Udi Finkelstein <[EMAIL PROTECTED]> wrote:
> On Fri, 17 Dec 1999 01:51:54 +0200 (IST), Evgeny Stambulchik
> <[EMAIL PROTECTED]> wrote:
>
> > As far as performance is considered, it's not RPM that usually matters
> > (inspite of a big hype), but the amount of cache on disk. Until
> > recently (1-2 years), the maximum cache on IDE disks was 128KB, while
> > about each SCSI disk had 0.5MB.
>
> I would speculate that all the on-disk cache you need is two full
> cylinder's worth of data.
>
> You want at least one cylinder's worth of data so you can read it all in
> one round, and really want two so you can read the next one while your
> buffers are still being transferred to the PC.
Applying this logics to RAM would suggest that level-2 cache should be just
twice as big as level-1 and the latter shouldn't exceed the doubled total
storage value of CPU registers. Actually, saying a fixed amount "is enough"
means there is no sense in multi-level caches at all.
> I have a hunch that with a computer fast enough, anything over 2 cylinder's
> worth of data will offer near zero improvement. After all, the system cache
> is more efficient than going to the drive and pulling data from the HD
> cache, so the HD cache can only help you if it lets you stream the data at
> the maximum possible speed (one rotation for each full cylinder).
The above reasonings are good, but you're silently assuming the OS has nothing
to do except writing/reading from the disk; moreover, from the _single_ disk.
It's definitely wrong in the multi-task OS's like Linux/Unix. I'd call your
estimate a "minimal reasonable value".
> The problem is that it's hard for us customers to really estimate how much
> cache is really in these drives (in terms of number of cylinders), since
> the numbers of cylinders pronted on the disk label has nothing to do with
> the actual number of physical cylinders it has.
?? It's a news for me. Anyway, what you're interested in is the amount of data
read in contiguously during one disk revolution, a ratio between (contiguous)
I/O read/write data access rate and RPM. This value is actually a function of
HD technology, and is pretty constant over the whole range of disks, both IDE
and SCSI, present today: with access rate ~8-10MB/s for 5400 RPM disks and
~13-15MB/s for 7200 RPM models, one gets ~100KB per disk revolution. Hence,
the minimal resonable amount of cache should be ~200KB, something that not all
currently available IDE disks exceed (and about none 1-2 years ago), while
0.5MB for SCSI disks has been a usual thing for years; many high-profile SCSI
disks can be (optionally) supplied with either 2 or 4MB - don't expect to
easily find them in Israel, though :)
Regards,
Evgeny
--
____________________________________________________________
/ Evgeny Stambulchik <[EMAIL PROTECTED]> \
/ Plasma Laboratory, Weizmann Institute of Science, Israel \ \
| Phone : (972)8-934-3610 == | == FAX : (972)8-934-3491 | |
| URL : http://plasma-gate.weizmann.ac.il/~fnevgeny/ | |
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|______________________________________________________________|
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