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SubjectRe: Why does the md/raid subsystem does not remap bad sectors in a raid array?
Robert Hancock wrote:
>> The controller does not drop the drive from the array when it hits an
>> error, the 3ware card "takes care of it" and the user need not worry
>> about it, whereas with md/raid every time it hits a bad sector, it
>> breaks the raid and it goes degraded, is this correct? Will/can
>> something like what 3ware does be possible in a sw-raid based
>> configuration or is a HW raid card required?
>
> Presumably all it's doing is writing that sector's contents back from
> the other drive(s) in the array when the read error is detected, this is
> something that software could do just as well. Drives only remap bad
> sectors when they are written over, as a read failure doesn't
> necessarily mean that the sector is entirely unreadable, but could be
> due to environmental factors such as high temperature, vibration, etc.
>
> Just rewriting the sector seems a bit questionable though, as if a drive
> in your array is growing read errors that's not really a good thing..

md has done this for a while now though. If it encounters a read error in the array it will make an
attempt to write the reconstructed data back to that disk attempting to force a reallocation. I've
seen it work quite well here on disks that have the occasional grown defect.

It's certainly _much_ nicer than having the disk booted from the array on a single read error.

If the disk is haemorrhaging sectors then you will find out about it sooner or later through other
means.

Brad
--
Dolphins are so intelligent that within a few weeks they can
train Americans to stand at the edge of the pool and throw them
fish.


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