skip_copy issue


themaxxz

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UnRaid is different than RAID. UnRaid is not involved in the writing directly to the individual disks (although it must pass writes through from the md# device to the sdX device). It monitors those writes, computes parity, and writes parity.  No data is striped across drives. It is a very difference paradigm from Raid. If parity were being corrupted, our parity checks would be showing that corruption. That's the only part that unRaid manages.

 

So unless the issue is related to a problem with the handing off of writes, I doubt very much this affects unRaid. @limetech might be interested to know about this, and may be able to comment further..

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First off, this does not affect unRAID becase the md/unraid driver diverged quite some time ago from 'stock' linux md driver.  Interestingly what 'skip_copy' optimization does in stock md is something I've wanted to put into md/unraid for quite a while - I guess I need to re-think that idea O.o

 

'skip_copy' is an optimization in stock linux md driver.  I believe it was introduced here:

https://www.spinics.net/lists/raid/msg46364.html

 

The specific issue seen by QNAP and Synology (and all linux md based systems I suppose) is talked about here:

https://www.spinics.net/lists/raid/msg59084.html

 

This looks like a hairy bug to fix properly, though workaround is to disable skip_copy feature.  (Again, not talking about unRAID here.)

 

Edit: to be clear: I don't mean to imply anything about linux md integrity issue.  The 'bug' was not introduced merely by adding the 'skip_copy' feature, I think it probably got introduced sometime after by other changes in the block layer in later kernels.

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