linux-kernel - Re: [PATCH 1/2] fs: add SEEK_HOLE and SEEK

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Message-ID: <20110425141540.GB20461@shareable.org>
Date:	Mon, 25 Apr 2011 15:15:40 +0100
From:	Jamie Lokier <jamie@...reable.org>
To:	Eric Blake <eblake@...hat.com>
Cc:	Sunil Mushran <sunil.mushran@...cle.com>,
	Markus Trippelsdorf <markus@...ppelsdorf.de>,
	Christoph Hellwig <hch@...radead.org>,
	Josef Bacik <josef@...icpanda.com>,
	linux-kernel@...r.kernel.org, linux-btrfs@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 1/2] fs: add SEEK_HOLE and SEEK_DATA flags

Eric Blake wrote:
> On 04/24/2011 11:49 AM, Jamie Lokier wrote:
> >> My problem with FIND_* is that we are messing with the well understood
> >> semantics of lseek().
> > 
> > fcntl() looks a better fit for FIND_HOLE/DATA anyway.
> 
> With fcntl(), it would have to be something like:
> 
> off_t offset = start;
> if (fcntl (fd, F_FIND_HOLE, &offset) != 0)
>     ; // find failed
> // offset is now set to the next location after start

Yes that, or a pointer-to-struct in the style of other fcntl()
operations.

A struct offers more flexibiliy such as a limit on search distance
(may be useful on filesystems where searching reads a lot of metadata
and you don't really want to scan all of a large file), and whether to
include unwritten prealloc space or written-known-zeros space.

I thought of fcntl() because historically it's often how you get quirky
information about files and how to read them, on many OSes.

-- Jamie
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