Why fibre disks

[Quin Hammes]

I have noticed that alot of array manufacturs are going to fibre disks over SCSI internally. Forgive me for my ingnorance but doesn't fibre go at 100 MB/s and UltraSCSI 360 goes at 360? Why is the industry moving towards fibre which seems to be slower (even 2 gig fibre is slower than Ultra SCSI 360, right). Please let me know if my speeds are off or something. Points for any good answers.

Quin

[Vincent Fleming · Accepted solution]

I think this is mostly due to the FC (as a bus structure) is much more reliable and handles errors more gracefully (due mostly to it's serial architecture). It's very efficient, can carry long distances, is switchable, and is not subject to electromagnetic interference.

SCSI 360 has, as it's biggest problem, a distance limitation.

FC also has a better roadmap - it's planned to be 1 GB/s in just a few years. (that's a 10x increase!)

If I were developing new products, I would invest in FC over SCSI for that reason alone.

[Vincent Farrugia]

Hello,

First of all, UltraSCSI 360 does not exist, that should be Ultra SCSI 320 (Ultra4).

The biggest advantage of Fibre Channel is distance and speed. Fibre channel can go for up to 10km, whereas SCSI a meagre 25m. Also, you can connect more devices on Fibre than on SCSI.

Bit error rate (BER) is very very low... 0.0000000000001 is the minimum acceptable rate for FC. That's a huge advantage as well.

HTH,
Vince

[Stefan Saliba]

I think what Quin is trying to say is not the advantages of SCSI over FIBRE channel but the use of Fibre channel disks instead of SCSI disks.

i.e. FC60 is a fibre channel array but inside still uses SCSI disks. The FC10 is a JBOD which uses Fibre CHannel disks.

The advantage that I see is that using FC disks you avoid the overhead needed to translate SCSI instructions to FIBRE channel ones. your disks have a loop ID and are part of the FC loop immideatly. Performance of these disks is better then the SCSI ones, I heard some of them have variable rotational speeds and they have better bandwidth then scsi. THE downside is the price TAG, a 72GB FC disk will cost you around $6,000

Hope I understood you well.
Of course the advnatages the others mentioned are also valid

Good luuck
stefan

[reg lustenhouwer]

Hello Quin:

One point to remember is that the speeds you are referring to sre the interface speeds. The sustained data rate coming from an individual disk is much less than 320MB/s for any random I/O (or even sequential). As the speed for the interface increases more devices can be placed on the same bus.
If you double the bus speed twice as many devoces can be serviced by the increased bus bandwidth if all else remains the same. This is where fibre channel disks excel. In a scsi parallel bus you can only have 16 devices. In an arbitrated loop you can theoretically have more than 200 on the one bus. This would not be possible on a parallel bus as they only have 16 id's and the distance limitation would also prohibit this.