linux-ext4 - Re: Status of META

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Message-ID: <20120318204153.GA31682@thunk.org>
Date:	Sun, 18 Mar 2012 16:41:53 -0400
From:	Ted Ts'o <tytso@....edu>
To:	Phillip Susi <psusi@...ntu.com>
Cc:	Andreas Dilger <adilger@...ger.ca>,
	ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: Status of META_BG?

On Thu, Mar 15, 2012 at 01:55:36PM -0400, Phillip Susi wrote:
> >META_BG addresses both of these issues by distributing the group
> >descriptor blocks into the filesystem for each "meta group" (= the
> >number of groups whose descriptors fit into a single block).
> 
> So it puts one GD block at the start of every several block groups?
> Wouldn't that drastically slow down opening/mounting the fs since
> the disk has to seek to every block group?

Not necessarily; right now we pull in every single block group
descriptor at mount time because we need to update s_free_inodes_count
and s_free_blocks_count.  If we change things so that we only pull in
the block group descriptors at mount time after a journal replay (but
not after a clean umount, when the last inodes count and free blocks
count should be correctly updated), that would avoid seeking to every
16th block group at mount time.  

						- Ted
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