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    SubjectRe: Blockbusting news, this is important (Re: Why are bad disk sectors numbered strangely, and what happens to them?)
    Norman Diamond wrote:

    >Friends in the disk drive section at Toshiba said this:
    >
    >When a drive tries to read a block, if it detects errors, it retries up to
    >255 times. If a retry succeeds then the block gets reallocated. IF 255
    >RETRIES FAIL THEN THE BLOCK DOES NOT GET REALLOCATED.
    >
    >This was so unbelievable to that I had to confirm this with them in
    >different words. In case of a temporary error, the drive provides the
    >recovered data as the result of the read operation and the drive writes the
    >data to a reallocated sector. In case of a permanent error, the block is
    >assumed bad, and of course the data are lost. Since the data are assumed
    >lost, the drive keeps the defective LBA sector number associated with the
    >same defective physical block and it does not reallocate the defective
    >block.
    >
    >I explained to them why the LBA sector number should still get reallocated
    >even though the data are lost. When the sector isn't reallocated, I could
    >repartition the drive and reformat the partition and the OS wouldn't know
    >about the defective block so the OS would try again to use it. At first
    >they did not believe I could do this, but I explained to them that I'm still
    >able to delete partitions and create new partitions etc., and then they
    >understood.
    >
    >They also said that a write operation has a chance of getting the bad block
    >reallocated. The conditions for reallocation on write are similar but not
    >identical to the conditions for reallocate on read. During a write
    >operation if a sector is determined to be permanently bad (255 failing
    >retries) then it is likely to be reallocated, unlike a read. But I'm not
    >sure if this is guaranteed or not. We agreed that we should try it on my
    >bad sector, but if the drive again detects a permantent error then it will
    >not reallocate the sector. First I still want to find which file contains
    >the sector; I haven't had time for this on weekdays.
    >
    >When I ran the "long" S.M.A.R.T. self-test, the number of reallocated
    >sectors and number of reallocation events both increased from 1 to 2, but
    >the known bad sector remained bad. This is entirely because of the behavior
    >as designed. The self-test detected a temporary error in some other
    >unrelated sector, rescued the data in that unreported sector number, and
    >reallocated it. That was only a coincidence. The known bad sector was
    >detected yet again as permanently bad and was not reallocated.
    >
    >In this mailing list there has been some discussion of whether file systems
    >should keep lists of known bad blocks and hide those bad blocks from
    >ordinary operations in ordinary usage. Of course historically this was
    >always necessary. As someone else mentioned, and I've done it too, when
    >formatting a disk drive, type in the list of known bad block numbers that
    >were printed on a piece of paper that came with the drive.
    >
    >In modern times, some people think that this shouldn't be necessary because
    >the drive already does its best to reallocate bad blocks. WRONG. THE BAD
    >BLOCK LIST REMAINS AS NECESSARY AS IT ALWAYS WAS.
    >
    >This design might change in the future, but it might not. My friends are
    >afraid that they might lose their jobs if they try to suggest such a change
    >in the high-level design of disk drive corporate politics. I only hope this
    >posting doesn't get them fired. (This is not a frivolous concern by the
    >way. The myth of lifetime employment is a less pervasive myth than it used
    >to be, and Toshiba is pretty much average in both world and Japanese
    >standards for corporate politics.)
    >
    >Regarding finding which file contains the known bad sector, someone in this
    >mailing list said that the badblocks program could help, but the manual page
    >for the badblocks program doesn't give any clues as to how it would help.
    >I'm still doing find of all files in the partition and cp them to /dev/null.
    >
    >Meanwhile, yes we do need to record those bad block lists and try to never
    >let them get allocated to user-visible files.
    >
    >
    >
    >
    >
    Instead of recording the bad blocks, just write to them.

    --
    Hans


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