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SubjectRe: bio too big device
Andries Brouwer wrote:

> We have seen *one* drive (a six years old Maxtor 7850AV) that could not
> sustain heavy load with max # secs set to 256, while it behaved better
> with max set to 255.

[...]

> Paul remarked: "So the 255 (or even the old 128) fixes things vs. 256,
> but I'd feel better being 100% sure why. Is 255 a "fix" or a perturbation
> that happens to paper over something else?"

As luck would have it, I kept that drive around, thinking it might be
interesting to have around if the above "why" part got revisited
someday.

> I think there is no good reason to limit us to 255 sectors.
>
> (And no reason for blacklists either - there is just no good evidence
> that something is systematically wrong with 256 sectors for any brand or
> model. Things would change if a second Maxtor 7850AV owner could confirm.)

Perhaps I can also act as the second owner in this case. :-) The drive is
currently in an old 16MB P133 on a PIIX3, so after tripping over this
thread, I did some testing to 1st see if I could still re-create any
problems in this different box. Of note, 2.4.21pre5 reports:

hda: 1667232 sectors (854 MB) w/64KiB Cache, CHS=1654/16/63, BUG DMA OFF
-------------------->^^^
which I haven't looked into yet. Anyway, I tweaked pre5 to re-allow 256
and then let it do two 2.5.64 compiles at the same time (in 16MB, this
takes a while) and nothing broke. Thinking that the original problem was
in an even older VLB box, I disabled DMA and did the same test. Still no
breakage.

What does this mean? I'm not sure :) Maybe this simple test isn't as
harsh as the prior one from several years ago. Realistically, there
are a fair number of changes that also may have a role: e.g. ide driver,
gcc, gas, kernel, CPU, mainboard, ide controller, PSU, cables, and so on
all have changed. It looks like I would need to jump back into the past
and test on the exact same system and then progressively rule out some
of these variables.

Paul.

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