Re: Why does the md/raid subsystem does not remap bad sectors in a raid array?

[Brad Campbell <brad@wasp.net.au>]

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
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