Re: Are linux-fs's drive-fault-tolerant by concept?

[John Bradford]

> I say:
> we already have the needed information inside every fs, why not use it?
> No space wasted, no double information.

Bad block remapping in software would be useful only on relatively
obscure devices which don't already do it.  I consider it to be
counter productive, and a bad thing to do on devices that already do
it themselves.

Therefore, it is an ideal candidate for being done in a separate
layer.

Your argument seems to be that if a write error is encountered, the
simplest thing to do is to try another block, and mark the first one
as unusable.

On devices which have a fixed physical area for each logical block,
and where no write caching was done, your approach might be the best
one.

However, modern operating systems often cache a lot of data to write
out when the disk is idle.  The filesystem will typically allocate
this data to various blocks immediately.  A write failiure 30 seconds
later would mean that the filesystem then has to change that
allocation from the bad block to a new block.  It's possible that
there might not even be a new block available by that time.  On disks
which do their own bad block management, if one block re-allocation
failed, because there is no space left, it's quite possible that all
future re-allocations will fail as well.

On devices which never report back bad blocks to the operating system,
the space for the bad block table is wasted.

John.