Re: bad sector in gmirror HDD

On Aug 20, 2011, at 06:24 , Jeremy Chadwick wrote:

> You might also be wondering "that dd command writes 512 bytes of zero =
to
> that LBA; what about the old data that was there, in the case that the
> drive remaps the LBA?"

If you write zeros at OS level to an LBA, you will end up with zeros at =
that LBA. What else did you expect???

The already remapped LBAs in ATA are not visible anymore to the user/OS. =
You get a perfectly readable sector. Of course not at the original =
location, but as you confirmed we are done with CHS addressing.

The pending bad sectors are almost always 'corrected', that is, remapped =
when you write to that LBA.

So your script will find only one readable sector and that will be the =
sector that is pending reallocation.

It may be that writing zeros to all free space, like

dd if=3D/dev/zero of=3D/filesystem/zero bs=3D1m; rm /filesystem/zero

is enough to remap the pending bad block and not have any unreadable =
sectors. But if the unreadable sector is in a file or directory -- bad =
luck -- these will need to be rewritten.

Once upon a time, BSD/OS had wonderful disk 'repair' utility. It could =
detect failing disks by reading every sector (had nice visual), or could =
re-write the drive by reading and writing back every sector. On bad =
blocks it would retry lots of times and eventually average what was read =
(with error).
Having said that, I doubt modern ATA drives will let anything be read by =
the pending bad block, but.. who knows.

Daniel

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