Re: Write and verify correct data to read-failed sectors before degrading array?

On Thursday September 16, linux@thirdmartini.com wrote:

    I have some experimental code that does the read-recovery piece for 
raid1 devices against kernel 2.4.26.  If an error is encountered on a 
read, the failure is delayed until the read is retried to the other 
mirror.  If the retried read succeeds it then writes the recovered block 
back over the previously failed block. 
    If the write fails then the drive is marked faulty otherwise we 
continue without setting the drive faulty.  ( The idea here is that 
modern disk drives have spare sectors, and will be automatically 
reallocate a bad sector to one of the spares on the next write ). 
    The caveat is that if the drive is generating lots of bad/failed 
reads it's most likely going south.. but that's what smart log 
monitoring is for.  If anyone is interested I can post the patch.

Certainly interested.

Do you have any interlocking to ensure that if a real WRITE is
submitted immediately after (or even during !!!) the READ, it does not
get destroyed by the over-write.
e.g.

application     drive0          drive1
READ request
                READ from drive 0
		fails
				READ from drive 1
				success. Schedule over-write on drive0
READ completes
WRITE block
		WRITE to drive0 WRITE to drive1

                overwrite happens.


It is conceivable that the WRITE could be sent even *before* the READ
completes though I'm not sure if it is possible in practice.

NeilBrown