[dm-devel] RE: [PATCH] dm mpath: Try recover from I/O failure by re-initializing the PG if device is running on one path

[Moger, Babu]

> > Problem: Device mapper fails the path for every I/O error.
> > It does not care about the type of error.
> 
> This is the fundamental problem.  Different layers of the block IO
> path have to agree on how to handle each possible type of error that
> can be returned. I don't know where to find such an agreement and
> think an implementation that does discriminate is needed.

> > There are certain errors which can be recovered by re-initializing the
> path again. I have seen this problem during my testing on rdac device
> handler. I have observed I/O errors when there is a change in Lun
> ownership. When Lun ownership changes device will return back with check
> condition with sense 0x05/0x94/0x01(SK/ASC/ASCQ -meaning Lun ownership
> changed). Currently, device mapper fails the path for this error and
> eventually this will lead to I/O error. We don't want to see I/O error for
> this reason.
> 
> 1) This patch isn't discriminating between transport, media, or other
> device errors. Wouldn't it make sense to discriminate?
> "LUN ownership changed" sounds like some of the events possible in
> multi-inititiator enviroment would want to be notified about and
> perhaps even take some action (renegotiate access to

We will not be able to discriminate the error because error specific information is not available to DM.

> 2) Will this result in resetting a SATA device?
> I ask because device reset may result in data loss due to WCE enabled.
> I just don't know the higher parts of the block SW stack and how
> errors flow up the stack.

I am not sure about this. Don’t know if re-activating (or calling activate_path) the device causes reset.