Re: [fuse-devel] [PATCH v2 00/14] fuse: An attempt to implement a write-back cache policy

[Feng S. <ste...@gm...>]

Thank you Maxim. This is very much clear and makes sense for me.


On Tue, Nov 27, 2012 at 3:56 PM, Maxim V. Patlasov
<mpa...@pa...>wrote:

> Hi Feng,
>
> 11/27/2012 05:04 AM, Feng Shuo пишет:
>
>  Hi Maxim,
>>
>> I'm new to fuse but have some experience with NFS. From my
>> understanding after reviewing your patchset, it seems only work with
>> local file system or a distributed file system whose file is never
>> modified (could be grown but no or very few modified) because it
>> doesn't exam the pre/post status of the writing object (e.g. a file).
>> So if a file is modified outside, fuse might not get any chance to
>> handle it...... Correct me if I got wrong since I'm really new to
>> fuse. :-)
>>
>
> This topic was discussed when Pavel sent initial version of patches (you
> can find it in fuse-devel archives). Brian asked:
>
>  Would this pose a problem for a filesystem in which the size of the
>> inode can change remotely (i.e., not visible to the local instance of
>> fuse)? I haven't tested this, but it seems like it could be an issue
>> based on the implementation.
>>
>
> And Pavel replied:
>
>  Yes, it will. The model of i_size management I implemented here is based
>> on an assumption that the userspace is just a storage for data and should
>> catch up with the kernel i_size value. In order to make it possible for
>> user
>> space to update i_size in kernel we'd have to implement some (probably)
>> tricky algorithm, I haven't yet thought about it.
>>
>
> The patch-set follows the model "trust kernel i_size only". This works
> fine at least in case of userspace fuse wtih exclusive write semantics. In
> case of mutual concurrent internal/external read/write access, sysad should
> not turn the feature on (it's turned off by default). I wouldn't like to
> complicate the patch-set further adding bits for that case. This area is
> opened for future enhancements :)
>
> Thanks,
> Maxim
>
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