Date: Wed, 29 Aug 2007 00:55:30 +1000
From: David Chinner <dgc@sgi.com>
To: Fengguang Wu <wfg@mail.ustc.edu.cn>
Cc: David Chinner <dgc@sgi.com>, Chris Mason <chris.mason@oracle.com>,
       Andrew Morton <akpm@osdl.org>, Ken Chen <kenchen@google.com>,
       linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
       Jens Axboe <jens.axboe@oracle.com>
Subject: Re: [PATCH 0/6] writeback time order/delay fixes take 3
Message-ID: <20070828145530.GD61154114@sgi.com>
References: <386910467.21100@ustc.edu.cn> <20070821202314.335e86ec@think.oraclecorp.com> <20070822011841.GA8090@mail.ustc.edu.cn> <20070823023306.GM61154114@sgi.com> <20070824135504.GA9029@mail.ustc.edu.cn>
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On Fri, Aug 24, 2007 at 09:55:04PM +0800, Fengguang Wu wrote:
> On Thu, Aug 23, 2007 at 12:33:06PM +1000, David Chinner wrote:
> > On Wed, Aug 22, 2007 at 09:18:41AM +0800, Fengguang Wu wrote:
> > > On Tue, Aug 21, 2007 at 08:23:14PM -0400, Chris Mason wrote:
> > > Notes:
> > > (1) I'm not sure inode number is correlated to disk location in
> > >     filesystems other than ext2/3/4. Or parent dir?
> > 
> > The correspond to the exact location on disk on XFS. But, XFS has it's
> > own inode clustering (see xfs_iflush) and it can't be moved up
> > into the generic layers because of locking and integration into
> > the transaction subsystem.
> >
> > > (2) It duplicates some function of elevators. Why is it necessary?
> > 
> > The elevators have no clue as to how the filesystem might treat adjacent
> > inodes. In XFS, inode clustering is a fundamental feature of the inode
> > reading and writing and that is something no elevator can hope to
> > acheive....
>  
> Thank you. That explains the linear write curve(perfect!) in Chris' graph.
> 
> I wonder if XFS can benefit any more from the general writeback clustering.
> How large would be a typical XFS cluster?

Depends on inode size. typically they are 8k in size, so anything from 4-32
inodes. The inode writeback clustering is pretty tightly integrated into the
transaction subsystem and has some intricate locking, so it's not likely
to be easy (or perhaps even possible) to make it more generic.

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group
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