linux-ext4 - Re: A proposal for making ext4's journal more SMR (and flash) friendly

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Message-ID: <20140110060429.GA26378@quack.suse.cz>
Date:	Fri, 10 Jan 2014 07:04:29 +0100
From:	Jan Kara <jack@...e.cz>
To:	Theodore Ts'o <tytso@....edu>
Cc:	Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org
Subject: Re: A proposal for making ext4's journal more SMR (and flash)
 friendly

On Wed 08-01-14 18:37:52, Ted Tso wrote:
> On Wed, Jan 08, 2014 at 11:14:30PM +0100, Jan Kara wrote:
> >   So when I was thinking about this (already couple of years ago) the thing
> > which stopped me was the question at which layer we should do the
> > translation. Ideally we would need something at submit_bh() level but just
> > wrapping submit_bh() calls inside ext4 isn't enough for stuff like symlinks
> > or journalled data... Do you have any thoughts on that?
> 
> I think there are two interfaces that should handle nearly all of our
> journal block mapping needs.  The functions that issue bio requests
> directly tend to use ext4_get_block*() functions, and functions which
> use the buffer cache uses submit_bh() (typically via ext4_getblk).
> There will probably be a few exceptions, but I don't think this should
> be an intractable problem.
  Surely not intractable :) It was just ugly. But you are right that
hooking in ext4_map_blocks() and then special-casing the few cases where we
get the block number by different means (xattrs, inode table, group
descriptor, superblock, traversal of extent tree & indirect block tree)
should be reasonably elegant.

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