🚀 go-pugleaf

In <43941@ames.arc.nasa.gov> lamaster@ames.arc.nasa.gov (Hugh LaMaster) wrote:
>In <10538@alice.UUCP> andrew@alice.UUCP (Andrew Hume) wrote:
>>[...] For example, the Sabre 2HP is only
>>a little more expensive than the regular sabre, a tad smaller
>>but twice as fast.

>I assume that you mean 6 MB/sec.  I guess Sun doesn't sell these.  Are
>there any problems with drivers/formatting/etc?  Can you just plug them
>in and run?

Well, the 2-headed (2HP) version does store the defect map bit interleaved.
If you are using the disk in byte or word interleaved mode, this might cause
you some annoyance in getting at the defect map.

>I thought the filesystem was the bottleneck.  The best you can get out
>of a 3 MB/sec disk is ~2MB/sec now.  What kind of speeds can you get out
>of a 6 MB/sec disk?

Not all the loss is the file system's fault.  The disk's transfer rate is
not the "usable data rate".  The disk handles data at 3 or 6 Mbyte/s, this
"data" has sync pulses, timing gaps (gaps, pads, and delays), ECC bytes,
and other little things like duplicate headers.  What you generally get is
about 512/600 of that (presuming your controller wants 600 byte physical
length sectors and you're using 512 data byte sectors).  This means you
will only get about ~2.56 Mbyte/s of usable data from a 3 Mbyte/s disk or
~5.12 Mbyte/s from a 6 Mbyte/s disk.  This is a lot like the difference
between formatted/newfs'd disk space and raw-disk space.

600 bytes is a rough number used by SMD-controllers-I-have-known.  On an
IPI-2 disk/controller there are additional pads/gaps, so the number probably
gets a little bigger.
--
/*    Jim Hutchison   		{dcdwest,ucbvax}!ucsd!celerity!hutch  */
/*    Disclaimer:  I am not an official spokesman for FPS computing   */