linux-ext4 - Re: [PATCH 1/6] fs: add hole punching to fallocate

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Date:	Tue, 11 Jan 2011 16:30:07 -0500
From:	Ted Ts'o <tytso@....edu>
To:	Lawrence Greenfield <leg@...gle.com>
Cc:	Dave Chinner <david@...morbit.com>, Josef Bacik <josef@...hat.com>,
	linux-kernel@...r.kernel.org, linux-btrfs@...r.kernel.org,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	xfs@....sgi.com, joel.becker@...cle.com, cmm@...ibm.com,
	cluster-devel@...hat.com
Subject: Re: [PATCH 1/6] fs: add hole punching to fallocate

On Tue, Jan 11, 2011 at 04:13:42PM -0500, Lawrence Greenfield wrote:
> > IOWs, all they want to do is avoid the unwritten extent conversion
> > overhead. Time has shown that a bad security/performance tradeoff
> > decision was made 13 years ago in XFS, so I see little reason to
> > repeat it for ext4 today....

I suspect things may have changed somewhat; both in terms of
requirements and nature of cluter file systems, and the performance of
various storage systems (including PCIe-attached flash devices).  

> I'd make use of FALLOC_FL_EXPOSE_OLD_DATA. It's not the CPU overhead
> of extent conversion. It's that extent conversion causes more metadata
> operations than what you'd have otherwise, which means systems that
> want to use O_DIRECT and make sure the data doesn't go away either
> have to write O_DIRECT|O_DSYNC or need to call fdatasync().
> 
> cluster file system implementor,

One possibility might be to make it an optional feature which is only
enabled via a mount option.  That way someone would have to explicit
ask for this feature two ways (via a new flag to fallocate) and a
mount option.

It might not make sense for XFS, but for people who are using ext4 as
the local storage file system back-end, and are doing all sorts of
things to get the best performance, including disabling the journal, I
suspect it really would make sense.  So it could always be an
optional-to-implement flag, that not all file systems should feel
obliged to implement.

					- Ted
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