From: geoff@zswamp.UUCP (Geoffrey Welsh)
Newsgroups: biz.sco.opendesktop
Subject: Re: SCSI-2 Adapters for SCO ODT
Date: 9 Jan 93 18:39:58 GMT
Organization: Izot's Swamp

brad@clarinet.com (Brad Templeton) writes:

> You're saying that DMA on intel machines shuts down the CPU?  I wasn't
> aware of this.  I thought DMA simply put the processor in a wait state
> if it happens to be going to main memory at the same time as a DMA
> transfer.   Since the processor goes to main memory on fewer cycles than
> you would think (that's what the cache is for) DMA is not supposed to
> impact the processor a great deal if it is done right.

   Now that you mention it, I can't think of a reason why you could not do it 
that way.  The question is: given the way that DOS usage affects PC compatible 
motherboard design, is it reasonable to expect that designers (with the 
possible exception of some brand names) would do this?

> This is not to
> say that EISA is not better than ISA, and that there isn't improvement.
> My goal here is to find out how much.  It is important to realize that
> going to a 32 bit wide bus at 33 mhz is not going to give you 8 times
> as much througput as a 16 bit bus at 8 mhz.  But what do you get?

   Why are manufacturers pushing local bus (aside of the obvious gimmick 
factor)?  We know that video cards are slow - it's only been in the past 
couple of years that commodity cards didn't impose *extra* wait states on I/O 
operations - and now we're putting them on the CPU bus?!?  PC Magazine thinks 
it's great (a recent issue benchmarked the speed improvement and, I have to 
admit, it's significant).

> If your disks are sending at 1 mb/second, then you would have to have
> a lot of disks going at once before saturating a 5 mb/s SCSI-1 bus and
> finding yourself in need of SCSI-2.  (Particularly since disks spend
> most of their time seeking and waiting for sectors to spin around)

   Conner's CP3100 and 3200F drives (SCSI-1) benchmark at roughly 700K/second 
on my 386/40 with a 1522 and the same ona friend's 486/33 with a 1542B.  The 
SCSI-2 CP3540 gave 1.25 MB/sec write and 1.5 MB/sec read (the latter aided by 
an on-board cache; the version of the benchmark used predated my adjustment to 
defeat the effects of 256K caches).  The benefits of SCSI-2 are not amazing, 
but they are significant. 

   BTW, the Micropolis 1624 and Adaptec 1740 on InSystems' NetWare server 
clocks in over 2 MB/sec; it looks like EISA has its advantages, too. 

> A surprisingly easy fix that's not done more is to
> read from multiple heads at the same time.  On a 16 head disk you can of
> course get 2 bytes per bit-time, 16 mb/second, though you become more
> vulnerable to errors.  Anybody doing this? 

   I would have thought so, but I haven't found any yet among commodity 
drives.  I guess that duplicating the circuitry for each R/W would make the 
drive too expensive to compete in a market where sales follow claims like the 
Plus Impulse '9 ms' drive (which is a relabelled Quantum ProDrive, which 
Quantum advertises as 'under 15 ms').  I've seen a prototype drive from Conner 
that uses two sets of read/write heads to reduce average seek time and read 
latency (the heads are at opposite ends of the drive, so you can save just 
under half a revolution's wait, and the drive can preposition the heads to 
read the next cylinder without stepping delay). 

> One scheme larger systems use is to have several disk drives and store one
> bit per disk drive!  With 19 disk drives, you can store 16 bits and 3 bits
> of error correction.  This makes it possible to take any one disk drive
> offline -- and the system still works.   Indeed, when a drive goes bad on
> these systems, you replace it and slowly rebuild the ecc onto it.  You
> are vulnerable to errors while it is off and being rebuilt, and you must of
> course keep the drives in sync.
> 
> Doubt anybody is doing this for SCO, but it's a way to get highly reliable
> disk operation and 16 megabyte/second throughput.

   RAID technology accomplishes similar results, and it's not uncommon.  Some 
of InSystems' clients use them for PC network servers.

  Geoffrey Welsh, 7 Strath Humber Court, Islington, Ontario, M9A 4C8 Canada
geoff@zswamp.uucp, [xenitec.on.ca|m2xenix.psg.com]!zswamp!geoff (416)258-8467
                Now I've lost everything, I give to you my soul
           And the meaning of all that I believed before escapes me
                  In this world of none, no thing, and no one
                            - Genesis, _Afterglow_