Re: Aligning file system data

["Barry K. Nathan" <barryn@pobox.com>]

On Tue, Mar 29, 2005 at 11:32:16PM -0500, John Richard Moser wrote:
> Does crossing a
> track boundary incur anything expensive?

AFAIK, yes. It's going to involve some kind of seeking (even a head
switch needs microjogging on modern drives), and it will certainly add
latency (although I don't remember how much, off the top of my head).

However, trying to control this from the kernel may be vastly harder
than you're expecting (assuming a modern hard drive). You may want to
look at these pages for more info:

http://www.storagereview.com/guide2000/ref/hdd/geom/tracksZBR.html
http://www.storagereview.com/guide2000/ref/hdd/geom/geomLogical.html

Also look at the last paragraph on this page -- not the paragraph with
the "Stop" sign, but the one after it:
http://www.storagereview.com/guide2000/ref/hdd/geom/formatDefect.html


I think this could in fact be done, but it would be a lot of effort,
and the kernel would need knowledge on a per-drive-model basis (or
at least it would need a way to obtain such knowledge from user space,
and the per-model knowledge would need to be stored there somehow).
For all I know, vendor-specific commands might also be needed in order
to find out which blocks are remapped, in order to use that knowledge to
avoid changing tracks spuriously. (And one other note: Since your device
almost certainly has many tracks with well over 256 sectors in reality,
your device is actually incapable of reading or writing a single track
with a single ATA command unless it supports LBA48.)

-Barry K. Nathan <barryn@pobox.com>
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