Re: Mail server
On Mon, 24 Feb 2003 18:34, Colin Ellis wrote:
> Email doesn't really need much processing, but does take surprisingly large
> amounts of disk space.
Obviously such things differ depending on exactly who is using the service and
what they are doing.
But my experience is that with modern disks a mail server will run out of seek
performance before it runs out of space.
The fastest drives (15000rpm) will take an average of 4ms for the disk to spin
to the correct location to start a transfer in addition to the seek times for
moving the heads. That gives a performance of something less than 100 IO
operations per second per disk. I am working on a bunch of Dell PowerEdge
2650 machines with 4*U160 15000rpm SCSI disks in a hardware RAID-5 with a
battery backed write-back cache. This gives a peak performance of about 130
disk writes per second.
If a message delivery takes 10 disk writes (actually it probably takes more
once you count writing to two files in the queue then writing it to the spool
and deleting the queue files with lots of fsync() along the way) then such a
machine can only deliver 13 messages per second.
I'm running a number of mail servers with lots of spare disk space that are
hitting the message delivery limits, which prevents me adding more users.
I have been considering modifying the Qmail and maildrop code to not use
fsync() etc to allow more users per server (yes I know about the reliability
issues, but there are lots of more important things to worry about).
If you need more space then there's lots of good options nowadays. 200G IDE
drives are getting cheap, I'll probably get a RAID-1 of them for my next home
machine. 70G U160 SCSI drives give better performance, and I'm finding that
their performance is a bottleneck not their size.
Of course bigger drives tend to be faster if all other things are equal. For
the servers I'm using I'd rather have 140G U160 drives, I'd still be using
<70G of them, but the performance would be better.
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