Indeed usually a disk should be able to finish the sector write with
remaining power. Actually most modern disks do have voltage shifters and
most parts operate on lower voltage. Thus a drop on the changer's input
does not immediately lead to a drop on the output of the voltage
shifter. If's theres enough power left for the physical layer scrambler
and the head to write, then everything should be fine. I was rather
wondering if there's definite sources on that?
BTW. The burst errors I mentioned did not happen on a power loss, but
rather during operation. Reading twice, one time with burst errors, one
time without. I checked the RAM for ages - no failures. That was really
weird.
Regards
-Sven
Am 08.02.2016 um 17:41 schrieb Arno Wagner:
On Mon, Feb 08, 2016 at 01:25:31 CET, Sven Eschenberg wrote:
[...]
Concerning disks, I thought with ACS2/ATA-8 real write barriers were
introduced. On the other hand I've seen disks returning successfull
reads with long zero-burst-errors undetected - no fun. I always
wondered how a HDD exactly behaves when power fails, while a sector
is in transit. My best hope is, that the CRC at the end of the
sector does not match and an error is returned on the next read?
For these you should have intact data on disk, but
your data never made it there. If data after the zeros
did get written fine, there is a simple explanation:
Modern disks may reorder sectors in order to be able
to begin writing as soon as the heads are stable in
the track.
Behavior on power failure used to be that the disk will
notice the power failing early enough that it has enough
time left with hood power to finish a sector-write in
progress. I think that still applies. The zeros would
then be sector-aligned and/or the data that was in
those sectors before, hence the checksums are fine.
The thing is that in a typical PC, power drops relatively
slowly and disks work non-seeking for a lower voltage
that the thresholds. Add to that that a single sector
write takes less than 1ms (probably much less), and
you get ample time to finish a write in progress.
Regards,
Arno
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