Re: [Qemu-devel] [PATCH 0/2] block: Write out internal caches even with cache=unsafe

[Paolo Bonzini]

On 10/22/2011 05:07 PM, Alexander Graf wrote:
> On 21.10.2011, at 11:44, Paolo Bonzini wrote:
>
> > On 10/21/2011 07:08 PM, Kevin Wolf wrote:
> > > Avi complained that not even writing out qcow2's cache on
> > > bdrv_flush() made cache=unsafe too unsafe to be useful. He's got
> > > a point.
> >
> > Why? cache=unsafe is explicitly allowing to s/data/manure/ on
> > crash.
>
> Exactly, but not on kill. By not flushing internal caches you're
> almost guaranteed to get an inconsistent qcow2 image.

This should be covered already by termsig_handler.  bdrv_close_all 
closes all block devices, and qcow2_close does flush the caches.

SIGKILL doesn't give any guarantee of course but it does not in general, 
even without cache=unsafe; you might get a SIGKILL "a moment before" a 
bdrv_flush even without cache=unsafe, and get unclean qcow2 metadata.

> By not flushing internal caches you're almost guaranteed to get an
> inconsistent qcow2 image.

Of course the inconsistencies with cache=unsafe will be massive if you 
don't have a clean exit, but that's expected.  If in some cases you want 
a clean exit, but right now you don't, the place to fix those cases 
doesn't seem to be the block layer, but the main loop.

Also,

1) why should cache=unsafe differentiate an OS that sends a flush from 
one that doesn't (e.g. MS-DOS), from the point of view of image metadata?

2) why should the guest actually send a flush if cache=unsafe?  Currently

    if (flags & BDRV_O_CACHE_WB)
        bs->enable_write_cache = 1;

covers cache=unsafe.  However, in the end write cache enable means "do I 
need to flush data", and the answer is "no" when cache=unsafe, because 
the flushes are useless and guests are free to reorder requests.

<shot-in-the-dark>Perhaps what you want is to make qcow2 caches 
writethrough in cache=unsafe mode, so that at least a try is made to 
write the metadata</shot-in-the-dark> (even though the underlying raw 
protocol won't flush it)?  I'm not sure that is particularly useful, but 
maybe it can help me understanding the benefit of this change.

Paolo