email applications.
IMAP and NFS service on the same machine have been done. In terms of IMAP
reliability and robustness, it is MUCH better than the IMAP server
accessing its data as an NFS client.
However, such a configuration is a wretched compromise, suitable only for
small sites. Although it's reliable and robust, its performance is not
particularly good. IMAP and NFS access the filesystem in very different
ways.
> It would
> raise the question of sendmail, the local delivery agent, spam filtering,
> virus filtering, and DNS MX records. Which ones would move or be
> duplicated on the NFS server and which ones would stay on the NFS
> client. Optimally I'd rather keep the NFS server handling users' home
> directories and the email server handling email.
That is the correct position to take. The missing part of the key is that
the NFS server should be out of the email business entirely, and dedicate
itself for the task of serving user files.
-- Mark --
http://panda.com/mrc
Democracy is two wolves and a sheep deciding what to eat for lunch.
Liberty is a well-armed sheep contesting the vote.
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