On Mon, Jul 17, 2000 at 10:24:53PM -0600, Steve Wolfe wrote:
> > With all of the emails I recieved, I get the impression that I'm going to
> > I/O bound instead of processor or memory bound. How much disk will be
> > sufficient for the queue? 1GB? More?
> It's not so much a matter of disk size (I don't think you'll have a 1 gig
> queue!), but of throughput. For example, a single IDE drive will get you a
> couple of megabytes of throughput per second, at a very high CPU cost. SCSI
> will yield more, with a lower CPU utilization, and with RAID arrays, you can
> move up to hundreds of megabytes per second if you want to.
Not entirely true. With UDMA mode, modern IDE drives get high
throughput with low CPU utilization. On my Celeron PC, I could get well
over 10MB/sec at well under 20% CPU, and it's hardly performance
hardware (5400RPM spindle). With a 10K RPM spindle and a faster chipset
(mine's a VIA) this will rival or beat fast SCSI disks in raw streaming
bandwidth. However, the majority of mail queues are not even bandwidth
bound -- they're seek bound, which is where SCSI disks still beat IDE.
The faster seek time, the better (which is the motivation behind DJB's
ingenious zeroseek proposal). Also, RAID5 arrays (the most common one
for large capacities) suffer a significant write penalty due to
recalculation and rewiting of the parity, and the mail queue is mostly
written (and subsequently cached). A RAID1+0 array works better, but
uses more disks.
--
Bruce Guenter <[EMAIL PROTECTED]> http://em.ca/~bruceg/
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