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>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 |
| Voice: (408) 656-3316 | 2800 Powder Mill Road |
| Fax: (408) 656-2814 | Adelphi, MD 20783-1197 |
A designer knows when he has achieved perfection not when there is
nothing left to add, but when there is nothing left to take away.
-- Antoine de Saint-Exupery