James Bottomley wrote:
On Thu, 2009-03-19 at 23:32 -0400, Mark Lord wrote:
Norman Diamond wrote:
For months I was wondering how a disk could do this:
dd if=/dev/hda of=/dev/null bs=512 skip=551540 count=4 # succeeds
dd if=/dev/hda of=/dev/null bs=512 skip=551544 count=4 # succeeds
dd if=/dev/hda of=/dev/null bs=512 skip=551540 count=8 # fails
This basically means the drive doesn't report where in the requested
transfer the error occurred. If we have that information, we'd return all
sectors up to that LBA as OK and all at or beyond as -EIO, so the
readahead wouldn't matter.
That's exactly what my submission suggested Linux should do, because that's
exactly what Linux isn't doing. The defective sector number is 551562.
Linux makes varying decisions on how much to read ahead, and when its
readahead includes the defective sector Linux doesn't do what you and I want
it to do.
The way I discovered the actual defective sector number is that one time
last week I noticed it in dmesg output. After noticing it, investigation
became a lot easier. I don't remember if I noticed it for hda (old IDE) or
sda (LIBATA) but either way the drive put the defective sector number in its
error report. When readahead was long enough to reach sector 551562 the
drive told the PC.
Regarding other threads of this discussion, I/Os are not being merged with
other processes. I'm running either Slax or Knoppix from a live CD, and the
only one accessing the hard drive is me.
In cases where Slax or Knoppix includes a sufficiently recent hdparm, I
could attempt reads of individual sectors. 551561 is OK. 551563 is OK.
551562 has an uncorrectable media error. I had mentioned that the drive has
egregiously bad firmware (which doesn't excuse Linux). That includes an
effort to relocate the sector by using hdparm to write sector 551562,
whereupon Hitachi drives me crazy. The drive reports success but subsequent
reads still fail.
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