Re: Application error handling with write-back caching

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Tue, 2016-05-10 at 18:42 +0200, Paolo Bonzini wrote:
> 
> On 10/05/2016 16:16, James Bottomley wrote:
> > > If "is performed" just means "completes", maybe with an error, 
> > > the application would have to resubmit write requests and then 
> > > try to flush the write cache again.
> > > 
> > > I'm not aware of applications that keep acknowledged write data 
> > > around until the cache flush completion in order to retry writes.
> > 
> > I think you may be misunderstanding the nature of the returned 
> > error. It will be permanent and fatal and usually signal that the 
> > device has a failed sector that can't be remapped and so the device 
> > itself has for most purposes failed.  The only recovery is if you 
> > happen to have RAID, in which case the RAID layer will mostly take
> > care of it.
> 
> What about a SPACE ALLOCATION FAILED error or a similar error that 
> can be fixed by administrator actions (or just by a concurrent 
> process doing an UNMAP)?  Would a subsequent cache flush cause data
> loss?

You're now asking about how these are handled?  It's not a SCSI
problem.  I believe if you look at the various layers, RAID would still
treat it as fatal and fail the drive and so would most filesystems. 
 The AEN warnings for TP are reported, but the admin has to sort it out
before they become a fatal error.

james