Re: [Xen-devel] Fatal crash on xen4.2 HVM + qemu-xen dm + NFS

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Tue, 22 Jan 2013, Stefano Stabellini wrote:
> > But this would explain why I'm still seeing the crash with O_DIRECT
> > apparently off (cache=writeback), as the cache setting is being ignored.
> > 
> > This would also explain why Ian might not have seen it (it went in
> > late and without O_DIRECT we think this crash can't happen).
> > 
> > Is the BDRV_O_NOCACHE | BDRV_O_CACHE_WB combination intentional or
> > should BDRV_O_NOCACHE be removed? Why would the default be different
> > for emulated and PV disks?
> 
> The setting is different from the one of emulated devices because after
> analyzing the IDE code, we thought that using BDRV_O_CACHE_WB would be
> safe enough because when the guest wants to make sure that the data hits
> the disk, it issues an IDE FLUSH_CACHE operation.
> 
> In the xen_disk case instead, we weren't quite sure about the
> assumptions of all the possible different PV frontend drivers, so we
> went for the safe choice, that is O_DIRECT.
> 
> In fact if we wanted to change the cache setting for xen_disk, we would
> probably have to go back to write-through (this setting is selected by
> passing neither BDRV_O_NOCACHE nor BDRV_O_CACHE_WB) that is quite slow.
> 
> Recently, thanks to Konrad's work on blkfront cache flushes, a new flush
> operation has been implemented in the block protocol:
> BLKIF_OP_FLUSH_DISKCACHE. BLKIF_OP_FLUSH_DISKCACHE was introduced in
> xen_disk by 7e7b7cba16faa7b721b822fa9ed8bebafa35700f "xen_disk:
> implement BLKIF_OP_FLUSH_DISKCACHE, remove BLKIF_OP_WRITE_BARRIER".
> Thanks to the new operation, maybe it is now safe to use write-back
> caching.
> Konrad, what do you think? Is blkback using the Linux disk cache by
> default? Or is it using O_DIRECT?

Looking more closely at xen_disk's flush operations,
even if the semantics of the old BLKIF_OP_WRITE_BARRIER is confused, the
implementation of it in xen_disk is strickly a superset of the flushes
performed by the new BLKIF_OP_FLUSH_DISKCACHE.

So unless blkfront issues fewer cache flushes when using
BLKIF_OP_WRITE_BARRIER, I am tempted to say that even with the old flush
operation, it might be safe to open the file write-back.

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