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

[Hans Reiser <reiser@namesys.com>]

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