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

[Peter Lieven <pl@kamp.de>]

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 <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?

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 :-)

Peter