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Date:	Tue, 13 Oct 2015 00:08:38 -0700
From:	"Darrick J. Wong" <darrick.wong@...cle.com>
To:	Andreas Dilger <adilger@...ger.ca>
Cc:	"tytso@....edu" <tytso@....edu>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH 02/12] libext2fs: automatically enable meta_bg to avoid
 filling up BG 0

On Mon, Oct 12, 2015 at 08:34:36PM -0600, Andreas Dilger wrote:
> The commit comment says enable meta_bg if 1/2 of the block group
> is full, but the code checks for 3/4 of the block group is full.  Which one
> Should it be?

Oops.  It should be 3/4 since it's generally advantageous to leave meta_bg off
as long as possible so as to keep the group descriptors nearby.

That said, I also toyed with the idea of making the switch if total overhead
(gdt + inodes) uses more than 3/4 of a BG.  But the inode size/ratio is
configurable, so I went with just checking gdt for now.

--D

> 
> Cheers, Andreas
> 
> > On Oct 12, 2015, at 15:56, Darrick J. Wong <darrick.wong@...cle.com> wrote:
> > 
> > If during formatting we'd lose more than half of a block group to
> > group descriptors and other metadata, enable the meta_bg feature.
> > This enables us to create >500T filesystems with default options.
> > 
> > Signed-off-by: Darrick J. Wong <darrick.wong@...cle.com>
> > ---
> > lib/ext2fs/initialize.c |    5 +++++
> > 1 file changed, 5 insertions(+)
> > 
> > 
> > diff --git a/lib/ext2fs/initialize.c b/lib/ext2fs/initialize.c
> > index f672a27..b5ca928 100644
> > --- a/lib/ext2fs/initialize.c
> > +++ b/lib/ext2fs/initialize.c
> > @@ -381,6 +381,11 @@ ipg_retry:
> >    overhead = (int) (3 + fs->inode_blocks_per_group +
> >              super->s_reserved_gdt_blocks);
> > 
> > +    /* Enable meta_bg if we'd lose more than 3/4 of a BG to GDT blocks. */
> > +    if (super->s_reserved_gdt_blocks + fs->desc_blocks >
> > +        super->s_blocks_per_group * 3 / 4)
> > +        fs->super->s_feature_incompat |= EXT2_FEATURE_INCOMPAT_META_BG;
> > +
> >    if (fs->super->s_feature_incompat & EXT2_FEATURE_INCOMPAT_META_BG)
> >        overhead++;
> >    else
> > 
> > --
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