Re: [Qemu-devel] [PATCHv2] block/nfs: cache allocated filesize for read-only files

[Jeff Cody <jcody@redhat.com>]

On Mon, Aug 24, 2015 at 10:13:16PM +0200, Max Reitz wrote:
> On 24.08.2015 21:34, Peter Lieven wrote:
> > Am 24.08.2015 um 20:39 schrieb Max Reitz:
> >> On 24.08.2015 10:06, Peter Lieven wrote:
> >>> If the file is readonly its not expected to grow so
> >>> save the blocking call to nfs_fstat_async and use
> >>> the value saved at connection time. Also important
> >>> the monitor (and thus the main loop) will not hang
> >>> if block device info is queried and the NFS share
> >>> is unresponsive.
> >>>
> >>> Signed-off-by: Peter Lieven <pl@kamp.de>
> >>> ---
> >>> v1->v2: update cache on reopen_prepare [Max]
> >>>
> >>>  block/nfs.c | 35 +++++++++++++++++++++++++++++++++++
> >>>  1 file changed, 35 insertions(+)
> >> Reviewed-by: Max Reitz <mreitz@redhat.com>
> >>
> >> I hope you're ready for the "Stale actual-size value with
> >> cache=direct,read-only=on,format=raw files on NFS" reports. :-)
> > actually a good point, maybe the cache should only be used if
> > 
> > !(bs->open_flags & BDRV_O_NOCACHE)
> 
> Good enough a point to fix it? ;-)
> 
> Max
> 

It seems more inline with expected behavior, to add the cache checking
in before using the size cache.  Would you be opposed to a v3 with
this check added in?

One other concern I have is similar to a concern Max raised earlier -
about an external program modifying the raw image, while QEMU has it
opened r/o.  In particular, I wonder about an NFS server making an
image either sparse / non-sparse.  If it was exported read-only, it
may be a valid assumption that this could be done safely, as it would
not change the reported file size or contents, just the allocated size
on disk.