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

[Jeff Cody]

On Wed, Aug 26, 2015 at 08:49:06PM +0200, Peter Lieven wrote:
> Am 26.08.2015 um 17:31 schrieb Jeff Cody:
> > 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 <address@hidden>
> >>>>> ---
> >>>>> v1->v2: update cache on reopen_prepare [Max]
> >>>>>
> >>>>>  block/nfs.c | 35 +++++++++++++++++++++++++++++++++++
> >>>>>  1 file changed, 35 insertions(+)
> >>>> Reviewed-by: Max Reitz <address@hidden>
> >>>>
> >>>> 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?
> 
> Of course, will send it tomorrow.
> 
> >
> > 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.
> 
> This might be a use case. But if I allow caching the allocated filesize
> might not always be correct. This is even the case on a NFS share mounted
> through the kernel where some attributes a cached for some time.
> 
> Anyway, would it hurt here if the actual filesize was too small?
> In fact it was incorrect since libnfs support was added :-)
> 

Yeah, I'm not sure what harm it would cause in practice.  It is a
fairly edge use case to begin with, and a relatively benign side
affect (especially since you added reopen() support).

With the cache flag checking, I am comfortable adding my r-b.

Jeff