[sclug] "Adding RAID" - what does that mean nowadays? I thought PCI bus was really slow

Alex Butcher lug at assursys.co.uk
Tue Mar 18 16:55:31 UTC 2008

On Tue, 18 Mar 2008, M.Blackmore wrote:

> Someone in an earlier thread mentioned adding RAID as being a major
> value-added increment he gave to computers he made up for clients, as
> this speeded up the disk access bottleneck.
> Now, as I've said in another posting, I'm really out of date since
> retiring in 2002 with ill health and a real big surprise baby for
> geriatric parents to become full time housedad. The only "raid" I've
> added to machine for our home use has been to ancient Pnetium class kit
> from the mid 90s we use as home nfs/samba servers, and that has just
> been the addition of a cheapo SATA PCI bus card and addition of some
> 320gb sata disks, software raided rather than using hardware raid cards.
> The PCI bus is, if I recall correctly, slow - about 290mbs/sec??

Standard PCI is:

 	- 32 bits wide
 	- 33MHz

giving a peak data transfer rate of 32*33*10^6=1056Mbit/s=132MByte/s. "Watch
your units!" as my Physics teacher often implored... :-)

There is also 64 bit PCI, 66MHz PCI (the last two may or may not be
combined), and 64 bit/133MHz PCI-X.

> - but for light home use by at most 4 or 5 users over a 100mbs network or
> a wireless network such speed limitations make little difference.
> So if one is "adding raid" and seeing a major performance boost, that
> indicates to me that this raid is not being added over the old PCI bus.
> Is there a new high speed data bus that has come into existence I know
> nothing about?

Yes, a few; most common are 64bit and/or 66MHz PCI (which has been around
since the inception of PCI), PCI-X and PCI-Express. Most RAID controllers
you'll find are 66MHz 64 bit PCI giving upto 533MByte/s, but there are some
PCI-X and PCI Express controllers out there. See
<http://en.wikipedia.org/wiki/List_of_device_bandwidths#Computer_buses> for
bus bandwidth specifications.

Hard discs usually give 40-80MByte/s each, so a RAID0 array of 8 should be
sufficient to saturate 64bit/66MHz PCI, at least on occasion, if not

Best Regards,
Alex Butcher, Bristol UK.                           PGP/GnuPG ID:0x5010dbff

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