Date: Thu, 10 Aug 2006 13:27:20 -0700
From: Andrew Morton <akpm@osdl.org>
To: Jeff Garzik <jeff@garzik.org>
Cc: Badari Pulavarty <pbadari@us.ibm.com>, cmm@us.ibm.com,
       linux-kernel@vger.kernel.org, ext2-devel@lists.sourceforge.net,
       linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH 1/5] Forking ext4 filesystem from ext3 filesystem
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On Thu, 10 Aug 2006 16:13:33 -0400
Jeff Garzik <jeff@garzik.org> wrote:

> The sooner we kill buffer heads and use submit_bio(), the better :)

A buffer_head is a caching entity and a bio is an IO container.  They're
quite separate concepts.

A buffer_head is the kernel's sole abstraction of a disk block. 
Filesystems use disk blocks a lot, and they need such an abstraction.

If one was to replace buffer_heads with direct-to-BIO operations then the
filesytem would need to internally track the mapping from

	page+offset+length -> disk block

and it would need to internally track the page+offset+length<->disk block
coherency state and it would need to internally perform serialisation of
access to each page+offset+length hunk of pagecache and etc and etc and
etc.  Create a data structure with which to do all that and voila,
buffer_heads reinvented.
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