Postfix and NFS

-------------------------------------------------------------------------------

Postfix support status for NFS

What is the status of support for Postfix on NFS? The answer is that Postfix
itself is supported when you use NFS, but there is no promise that an NFS-
related problem will promptly receive a Postfix workaround, or that a
workaround will even be possible.

That said, Postfix will in many cases work very well on NFS, because Postfix
implements a number of workarounds (see below). Good NFS implementations seldom
if ever give problems with Postfix, so Wietse recommends that you spend your
money wisely.

Postfix file locking and NFS

For the Postfix mail queue, it does not matter how well NFS file locking works.
The reason is that you cannot share Postfix queues among multiple running
Postfix instances. You can use NFS to switch a Postfix mail queue from one NFS
client to another one, but only one NFS client can access a Postfix mail queue
at any particular point in time.

For mailbox file sharing with NFS, your options are to use fcntl (kernel
locks), dotlock (username.lock files), to use both locking methods
simultaneously, or to switch to maildir format. The maildir format uses one
file per message and needs no file locking support in Postfix or in other mail
software.

Many sites that use mailbox format play safe and use both locking methods
simultaneously.

    /usr/local/etc/postfix/main.cf:
        virtual_mailbox_lock = fcntl, dotlock
        mailbox_delivery_lock = fcntl, dotlock

Postfix NFS workarounds

The list below summarizes the workarounds that exist for running Postfix on NFS
as of the middle of 2003. As a reminder, Postfix itself is still supported when
it runs on NFS, but there is no promise that an NFS-related problem will
promptly receive a Postfix workaround, or that a workaround will even be
possible.

  * Problem: when renaming a file, the operation may succeed but report an
    error anyway[1].

    Workaround: when rename(old, new) reports an error, Postfix checks if the
    new name exists and the old name is gone. If the check succeeds, Postfix
    assumes that the rename() operation completed normally.

  * Problem: when creating a directory, the operation may succeed but report an
    error anyway[1].

    Workaround: when mkdir(new) reports an EEXIST error, Postfix checks if the
    new name resolves to a directory. If the check succeeds, Postfix assumes
    that the mkdir() operation completed normally.

  * Problem: when creating a hardlink to a file, the operation may succeed but
    report an error anyway[1].

    Workaround: when link(old, new) fails, Postfix compares the device and
    inode number of the old and new files. When the two files are identical,
    Postfix assumes that the link() operation completed normally.

  * Problem: when creating a dotlock (username.lock) file, the operation may
    succeed but report an error anyway[1].

    Workaround: in this case, the only safe action is to back off and try again
    later.

  * Problem: when a file server's "time of day" clock is not synchronized with
    the client's "time of day" clock, email deliveries are delayed by a minute
    or more.

    Workaround: Postfix explicitly sets file time stamps to avoid delays with
    new mail (Postfix uses "last modified" file time stamps to decide when a
    queue file is ready for delivery).

[1] How can an operation succeed and report an error anyway?

Suppose that an NFS server executes a client request successfully, and that the
server's reply to the client is lost. After some time the client retransmits
the request to the server. Normally, the server remembers that it already
completed the request (it keeps a list of recently-completed requests and
replies), and simply retransmits the reply.

However, when the server has rebooted or when it has been very busy, the server
no longer remembers that it already completed the request, and repeats the
operation. This causes no problems with file read/write requests (they contain
a file offset and can therefore be repeated safely), but fails with non-
idempotent operations. For example, when the server executes a retransmitted
rename() request, the server reports an ENOENT error because the old name does
not exist; and when the server executes a retransmitted link(), mkdir() or
create() request, the server reports an EEXIST error because the name already
exists.

Thus, successful, non-idempotent, NFS operations will report false errors when
the server reply is lost, the client retransmits the request, and the server
does not remember that it already completed the request.

