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

Andy Smith andy at lug.org.uk
Tue Mar 18 16:54:03 UTC 2008


On Tue, Mar 18, 2008 at 04:19:08PM +0000, M.Blackmore wrote:
> The PCI bus is, if I recall correctly, slow - about 290mbs/sec?? - 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.

A 32-bit 33MHz PCI bus has a maximum theoretical transfer rate of
133MB/sec.  That is megabyte not megabit.

A 64-bit 66MHz PCI bus would be 4 times that.

PCI-X is 64-bit and can run at various frequencies from 66MHz to
133MHz (more is possible but rare).

PCI-E is a serial interconnect like serial ATA or firewire composed
of multiple lanes.  So a PCIe x8 slot has 8 lanes.  Devices will use
as many lanes as they support and are available.  4 lanes is about
equal to 1000MB/s although it scales bettwe with multiple devices.

Even 133MB/s is more than a single SATA disk can do in sustained or
random seek mode.  However...

> 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.

...I would dispute that RAID is always for the purpose of
performance.  Usually you want redundancy as much as if not more
than performance.  A RAID-5 *can be* slower than a single disk no
matter what type of bus it is connected to.

> As I've said in another post just a moment ago, any guidance for
> "catching up" literature for modern hardware would be gratefully
> received, I don't know where to start, and am stuck in a 2002 knowledge
> timewarp!

Putting the acronyms into wikipedia works usually. :)


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