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At 10:36 AM 5/18/2001, you wrote:
>Do you know the logical LBA to physical LBA mapping for a drive?
>Do you know the zone layout? Do you know if the drive does
>small seeks faster than a head switch (most do!)? Do you
>realize that with write caching on all write commands have
>virtually zero "command overhead"? Do you realize that many
>drives can execute 5, 10, maybe a 100 commands all in hardware
>without the drive's firmware getting involved? Do you know that
>many drives will recognize the seek and read/write patterns of
>the most popular "performance measurement" software and adjust
>their caching operations so they look better while running that
>test? If you issue a bunch of seek commands without reading or
>writing data do you know if the drive is really doing the seek
>you think it should? Do you know if it is doing any seek at all?
>It might not even be spinning while you think you are doing those
>seek commands!
HALE HALE HALE!!! People are NOT supposed to know our secrets!!
If you tell everybody our secrets, then everybody will know our secrets!
TSK TSK TSK.
Now, we'll have to redo all our performance measurement detection and adjustment code!
=ron
(just kidding!)
But, Hale is right: there is a whole lot of (1) hardware assist, (2) data location
mangling due to defect management, (3) data caching, including write-back!,
(4) cyl/head/zone/slip stuff going on depending on the TYPE of drive AND quality
of drive, and (5) cache management adjustments based on past behaviour! [for example,
if suddenly "writes" prevail, more and more cache may be assigned to writing...]
I'd have to say that just looking from the outside interface, a lot of it would appear
to
be non-deterministic in a short term test.
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