Date:	Wed, 7 Feb 2007 13:46:57 -0700
From:	Andreas Dilger <adilger@...sterfs.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC][PATCH 2/3] Move the file data to the new blocks

On Feb 06, 2007  17:35 -0800, Andrew Morton wrote:
> On Mon, 5 Feb 2007 14:12:04 +0100
> Jan Kara <jack@...e.cz> wrote:
> > > Move the blocks on the temporary inode to the original inode
> > > by a page.
> > > 1. Read the file data from the old blocks to the page
> > > 2. Move the block on the temporary inode to the original inode
> > > 3. Write the file data on the page into the new blocks
> >   I have one thing - it's probably not good to use page cache for
> > defragmentation.
> 
> Then it is no longer online defragmentation.  The issues with maintaining
> correctness and coherency with ongoing VFS activity would be truly ghastly.
> 
> If we're worried about pagecache pollution then it would be better to control
> that from userspace via fadvise().

It should be possible to have the online defrag tool lock the inode against
any changes, flush all pages out of the cache for that inode, and then do
the reallocated outside of the page cache.  For inodes not already in cache
this is a no-op.  For the (hopefully rare) case were the inode already has
cached pages and also needs to be reallocated it would be a performance hit.

Alternately, we could skip files currently being modified (or mmaped), or
even recently modified (e.g. within the last 30 minutes) in the default case,
on the assumption that they might be deleted soon anyways.

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

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