Re: Cheap RAID system recommendation?

Rick Denoire <100.17706_at_germanynet.de> wrote in message news:<fq18muc22beh9r7cnc6mb423rvkan3cijp_at_4ax.com>...

> That's a really strange argumentation. If a bus can transfer 160 MB/s,
> who cares if it is doing it serially?

I do. If it is serial, it means I need a device that can transfer at that speed in order to fully use the speed of the bus. If it is parallel then I don't care: I just jam as many operations as the bus can cope with and rip the aggregate speed out.

>
> >You'd need some purty synchronized I/O requests to achieve the bus speed
> >capacity and streaming with parallel disk ops.
>
> That is what is called "RAID". That is what we are talking about.

No, it most definitely is not RAID. RAID is Redundant Array of Inexpensive Disks. It isn't "parallel operation". Nor SCSI. You can quite happily have RAID with IDE/ATA or Fibre channel disks/controllers. And RAID has no guarantee of any synchronization or sequencing of op requests to disks in a bus. It delivers a consistent result to the mainboard using the RAID controller. That does not mean the controller will sequence the I/O to the disks in any particular order or sequence.

In most cases it won't. That is why RAID controllers usually have fat, huge caches: they help even out the effects of non-sequenced disk I/O requests. Don't confuse that with what bus speed buys you.

>
> Do you know ANY disk that does not have a built-in cache nowadays?
> Not to mention the RAID's own cache.
>

I said a fast cache. Of sufficient size. All modern SCSI disks have a cache, that is neither fast enough nor sufficiently large to cope with a 160Mb speed bus.

If you pour through the Seagate website, you'll find the same disk model with various different cache sizes. Guess which are the really expensive ones you cannot get at the corner computer store? Same for IBM disks.

The only thing a 160Mb SCSI bus buys you is the guarantee that SCSI packets will travel at that speed between a disk and the system mainboard - in the bus. Aggregate I/O speed is a lot more than just that bit.

Cheers
Nuno Souto
nsouto_at_aspect.com.au Received on Fri Aug 23 2002 - 01:01:09 CDT