Re: software RAID

[Grzegorz Kulewski <kangur@polcom.net>]

On Fri, 20 May 2005, Lennart Sorensen wrote:

> On Fri, May 20, 2005 at 12:56:13PM -0500, Adam Miller wrote:
>>   We're looking to set up either software RAID 1 or RAID 10 using 2 SATA
>> disks.  If a disk in drive A has a bad sector, can it be setup so that the
>> array will read the sector from drive B and then have it rewrite the
>> bad sector on drive A?  Please CC me in the response.
>
> If a harddisk has a bad sector that is visible to the user (and hence
> not remapped by the drive) then it is time to retire the drive since it
> is out of spares and very damaged by that point.
>
> 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.

What do you mean "see bad sectors"?

Modern drives are trying to relocate sectors that can become bad in short 
time. But this does not work 100% reliably. And sometimes disk wants to 
relocate sector but the sector can not be read anymore. If this happens 
disk will return read error when reading the sector _but_ when you will 
write it again it will relocate it (with new data). And I think this was 
the idea behind first post... To allow disk A to relocate not readable 
sectors with correct data from disk B.


Grzegorz Kulewski