RE: Hard drive on the EPP

[george@cs.nps.navy.mil (Robert George)]

>Did my message not get out?  The 5-10MB/sec specification being waved around
>for many new drives is the burst transfer rate - i.e. moving data from the
>built-in cache to the computer.  The vast majority of disks can only read

Since the IBM ThinkPad hard drives have a 32KByte cache, the cache is 
practically insignificant for most throughput calculations.

>data off the platter at 500k/s - 1MB/s.  If your drive is rated at 10MB/sec
>and it has a 256k cache, the maximum time penalty for using EPP is waiting
>0.25 seconds instead of .025 seconds.  In typical use, you'll barely notice
>the difference.

Wrong!
        The ThinkPad's disks have a data transfer rate off the platter of
24.5 - 35.8 Mbits/sec (quoting from the IBM internal technical documents).
This, of course, equates to ~ 3 - 4.5 MBytes/sec. 

> 
> Here's a quick chart based on the disk's physical specifications (sectors
> per track i.e. sectors in one revolution, and rpm) based on the standard
> 512 byte sectors:
> 
>     kilobytes passing under disk head each second
> 
>             RPM   1800     3600     5400
>    Sec/Trk ------------------------------
>      17    |      255k     510k     765k
>      33    |      495k     990k    1485k
>      43    |      645k    1290k    1935k
>      63    |      945k    1890k    2835k

Yes, I agree with this table, however these specifications were state-of-the
-art 3 years ago.

> 
> Most of the older disks I know of are 33 sectors/track and 1800 rpm.
> 3600rpm disks started showing up in force a couple years ago, and 5400 rpm
> disks are still pretty rare (usually advertised as optimized for

The Micropolis SCSI-2 drive I just bought for my PC rotates at 5400 RPM --
and it wasn't outrageously expensive.

> multimedia).  33 sectors/track seems to be the most popular.  You can
> figure out the rest.  The rate on the chart is the FASTEST the drive can
> read data off the disk - 1:1 interleave, data in sequential order on only
> one track, no bad sectors, etc.  The drive can exceed this speed for a
> fraction of a second via a cache hit, but the sustained data transfer rate
> will not be much higher.

The IBM drives spin at 3800 RPM, and have 112 sectors per track.

> I have no idea what the specs on the TP's hard drives are.

> In typical use, you'll barely notice the difference. [...by using EPP]

Empirically, I can say unequivocally that this is not true.  Those of you out
there who have desktop PC's with Fast SCSI-2 controllers, try turning off
synchronous transfers, which will drop your bus transfer rate from
10 MBytes/sec to 5 MBytes/sec.  You will notice a severe performance 
degredation under _normal_ DOS and Windows applications.

The bottom line is, if you are using a modern disk drive, don't waste it
on an EPP port.  Invest in a Fast SCSI-2 PCMCIA adaptor!

Sorry my posts are at spurious intervals, I'm usually juggling multiple
research projects as well as flying around the country... 



| Robert George            |  Army Research Laboratory              |
| robertg@assb01.arl.mil   |  AMSRL-SS-IC                           |
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