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SubjectRe: Blockbusting news, this is important (Re: Why are bad disk sectors numbered strangely, and what happens to them?)
Quote from "Norman Diamond" <ndiamond@wta.att.ne.jp>:
> Now, maybe there is a technique to force it anyway. When a partition is
> newly created and is being formatted with the intention of writing data a
> few minutes later, do writes that "should" have a better chance of being
> detected. The way to start this is to simply write every block, but this is
> obviously insufficient because my block did get written shortly after the
> partition was formatted and that write didn't cause the block to be
> reallocated. So in addition to simply writing every block, also read every
> block. For each read that fails, proceed to do another write which "should"
> force reallocation.

I am just imagning how many Flash devices will be worn out
unnecessarily by any filesystem utility that does this transparently
to the user :-(.

> Russell King replied to me:
>
> > > 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 is perfectly reasonable. If the drive can't recover your old data
> > to reallocate it to a new block, then leaving the error present until you
> > write new data to that bad block is the correct thing to do.

I 99% agree with that. The 1% where I don't is that there may be
situations where there is no interest in doing any data recovery from
the drive, (you have backups, it is part of a RAID array, or storing
temporary data that can be re-generated whenever necessary), and also,
any read errors that occur during a S.M.A.R.T. read test should result
in a re-mapping of the block.

> > Think about what would happen if it did get reallocated. What data would
> > the drive return when requested to read the bad block?
>
> Why does it matter? The drive already reported a read failure. Maybe Linux
> programs aren't all smart enough to inform the user when a read operation
> results in an I/O error, but drivers could be smarter. I think there's
> probably a bit of room in an inode to add a flag saying that the file has
> been detected to be partially unreadable. Sorry for the digression.
> Anyway, it is 100% true that the data in that block are gone. The block
> should be reallocated and the new physical block can either be zeroed or
> randomized or anything, and that's what subsequent reads will get until the
> block gets written again.

100% agreed.

> > If the error persists during a write to the bad block, then yes, I'd
> > expect it to be reallocated at that point - but only because the drive has
> > the correct data for that block available.
>
> We agree in our moral expectations and our technical analysis that correct
> data will be available at that time. But if your word "expect" means you
> have confidence that the drive will perform correctly, I do not share your
> confidence (I think it is possible but highly unlikely that the drive did
> its job correctly during the previous write).

If the drive is not doing it's job properly DRIVE -> BIN.

> > Your description of the way Toshibas drive works seems perfectly sane.

I disagree - we haven't confirmed what happens in the error-on-write
situation. If it does indeed always remap the block, then I'd agree
that that aspect was perfectly sane.

John.
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