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Re: Slow disk performance (was: Re: install help 6400/200)

On  26 Jul, this message from rsaran echoed through cyberspace:
> The standard internal SCSI bus on OldWord PowerMacs is quite slow
> for today standards, with 10 Mb/s of nominal bandwitdh.
Make that MB/s (Megabytes, not Megabits)

Few machines had that 10 MB/s internal bus: only those with _separate_
internal and external buses had the 10MB/s internal (which limits it to
the 7300/7500/7600/8500/8600/9x00; not sure about the later Quadras).

> To make it worse factory installed hard disks where specially slow.
> Tipical sustained transfer read ates under MacOS are in the 3-5 Mb/s
> range.

True. In my experience the max you can get on the internal (10MB/s) bus
is around 6 MB/s, even using fast disks.

> This same machine supports a ten-fold increase in transfer rates using
> newer SCSI appliances. The 9500 provides 6 PCI slots and two of them are
> 64 bit wide.

Errmm.. no, the 9x00 series have no 64-bit slots. Those were the
first-generation PCI machines. You're a generation too far :-).

> Just add a good SCSI card (Ultra-2 LVD or Ultra160) and a
> newer disk to see real (not bus nominal) transfer speeds of 25-40 Mb/s.

Or an IDE card, for that matter. I get close to 15 MB/s on a (now rather
old) 10 Gig Maxtor 7200 rpm disk with a Promise Ultra/66.

> Some disk controllers even provide striping features, letting you read
> file fragments in parallel from different physical disks. Some setups
> deliver sustained 130 Mb/s read data rates.

You will never get that much (supposing you mean 130 MB/s). For one, a
single disk today will not go above maybe 40 MB/s. Anything above can
only be obtained with expensive high-end RAID systems; and then you're
starting to hit the limits of your storage attachment system, be it
SCSI/320, Ultra/133 for IDE, or Fibre Channel (1 or 2 Gbit/s).

Coming back to the 9500, you'll also hit the limit of that system's PCI
buses, which can do (according to Apple's doc: see technote 1008) a
maximum of 80 MB/s under optimal conditions.

> On memory and crashes: OldWorld PowerMacs are very sensitive to memory and
> bus speed. I am not sure but your machine bus probably runs at 45 MHz.
> Defective memory chips may fail even in standard speed, and certainly
> fail if you overclock the bus. Just forget about using your old memory,
> give the dimms away for someone with a PowerMac with lower bus speed.

This is true as well. I have enough mixed experience with a 7600 :-(.

> The 9500 has an additional problem, because its 512 Kb L2 cache is soldered
> in the mainboard and can't be replaced, but usually it is a good quality
> component.

Sure about that? All the other first-gen systems have their L2 cache on
a DIMM....



Michel Lanners                 |  " Read Philosophy.  Study Art.
23, Rue Paul Henkes            |    Ask Questions.  Make Mistakes.
L-1710 Luxembourg              |
email   mlan@cpu.lu            |
http://www.cpu.lu/~mlan        |                     Learn Always. "

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