linux-ext4 - Re: [PATCH 02/12] libext2fs: automatically enable meta

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Message-Id: <6D149520-CD92-4409-B155-8FF587130557@dilger.ca>
Date:	Mon, 12 Oct 2015 20:34:36 -0600
From:	Andreas Dilger <adilger@...ger.ca>
To:	"Darrick J. Wong" <darrick.wong@...cle.com>
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

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?

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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