Messages in this thread | | | Date | Fri, 20 May 2005 16:17:27 -0500 (CDT) | From | Adam Miller <> | Subject | Re: software RAID (fwd) |
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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.
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