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

[Peter Lieven <pl@kamp.de>]

Am 21.08.2015 um 18:46 schrieb Max Reitz:
> On 2015-08-21 at 00:49, 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>
>> ---
>>   block/nfs.c | 6 ++++++
>>   1 file changed, 6 insertions(+)
>
> First, I don't like the idea of this patch very much, but since I've never used qemu's native NFS client, it's not up to me to decide whether it's worth it.

I am trying to solve that a stale NFS Server with a CDROM ISO on it can hang Qemus main loop. One of the things that happens is that
you query "info block" in hmp or "query-block" via QMP and indirectly call bdrv_get_allocated_file_size and bang, Qemu hangs. Also I don't
know if its worth to issue an RPC call for each executing of info block.


>
> When it comes to breaking this, what comes to mind first is some external program opening the image read-write outside of qemu and writing to it. Maybe that's a case we generally don't want, but maybe that's something some people do on purpose, knowing what they're doing (with raw images), you never know.

I would consider this bad behaviour. However, allocated file size shouldn't matter for raw images. If you resize the image from external you have to call bdrv_truncate anyway to make Qemu aware
of that change.

>
> Other than that, there's reopening. As far as I'm aware, qemu can reopen a R/W image read-only, and if that happens, st_blocks may be stale.

Thats a valid point. But it can be solved be implementing .bdrv_reopen_prepare and update st_blocks there.

Thanks for you thoughts,
Peter