linux-kernel - Re: [PATCH] xfstests 255: add a seek_data/seek

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Message-ID: <4E58AB28.1080009@gmail.com>
Date:	Sat, 27 Aug 2011 10:30:32 +0200
From:	Marco Stornelli <marco.stornelli@...il.com>
To:	Zach Brown <zab@...bo.net>
CC:	Dave Chinner <david@...morbit.com>,
	Andreas Dilger <adilger@...ger.ca>,
	Christoph Hellwig <hch@...radead.org>,
	Josef Bacik <josef@...hat.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-btrfs@...r.kernel.org,
	xfs@....sgi.com, viro@...iv.linux.org.uk, dchinner@...hat.com
Subject: Re: [PATCH] xfstests 255: add a seek_data/seek_hole tester

Il 26/08/2011 16:41, Zach Brown ha scritto:
>>> 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.
>
>> No for me. A hole is made up of zero data? It's a strange definition
>> for me.
>
> It's a very natural definition for me.  It mirrors the behaviour of
> read() of sparse data inside i_size that file system authors already
> have to consider.
>
> It's also a reminder for people that this interface is about avoiding
> reading zeros.  Systems that track contents can do this for files that
> had tons of zeros written.  The data is there but the app is
> specifically asking us to skip it by using SEEK_DATA.
>
> - z
>

I think we need to consider a hole and "data not present/not written 
yet" as two different cases even they are related. For example, if I do 
an fallocate without keep size option, then I do a read, I have the same 
behavior of sparse data inside i_size, but the blocks are allocated so 
no sparse data in this case. Simply there are no difference from app 
point of view.

Marco
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