Re: [RFC] new ->perform_write fop

From: Jan Kara
Date: Mon May 17 2010 - 19:35:54 EST

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.

Jan Kara <jack@xxxxxxx>
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