linux-ext4 - Re: [RFC][Patch 1/1] Persistent preallocation in ext4

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Message-ID: <20061213153838.GB7193@in.ibm.com>
Date:	Wed, 13 Dec 2006 21:08:38 +0530
From:	Suparna Bhattacharya <suparna@...ibm.com>
To:	Dave Kleikamp <shaggy@...ux.vnet.ibm.com>
Cc:	"Amit K. Arora" <aarora@...ux.vnet.ibm.com>,
	Mingming Cao <cmm@...IBM.COM>, linux-ext4@...r.kernel.org,
	suzuki@...ibm.com
Subject: Re: [RFC][Patch 1/1] Persistent preallocation in ext4

On Wed, Dec 13, 2006 at 07:36:29AM -0600, Dave Kleikamp wrote:
> On Wed, 2006-12-13 at 15:31 +0530, Amit K. Arora wrote:
> > On Tue, Dec 12, 2006 at 04:20:38PM -0800, Mingming Cao wrote:
> > > On Tue, 2006-12-12 at 11:53 +0530, Amit K. Arora wrote:
> 
> > > Supporting preallocation for extent based files seems fairly
> > > straightforward.  I agree we should look at this first.  After get this
> > > done, it probably worth re-consider whether to support preallocation for
> > > non-extent based files on ext4. I could imagine user upgrade from ext3
> > > to ext4, and expecting to use preallocation on those existing files....
> 
> I disagree here.  Why add the complexity for what is going to be a rare
> case?  In cases where a user is going to benefit from preallocation,
> she'll probably also benefit from extents, and would be better off
> making a copy of the file, thus converting it to extents.
> 
> > I gave a thought on this initially. But, I was not sure how we should
> > implement preallocation in a non-extent based file. Using extents we can
> > mark a set of blocks as unitialized, but how will we do this for
> > non-extent based files ? If we do not have a way to mark blocks
> > uninitialized, when someone will try to read from a preallocated block,
> > it will return junk/stale data instead of zeroes.
> 
> If anything, the block-based preallocation could initialize all of the
> data to zero.  It would be slow, but it would still provide the correct
> function and result in contiguous allocation.

And posix_fallocate does that already ... 

Regards
Suparna

> 
> Shaggy
> --
> David Kleikamp
> IBM Linux Technology Center
> 
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-- 
Suparna Bhattacharya (suparna@...ibm.com)
Linux Technology Center
IBM Software Lab, India

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