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

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Message-ID: <4E5A15C0.3020205@gmail.com>
Date:	Sun, 28 Aug 2011 12:17:36 +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 27/08/2011 10:30, Marco Stornelli ha scritto:
> 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

Please don't care about the last part, when reading in this case the app 
will have a return value different from zero obviously, I was under the 
effect of a beer :) However I'd add to the definition, that we consider 
holes only inside i_size, as Zack said. In this way, we haven't got any 
ambiguity for preallocated space beyond eof.

Marco
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