From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Maxim V. Patlasov" <mpatlasov@parallels.com>
Subject: Re: [PATCH v2 00/14] fuse: An attempt to implement a write-back cache
 policy
Date: Tue, 27 Nov 2012 11:56:51 +0400
Message-ID: <50B47243.9080706@parallels.com>
References: <20121116170123.3196.93431.stgit@maximpc.sw.ru> <CALmpHyb5wpn-dABuEwpypUggRY=1aULvJW0z17HpH50pr1=HKg@mail.gmail.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=UTF-8;
	format=flowed
Content-Transfer-Encoding: QUOTED-PRINTABLE
Cc: <miklos@szeredi.hu>, <dev@parallels.com>,
	"fuse-devel@lists.sourceforge.net" <fuse-devel@lists.sourceforge.net>,
	<linux-kernel@vger.kernel.org>, <jbottomley@parallels.com>,
	<viro@zeniv.linux.org.uk>, <linux-fsdevel@vger.kernel.org>,
	<xemul@openvz.org>
To: Feng Shuo <steve.shuo.feng@gmail.com>
Return-path: <linux-kernel-owner@vger.kernel.org>
In-Reply-To: <CALmpHyb5wpn-dABuEwpypUggRY=1aULvJW0z17HpH50pr1=HKg@mail.gmail.com>
Sender: linux-kernel-owner@vger.kernel.org
List-Id: linux-fsdevel.vger.kernel.org

Hi Feng,

11/27/2012 05:04 AM, Feng Shuo =D0=BF=D0=B8=D1=88=D0=B5=D1=82:
> 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 (yo=
u=20
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 ba=
sed
> on an assumption that the userspace is just a storage for data and sh=
ould
> catch up with the kernel i_size value. In order to make it possible f=
or user
> space to update i_size in kernel we'd have to implement some (probabl=
y)
> tricky algorithm, I haven't yet thought about it.

The patch-set follows the model "trust kernel i_size only". This works=20
fine at least in case of userspace fuse wtih exclusive write semantics.=
=20
In case of mutual concurrent internal/external read/write access, sysad=
=20
should not turn the feature on (it's turned off by default). I wouldn't=
=20
like to complicate the patch-set further adding bits for that case. Thi=
s=20
area is opened for future enhancements :)

Thanks,
Maxim