linux-ext4 - Re: [PATCH 4/7][TAKE5] support new modes in fallocate

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Date:	Tue, 26 Jun 2007 11:42:50 -0400
From:	Andreas Dilger <adilger@...sterfs.com>
To:	"Amit K. Arora" <aarora@...ux.vnet.ibm.com>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-ext4@...r.kernel.org, David Chinner <dgc@....com>,
	suparna@...ibm.com, cmm@...ibm.com, xfs@....sgi.com
Subject: Re: [PATCH 4/7][TAKE5] support new modes in fallocate

On Jun 26, 2007  16:15 +0530, Amit K. Arora wrote:
> On Mon, Jun 25, 2007 at 03:52:39PM -0600, Andreas Dilger wrote:
> > In XFS one of the (many) ALLOC modes is to zero existing data on allocate.
> > For ext4 all this would mean is calling ext4_ext_mark_uninitialized() on
> > each extent.  For some workloads this would be much faster than truncate
> > and reallocate of all the blocks in a file.
> 
> In ext4, we already mark each extent having preallocated blocks as
> uninitialized. This is done as part of following code (which is part of
> patch 5/7) in ext4_ext_get_blocks() :  

What I meant is that with XFS_IOC_ALLOCSP the previously-written data
is ZEROED OUT, unlike with fallocate() which leaves previously-written
data alone and only allocates in holes.

So, if you had a sparse file with some data in it:

     AAAAA         BBBBBB

fallocate() would allocate the holes:

00000AAAAA000000000BBBBBB00000000

XFS_IOC_ALLOCSP would overwrite everything:

000000000000000000000000000000000

In order to specify this for allocation, FA_FL_DEL_DATA would need to make
sense for allocations (as well as the deallocation).  This is farily easy
to do - just mark all of the existing extents as unallocated, and their
data disappears.

Cheers, Andreas
--
Andreas Dilger
Principal Software Engineer
Cluster File Systems, Inc.

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