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

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Message-ID: <50B47243.9080706@parallels.com>
Date:	Tue, 27 Nov 2012 11:56:51 +0400
From:	"Maxim V. Patlasov" <mpatlasov@...allels.com>
To:	Feng Shuo <steve.shuo.feng@...il.com>
CC:	<miklos@...redi.hu>, <dev@...allels.com>,
	"fuse-devel@...ts.sourceforge.net" <fuse-devel@...ts.sourceforge.net>,
	<linux-kernel@...r.kernel.org>, <jbottomley@...allels.com>,
	<viro@...iv.linux.org.uk>, <linux-fsdevel@...r.kernel.org>,
	<xemul@...nvz.org>
Subject: Re: [PATCH v2 00/14] fuse: An attempt to implement a write-back cache
 policy

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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