linux-ext4 - Re: [PATCH 6/6] ext4/242: Add ext4 specific test for fallocate zero range

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Date:	Tue, 25 Feb 2014 22:27:40 +0100 (CET)
From:	Lukáš Czerner <lczerner@...hat.com>
To:	Dave Chinner <david@...morbit.com>
cc:	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	xfs@....sgi.com
Subject: Re: [PATCH 6/6] ext4/242: Add ext4 specific test for fallocate zero
 range

On Tue, 25 Feb 2014, Lukáš Czerner wrote:

> Date: Tue, 25 Feb 2014 22:01:06 +0100 (CET)
> From: Lukáš Czerner <lczerner@...hat.com>
> To: Dave Chinner <david@...morbit.com>
> Cc: linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org, xfs@....sgi.com
> Subject: Re: [PATCH 6/6] ext4/242: Add ext4 specific test for fallocate zero
>     range
> 
> On Wed, 26 Feb 2014, Dave Chinner wrote:
> 
> > Date: Wed, 26 Feb 2014 07:53:49 +1100
> > From: Dave Chinner <david@...morbit.com>
> > To: Lukas Czerner <lczerner@...hat.com>
> > Cc: linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org, xfs@....sgi.com
> > Subject: Re: [PATCH 6/6] ext4/242: Add ext4 specific test for fallocate zero
> >     range
> > 
> > On Tue, Feb 25, 2014 at 08:15:28PM +0100, Lukas Czerner wrote:
> > > This is copy of xfs/242. However it's better to make it file system
> > > specific because the range can be zeroes either directly by writing
> > > zeroes, or converting to unwritten extent, so the actual result might
> > > differ from file system to file system.
> > 
> > You could say the same thing about preallocation using unwritten
> > extents. Yet, funnily enough, we have generic tests for them because
> > all filesystems currently use unwritten extents for preallocation
> > and behave identically....
> > 
> > This test is no different - all filesystems currently use unwritten
> > extents, and so this test should be generic because all existing
> > filesystems *should* behave the same.
> > 
> > When we get a filesystem that zeros rather uses unwritten extents,
> > we can add a new *generic* test that tests for zeroed data extents
> > rather than unwritten extents. All that we will need is a method of
> > checking what behaviour the filesystem has and adding that to a
> > _requires directive to ensure the correct generic fallocate tests
> > are run...
> 
> Currently xfs/242 fails on xfs for me and it does behave differently
> than ext4. Also I had to change to 242.out a bit because ext4 was
> a little different. It seems to me that it was expected that when
> the extent is small enough it would be overwritten by zeroes rather
> than converted to unwritten, but I have not looked into
> implementation.
> 
> Btw this kind of optimization is actually something I've been
> thinking of as well for ext4. Rather than going though the hassle of
> changing extents around it might be worth in some situation to zero
> out. But that's an optimization I have not implemented yet.

Oops, I am taking it back. It's just too late and apparently I've
overlooked something.

-Lukas

> 
> -Lukas
> 
> 
> > 
> > Cheers,
> > 
> > Dave.
> > 
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