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

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Date:	Mon, 12 Oct 2015 14:56:42 -0700
From:	"Darrick J. Wong" <darrick.wong@...cle.com>
To:	tytso@....edu, darrick.wong@...cle.com
Cc:	linux-ext4@...r.kernel.org
Subject: [PATCH 02/12] libext2fs: automatically enable meta_bg to avoid
 filling up BG 0

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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