Re: Application error handling with write-back caching

[Paolo Bonzini <pbonzini@redhat.com>]

On 10/05/2016 19:31, James Bottomley wrote:
> > 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.

Thanks, fatal errors are fine I guess.  We were worried that the next
SYNCHRONIZE CACHE would succeed and throw away the writes because it has
already "performed a write medium operation".

POSIX fsync is pretty underspecified in this respect too; gluster has
been throwing away those writes for a long time!  It stopped now because
we explained the issue to them, but it's pointless if the next layer
below does the same---hence Stefan's question.

(In our case the next layer is not the page cache, because we generally
use O_DIRECT.  Evicting dirty pages from the page cache would be okay if
the process(es) that wrote them are SIGKILLed, but in general it would
be a problem too).

Thanks,

Paolo