Bus architecture was RE: [SunRescue] Re: NetApps?

[Björn Ramqvist]

> Note, this is a major reason why an u160 device in your PC isn't generally
> any faster than an FC-AL device (probably not much faster than your 80MB/s
> stuff), and one of the reasons why full duplex gig-e is pointless to the
> desktop.

No, but the U160 standard unlocks another rather impopular phenomena, a
choked bus.
Many of us already know that you can hook up to 15 devices on a standard
16-bit (Wide) SCSI-bus. However, even if you have all this Ultra2 fancy
stuff (80 MB/s), you could choke the bus if you read/write aggresivly
with large sequential files. (typical fileserving)
So, practicly you can only hang upto 3,4 or maybe 5 quick devices on a
heavily used SCSI-bus. If we don't wanna go for dual- or maybe
triple-channel, we go for even higher speeds (even though the drives
won't ever come close to the speed). This enables higher "burts" (direct
cached reads) and more devices on the same bus, without choking it.
Compaq (and others) are already looking at the next proposed standard,
SCSI-3, or "Ultra320".

This scenario isn't really on the desktop either, where we have (as
you've said) already bottlenecks in the architecture. But for big fat FC
RAID-controllers (external), this could cut the pricetag down when
volume goes up, and still make a tremendous bandwidth out of just one
channel. Right now it's popular spreading bandwidth among several
SCSI-buses, 2, 3, or even 6 buses, just to keep the FC busy and still
manage to get away with SCSI-devices.

> Now if the infiniband stuff ever gets off the ground, or if manufacturers
> would finally switch to 64 bit 66Mhz bus architecture, then maybe we would
> be able to effectivley use high bandwidth devices like GigE, and the higher
> end SCSI. Now prototyes are in the works for 2Gb/s ethernet, and 400MB/s FC
> devices. Nothing we have available on the desktop and low-midrange server
> market today could effectivley use these things.

It's a good thing we are almost stucked to 64-bit/66MHz devices here at
work, both on SGI and the Alphas. It's just those PeeCee-servers that,
for some reason, sometimes comes with 32-bit cards, even the Fibre
Channel adapters.
PCI-64 in all it's glory, but infiniband seems to be somewhat
interesting.

/Bjorn