linux-kernel - Re: [RFC] new ->perform

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Message-ID: <20100517233540.GM3364@quack.suse.cz>
Date:	Tue, 18 May 2010 01:35:41 +0200
From:	Jan Kara <jack@...e.cz>
To:	Dave Chinner <david@...morbit.com>
Cc:	Josef Bacik <josef@...hat.com>, linux-fsdevel@...r.kernel.org,
	chris.mason@...cle.com, hch@...radead.org,
	akpm@...ux-foundation.org, npiggin@...e.de,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC] new ->perform_write fop

On Fri 14-05-10 16:41:45, Dave Chinner wrote:
...
> Well, the "RESERVE" callout I proposed should mean that the
> allocation that follows the copy should never fail with ENOSPC. i.e.
> if there isn't space, the RESERVE call will fail with ENOSPC, not
> the actual ALLOCATE call that occurs after the data is copied.
> 
> Basically I was thinking that in the XFS case, RESERVE would scan the
> range for holes, and reserve all blocks needed to fill all the holes
> in the range. Then the ALLOCATE call would mark them all delalloc.
> The space reservation and delalloc is done in a single call right
> now, but splitting them is not hard...
  But this would mean that you would have to implement at least a basic
delayed allocation support for filesystems like ext2, ext3, udf, vfat, etc.
They currently do not have a way to provide a functionality you require
from RESERVE call...
  Not that it would be so hard to do - I actually have some patches for
ext2/3 doing that because of problems with ENOSPC during mmaped writes but
it's not completely trivial either.

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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