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Re: [PATCH] xfstests 255: add a seek_data/seek_hole tester

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH] xfstests 255: add a seek_data/seek_hole tester
From: Sunil Mushran <sunil.mushran@xxxxxxxxxx>
Date: Tue, 30 Aug 2011 18:17:02 -0700
Cc: Andreas Dilger <adilger@xxxxxxxxx>, Christoph Hellwig <hch@xxxxxxxxxxxxx>, Josef Bacik <josef@xxxxxxxxxx>, linux-fsdevel@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, linux-btrfs@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx, viro@xxxxxxxxxxxxxxxxxx, dchinner@xxxxxxxxxx
In-reply-to: <20110826013528.GW3162@dastard>
References: <1309275199-10801-1-git-send-email-josef@xxxxxxxxxx> <1309275199-10801-5-git-send-email-josef@xxxxxxxxxx> <20110825060632.GA9933@xxxxxxxxxxxxx> <20110825064039.GO3162@dastard> <0A267E55-7772-438D-B6A7-89B73020F311@xxxxxxxxx> <20110826013528.GW3162@dastard>
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On 08/25/2011 06:35 PM, Dave Chinner wrote:
Agreed, that's the way I'd interpret it, too. So perhaps we need to
ensure that this interpretation is actually tested by this test?

How about some definitions to work by:

Data: a range of the file that contains valid data, regardless of
whether it exists in memory or on disk. The valid data can be
preceeded and/or followed by an arbitrary number of zero bytes
dependent on the underlying implementation of hole detection.

Hole: a range of the file that contains no data or is made up
entirely of  NULL (zero) data. Holes include preallocated ranges of
files that have not had actual data written to them.

Does that make sense? It has sufficient flexibility in it for the
existing generic "non-implementation", allows for filesystems to
define their own hole detection boundaries (e.g. filesystem block
size), and effectively defines how preallocated ranges from
fallocate() should be treated (i.e. as holes). If we can agree on
those definitions, I think that we should document them in both the
kernel and the man page that defines SEEK_HOLE/SEEK_DATA so everyone
is on the same page...

We should not tie in the definition to existing fs technologies. Instead
we should let the fs weigh the cost of providing accurate information
with the possible gain in performance.

Data:
A range in a file that could contain something other than nulls.
If in doubt, it is data.

Hole:
A range in a file that only contains nulls.

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