System Bottlenecks

[Julian Field]

At 17:19 02/09/2003, you wrote:
>Errol Neal wrote:
> > At 04:47 PM 9/2/2003 +0100, you wrote:
> >> The best I/O improvement is by making sure you've got plenty of RAM
> >> and putting the MailScanner work directory in tmpfs (not either of
> >> the mail queues though!)
> >
> > That is a bit scary for us. Unpacking messages in a memory based
> > file  system could be catastrophic. *Shudders*. Too scary to even
> > think about it if for example,
> > MailScanner dies and leaves a bunch of mail in the tmpfs and we
> > unknowingly reboot the system... for us.. instant law suit.
> > Can anyone explain how this works? Does MailScanner unpack messages 1
> > at a time, does it unpack all the messages bulky in this directory?
>
>No, its absolutely safe, so long as you only do this for the _work_
>directory (/var/spool/MailScanner/incoming) and NOT the _queue_
>directories (mqueue and mqueue.in).  MailScanner never removes the queue
>files (even when it moves the files it actually just links them into the
>outgoing directory then unlinks them in the incoming directory IIRC).  The
>MailScanner incoming directory is used to unpack batches of messages to be
>scanned.  Should the System crash only these unpacked _copies_ of the
>original message will be lost, the original message will still be sitting
>there in mqueue.in (on a disk based filesystem) ready to be processed by
>MailScanner when it is restarted.

Just to confirm this, the above explanation is absolutely correct. There is
no critical data ever stored in the MailScanner/incoming directory.
Furthermore, MailScanner never takes responsibility for any message, there
is always either the original in mqueue.in or the finished version in
mqueue or both. There is no situation in which the message is in neither
queue. You can safely pull the plug on MailScanner at any time, you will
not lose any mail, even if you do use tmpfs for MailScanner/incoming.

As for the size of tmpfs, the usual maximum figure is half your physical
RAM. But as it expands and contracts as needed, you are best leaving it to
the operating system to manage. It's better at adjusting it than you are.
--
Julian Field
www.MailScanner.info
Professional Support Services at www.MailScanner.biz
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