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Re: [Qemu-devel] [PATCHv2] block/nfs: cache allocated filesize for read-
From: |
Peter Lieven |
Subject: |
Re: [Qemu-devel] [PATCHv2] block/nfs: cache allocated filesize for read-only files |
Date: |
Wed, 26 Aug 2015 20:49:06 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.4.0 |
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 :-)
Peter