[CentOS] OT - Offline uncorrectable sectors

Fri Aug 22 16:33:22 UTC 2008
nate <centos at linuxpowered.net>

William L. Maltby wrote:

> ?? Uncertain about "spares has been exhausted".

I don't recall where I read it, and I suppose it may be
misinformation, but it made sense at the time. The idea is
the disks are not made to hold EXACTLY the amount of blocks
that the specs are for. There are some extra blocks, that
the disk "hides" from the disk controller. The disk automatically
re-maps these hidden blocks(making them visible again). By
the time bad blocks start showing up on the OS level these
extra blocks are already full, an indication that there is
far more bad blocks on the disk than just the ones that you
can see at the OS level.

> Now, I don't know (or care) if an alternate sector was assigned, just
> that the sector was flagged unusable. For my use (temporary use - no
> permanent or critical data) this is fine. Last several mke2fs runs have
> produced the same amount of usable blocks and i-nodes, so I don't see
> evidence that no spare was available.

Note that mke2fs doesn't write over the entire disk, I doubt it
even scans the entire disk. I've used a technology called thin
provisioning where only data that is written to disk is actually
allocated on disk(e.g. you can create a 1TB volume, if you only
write 1GB to it, it only uses 1GB, allowing you to oversubscribe
the system, and dynamically grow physical storage as needed). When
allocating thinly provisioned volumes and formatting them with
mke2fs, even on multi hundred gig systems only a few megs are written
to disk(perhaps a hundred megs).