linux-ext4 - Re: Testing ext4 persistent preallocation patches for 64 bit features

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Date:	Wed, 7 Feb 2007 14:11:17 -0700
From:	Andreas Dilger <adilger@...sterfs.com>
To:	Suparna Bhattacharya <suparna@...ibm.com>
Cc:	Mingming Cao <cmm@...ibm.com>,
	"Amit K. Arora" <aarora@...ux.vnet.ibm.com>,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	alex@...sterfs.com, suzuki@...ibm.com
Subject: Re: Testing ext4 persistent preallocation patches for 64 bit features

On Feb 07, 2007  16:06 +0530, Suparna Bhattacharya wrote:
> On Wed, Feb 07, 2007 at 12:25:50AM -0800, Mingming Cao wrote:
> > - disable preallocation if the filesystem free blocks is under some low
> > watermarks, to save space for near future real block allocation?
> 
> A policy decision like this is probably worth a discussion during today's call.
> 
> > - is de-preallocation something worth doing?

As discussed in the call - I don't think we can remove preallocations.
The whole point of database preallocation is to guarantee that this space
is available in the filesystem when writing into a file at random offsets
(which would otherwise be sparse).

Similarly, persistent preallocation shouldn't be considered differently
than an efficient way of doing zero filling of blocks.  At least that is
my understanding...  Is this code implementing the "uninitialized extents"
for databases (via explicit preallocation via fallocate/ioctl) so that
they don't have to zero-fill large files, or is there also automatic
preallocation of space to files (e.g. for O_APPEND files)?

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

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