Effect in guest of a hard read error from the host's storage ?

[Simon Hobson <simon@xxxxxxxxxxxxxxxx>]

Not having experienced it on any of my Xen systems I’ve managed in the past, I 
just wanted to check what happens when the host’s storage encounters an 
unrecoverable read error ?
My expectation would be that the error would be passed up the chain (host’s OS, 
virtual disk emulator, guest OS disk driver, guest OS filesystem, guest OS 
program), appear as an unrecoverable error to the guest, and hence passed up to 
the program attempting to read the file - so really nothing different to the 
same error on bare metal.

Has anyone encountered this, and can confirm if that’s how Xen+Linux handles it 
?

The reason for asking is that I also use Parallels on a Mac and have this 
situation. Cloning the host’s drive (Carbon Copy Cloner) “does the right thing” 
- CCC copies the files it can and reports the ones it can’t. A couple of the 
ones it can’t are virtual disk files for Parallels guests.
When I try to recover what I can (and any unreadable files might not be 
important anyway but I don’t know which they are) using the same technique in 
the guest, Parallels doesn’t behave like that. It pops up a dialog to say a 
critical error has occurred, with two options - retry (which of course doesn’t 
do anything useful), or to stop the guest. So the guest OS never gets to see 
the error, it just gets killed. Thus meaningful recovery is impossible as the 
guest gets stopped without being able to tell me what file(s) is affected.
I’ve been “discussing” this with Parallels support and at the moment I’ve 
reached the point where they’re telling me that this is normal when things are 
virtualised - which is setting off the BS detector for me.

Simon