David Carter wrote:
On Sat, 4 Nov 2006, Joel Reicher wrote:
Just out of curiosity, why is there *any* interest in getting NFS and
IMAP interoperate? They both seem to be different solutions to the
same problem, i.e. distributed access to storage, but IMAP is
application-specific.
It's a easy (if inefficient) way to do horizontal scaling: you have
several NFS backends and add more to get additional capacity without
having to touch the existing systems. It used to be fairly common for
large (multi million user) ISPs to store email on NFS toaster devices,
even though the only presentation to users was POP and Webmail.
I inherited a mail system based on NFS. It worked reasonably well,
though I certainly wouldn't want to try running GByte size mbox files
over NFS.
These days we use a cluster of Cyrus systems hidden behind an IMAP
proxy. This works really well, though we did have to add replication
to Cyrus (a version of which has been merged into Cyrus 2.3) to get
the same level of reliability as the old NFS toasters.
In our case the situation is sort of the inverse... I inherited a server
room
long ago where all of the user data is centralized onto a large central
file
server (raid array, tape library, etc) and then distributed via NFS to
quite
a few other servers that are application specific. No user applications are
run on the central fileserver itself.
The application servers range widely in purpose (email, http, ftp, and a
wide range of users applications). There are quite a few application
servers but only one of them acts as an imap server for user email.
Due to performance reasons (1GB+ inboxes) we recently broke the
above model and started mounting /var/mail as local disk directly on
the imap server vs. NFS from the central file server. But, the rest of
the users' imap folders are in their home directories and are still
mounted via NFS.
I know I've read that imap shouldn't be used over NFS, but it's worked
well for us for many years (other than speed/performance issues).
Mark, after reading your last email, I guess it's just luck that we're
using
traditional unix mailboxes. I wasn't previously aware that there are
additional negative issues related to using the other various folder
formats
with respect to NFS that make traditional unix mailboxes the proper
choice (if it's necessary to use NFS at all).
My question is, in our situation where there's only one NFS server
and one NFS client dealing with the imap folders, are any of the
negative effects of NFS alleviated? Or, is it NFS itself that's the
problem (unrelated to concurrent access by multiple NFS clients).
Also, if we were to switch over to "mix" format imap folders, will that
cause us additional problems in our scenario compared to traditional
unix format folders, considering that we only have one NFS server
and one NFS client touching the imap folders in the users' home
directories and the inbox (/var/mail) is on local disk?
Obviously any particular user might have a home computer, work
computer, and notebook all trying to access the same imap folders
at the same time. Anyway, I guess I'm still not clear on whether the
NFS issues that have been warned against are related to multiple
imap clients, multiple NFS clients, or simply the NFS protocol itself.
Maybe our situation isn't any better than one where multiple NFS
client machines exist.
Thanks,
Brian
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