Fwd: How to read bad blocks error message & marking of same
das at FreeBSD.ORG
Fri Aug 6 17:11:28 PDT 2004
On Fri, Aug 06, 2004, Gary Corcoran wrote:
> Dan Nelson wrote:
> >In the last episode (Aug 06), Gary Corcoran said:
> >>Mike Meyer wrote:
> >>>Modern drives deal with bad block substitution all by themselves.
> >>Umm - not quite, right? That is, if a block "goes bad" and you get a
> >>read error, the drive isn't going to do any "substituting" at that
> >>point. You'll just continue to get the read error if you try to
> >>access (read) that block. It's only when you allow another *write*
> >>to that block (e.g. by deleting the original file and writing new
> >>files) that the drive will automatically substitute a spare block for
> >>the one that went bad.
> >SCSI drives, at least, may do automatic reallocation on both reads and
> >writes ( camcontrol mode da0 -m 1, the ARRE and AWRE flags ). If the
> >drive had to reread the block or had to use ECC to recover data, AND
> >the entire block was recovered, it will relocate the data if ARRE is
> Good to know, although I stopped buying SCSI disks (for home use)
> years ago. I presumed the more common case these days, that we
> were talking about IDE disks. In fact doesn't this (from the original
> ad0s1a: hard error
> necessarily refer to an ATA (IDE) disk? I don't believe any (current)
> ATA disks will do automatic reallocation on reads, will they? Though
> of course serial ATA drives seem to be "the future" and are taking
> on more and more SCSI-like features as time goes by.
Both ATA and SCSI drives may relocate blocks that were difficult
to read (e.g. correctable errors, took multiple attempts, etc).
But if the block can't be recovered at all, the drive will still
report an error to the OS (in addition to relocation).
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