Here is a response from Bruce Allen.
>> If you have a bad sector, it doesn't go away by writing to it again. On
>> modern drives, if you see bad sectors the disk is just about dead, and
>> will probably be seen as such by the raid system which will then stop
>> using the disk entirely and expect you to replace it ASAP.
This is false.
Modern ATA and SCSI disk drives have several thousand spare sectors.
When a sector is unreadable (UNC) which means that the ECC codes are
inconsistent, the drive will REALLOCATE the sector, assigning a spare
sector the LBA of the failed sector. However it will only do this when
you WRITE to the LBA of the failed sector.
> The one exception here is if you have a miswritten sector (usually
> caused by unexpected power-down), which won't read back correctly -
> but running badblocks with one of the 'write-verify' options will
> resurrect it.
Sectors can have inconsistent ECC codes for a number of reasons:
-- failed write during sudden power-down
-- damage to magnetic media at this LBA
-- other reasons
> If you have a drive that has a bad block in it even *after* badblocks has
> re-written it, it's time to replace the drive *now*....
Not true. Disks which have reallocated large numbers of blocks are
usually failing. But most good disks have some reallocated blocks.
> For the original poster: Breaking the mirror and then re-mirroring
> from the "good" drive *might* recover the bad block when it re-writes
> it. But don't bet on it...
It won't 'recover' the bad block. It will write the data (obtained from
the good drive) to a newly allocated spare sector on the bad drive.
Cheers,
Bruce