from: Andreas Dilger <adilger@clusterfs.com>
In pass1 of e2fsck, every inode table in the fileystem is scanned and checked,

regardless of whether it is in use.  This is this the most time consuming part

of the filesystem check.  The unintialized block group feature can greatly

reduce e2fsck time by eliminating checking of uninitialized inodes.  
With this feature, there is a a high water mark of used inodes for each block

group.  Block and inode bitmaps can be uninitialized on disk via a flag in the

group descriptor to avoid reading or scanning them at e2fsck time.  A checksum

of each group descriptor is used to ensure that corruption in the group

descriptor's bit flags does not cause incorrect operation.
The feature is enabled through a mkfs option
	mke2fs /dev/ -O uninit_groups
A patch adding support for uninitialized block groups to e2fsprogs tools has

been posted to the linux-ext4 mailing list.
The patches have been stress tested with fsstress and fsx.  In performance

tests testing e2fsck time, we have seen that e2fsck time on ext3 grows

linearly with the total number of inodes in the filesytem.  In ext4 with the

uninitialized block groups feature, the e2fsck time is constant, based

solely on the number of used inodes rather than the total inode count.

Since typical ext4 filesystems only use 1-10% of their inodes, this feature can

greatly reduce e2fsck time for users.  With performance improvement of 2-20

times, depending on how full the filesystem is.
The attached graph shows the major improvements in e2fsck times in filesystems

with a large total inode count, but few inodes in use.  
In each group descriptor if we have
EXT4_BG_INODE_UNINIT set in bg_flags:

        Inode table is not initialized/used in this group. So we can skip

        the consistency check during fsck.

EXT4_BG_BLOCK_UNINIT set in bg_flags:

        No block in the group is used. So we can skip the block bitmap

        verification for this group.
We also add two new field...
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On Tue, 18 Sep 2007 17:25:31 -0700
This needed a few fixups due to conflicts with

ext2-ext3-ext4-add-block-bitmap-validation.patch but they were pretty

straightforward.  Please check that the result is OK.
-

And is we really really have to do this, then the ext4-private crc16()

should have static scope.

-

That's rather sad.  A plain old "depends on" would be better.

-

My bad.  We wrote this patch and it had to run on older kernels that might

not even have lib/crc16.c (it was added around 2.6.15 or so, so e.g. RHEL4

doesn't have it).  I forgot to remove it from the upstream submission,

and since it didn't cause problems nobody else complained...
Cheers, Andreas

--

Andreas Dilger

Principal Software Engineer

Cluster File Systems, Inc.
-

The incremental patch below removes the local crc16 code, and also resolves an

issue with properly updating bg_itable_unused when an inode is allocated in an

unused block groups.
Thanks

Avantika
---

 fs/Kconfig       |    1 +

 fs/ext4/ialloc.c |    8 +++++++-

 fs/ext4/super.c  |   51 +--------------------------------------------------

 3 files changed, 9 insertions(+), 51 deletions(-)
Index: linux-2.6.23-rc6/fs/ext4/ialloc.c

===================================================================

--- linux-2.6.23-rc6.orig/fs/ext4/ialloc.c	2007-09-19 15:38:21.000000000 -0700

+++ linux-2.6.23-rc6/fs/ext4/ialloc.c	2007-09-19 15:41:11.000000000 -0700

@@ -635,12 +635,18 @@

 	if (EXT4_HAS_RO_COMPAT_FEATURE(sb, EXT4_FEATURE_RO_COMPAT_GDT_CSUM)) {

 		if (gdp->bg_flags & cpu_to_le16(EXT4_BG_INODE_UNINIT)) {

 			gdp->bg_flags &= cpu_to_le16(~EXT4_BG_INODE_UNINIT);

-			free = EXT4_INODES_PER_GROUP(sb);

+			free = 0;

 		} else {

 			free = EXT4_INODES_PER_GROUP(sb) -

 				le16_to_cpu(gdp->bg_itable_unused);

 		}
+		/*

+		 * Check the relative inode number against the last used

+		 * relative inode number in this group. if it is greater

+		 * we need to  update the bg_itable_unused count

+		 *

+		 */

 		if (ino > free)

 			gdp->bg_itable_unused =

 				cpu_to_le16(EXT4_INODES_PER_GROUP(sb) - ino);

Index: linux-2.6.23-rc6/fs/ext4/super.c

===================================================================

--- linux-2.6.23-rc6.orig/fs/ext4/super.c	2007-09-19 15:38:21.000000000 -0700

+++ linux-2.6.23-rc6/fs/ext4/super.c	2007-09-19 15:38:51.000000000 -0700

@@ -37,6 +37,7 @@

 #include <linux/quotaops.h>

 #include <linux/seq_file.h>

 #include <linux/log2.h>

+#include <linux/crc16.h>
 #include <asm/uaccess.h>
@@ -1248,56 +1249,6 @@

 	return res;

 }
-#if !defined(CONFIG_CRC16)

-/** CRC table for the CRC-16. The poly is 0x8005 (x16 + x15 + x2 + 1) */

-__u16 const crc16_table[256] = {

-	0x0000, 0xC0C1, 0xC181, 0x0140, 0xC30...
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