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Date:	Sat, 09 Apr 2011 02:45:24 +0800
From:	Tao Ma <tm@....ma>
To:	Amir Goldstein <amir73il@...il.com>
CC:	Theodore Tso <tytso@....edu>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: [RFC] exclude bitmap and 32bit bitmap cheksum fields

Hi Amir,
On 04/09/2011 02:00 AM, Amir Goldstein wrote:
> Hi Ted,
> 
> Following our conversation, here is a proposal how to squeeze:
> - 32bit exclude bitmap block address
> - 32bit block+exclude bitmap checksum
> - 32bit inode bitmap checksum
> into the reaming 8 bytes in the group descriptor.
> 
> The idea is that the 16bit persistent free inode/block counters
> are redundant to the inode/block bitmap information
> and are needed in 2 use cases:
> 1. sanity checks on fsck
> 2. quick load of in-memory counters
> 
> The first use case is nulled by the introduction of inode/block bitmap
> checksums.
> The second use case can be bypassed with no substantial penalty:
> in-memory counters can be calculated on first inode/block bitmap access,
> when the GRP_NEED_INIT (or another) flag is set in the group_info struct,
> just like their cousins, the buddy bitmap counters.
> 
> Amir.
> 
> diff --git a/lib/ext2fs/ext2_fs.h b/lib/ext2fs/ext2_fs.h
> index 0deb554..5cbaeb2 100644
> --- a/lib/ext2fs/ext2_fs.h
> +++ b/lib/ext2fs/ext2_fs.h
> @@ -153,11 +153,11 @@ struct ext2_group_desc
>  	__u32	bg_block_bitmap;	/* Blocks bitmap block */
>  	__u32	bg_inode_bitmap;	/* Inodes bitmap block */
>  	__u32	bg_inode_table;		/* Inodes table block */
> -	__u16	bg_free_blocks_count;	/* Free blocks count */
> -	__u16	bg_free_inodes_count;	/* Free inodes count */
> +	__u32	bg_exclude_bitmap;	/* Exclude bitmap block */
>  	__u16	bg_used_dirs_count;	/* Directories count */
>  	__u16	bg_flags;
> -	__u32	bg_reserved[2];
> +	__u32	bg_block_bitmap_csum;	/* Blocks+exclude bitmap checksum */
> +	__u32	bg_inode_bitmap_csum;	/* Inodes bitmap checksum */
>  	__u16	bg_itable_unused;	/* Unused inodes count */
>  	__u16	bg_checksum;		/* crc16(s_uuid+grouo_num+group_desc)*/
>  };
> @@ -170,18 +170,17 @@ struct ext4_group_desc
>  	__u32	bg_block_bitmap;	/* Blocks bitmap block */
>  	__u32	bg_inode_bitmap;	/* Inodes bitmap block */
>  	__u32	bg_inode_table;		/* Inodes table block */
> -	__u16	bg_free_blocks_count;	/* Free blocks count */
> -	__u16	bg_free_inodes_count;	/* Free inodes count */
> +	__u32	bg_exclude_bitmap;	/* Exclude bitmap block */
>  	__u16	bg_used_dirs_count;	/* Directories count */
>  	__u16	bg_flags;
> -	__u32	bg_reserved[2];
> +	__u32	bg_block_bitmap_csum;	/* Blocks+exclude bitmap checksum */
> +	__u32	bg_inode_bitmap_csum;	/* Inodes bitmap checksum */
>  	__u16	bg_itable_unused;	/* Unused inodes count */
>  	__u16	bg_checksum;		/* crc16(s_uuid+grouo_num+group_desc)*/
>  	__u32	bg_block_bitmap_hi;	/* Blocks bitmap block MSB */
>  	__u32	bg_inode_bitmap_hi;	/* Inodes bitmap block MSB */
>  	__u32	bg_inode_table_hi;	/* Inodes table block MSB */
> -	__u16	bg_free_blocks_count_hi;/* Free blocks count MSB */
> -	__u16	bg_free_inodes_count_hi;/* Free inodes count MSB */
> +	__u32	bg_exclude_bitmap;	/* Exclude bitmap block MSB */
>  	__u16	bg_used_dirs_count_hi;	/* Directories count MSB */
>  	__u16   bg_pad;
>  	__u32	bg_reserved2[3];
> @@ -190,6 +189,7 @@ struct ext4_group_desc
>  #define EXT2_BG_INODE_UNINIT	0x0001 /* Inode table/bitmap not initialized */
>  #define EXT2_BG_BLOCK_UNINIT	0x0002 /* Block bitmap not initialized */
>  #define EXT2_BG_INODE_ZEROED	0x0004 /* On-disk itable initialized to zero */
> +#define EXT2_BG_EXCLUDE_UNINIT	0x0008 /* Exclude bitmap not initialized */
> 
>  /*
>   * Data structures used by the directory indexing feature
> @@ -751,6 +751,7 @@ struct ext2_super_block {
>  #define EXT4_FEATURE_RO_COMPAT_DIR_NLINK	0x0020
>  #define EXT4_FEATURE_RO_COMPAT_EXTRA_ISIZE	0x0040
>  #define EXT4_FEATURE_RO_COMPAT_HAS_SNAPSHOT	0x0080
> +#define EXT4_FEATURE_RO_COMPAT_BITMAP_CSUM	0x0100
I guess this should be in FEATURE_INCOMPAT, not RO_COMPAT?
Since you have changed free_{inodes, blocks}_count to exclude_map. an
old kernel will interpret these values wrongly.

Regards,
Tao
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