On Wed, Sep 21, 2011 at 11:03:57AM +0300, Amir Goldstein wrote: > On Tue, Sep 20, 2011 at 9:56 PM, Darrick J. Wong <djwong@xxxxxxxxxx> wrote: > > On Thu, Sep 15, 2011 at 06:50:51PM -0400, Theodore Ts'o wrote: <snip> > >> diff --git a/lib/ext2fs/ext2_fs.h b/lib/ext2fs/ext2_fs.h > >> index 4fec5db..1c86cb2 100644 > >> --- a/lib/ext2fs/ext2_fs.h > >> +++ b/lib/ext2fs/ext2_fs.h > >> @@ -142,7 +142,9 @@ struct ext2_group_desc > >> __u16 bg_free_inodes_count; /* Free inodes count */ > >> __u16 bg_used_dirs_count; /* Directories count */ > >> __u16 bg_flags; > >> - __u32 bg_reserved[2]; > >> + __u32 bg_exclude_bitmap_lo; /* Exclude bitmap for snapshots */ > >> + __u16 bg_block_bitmap_csum_lo;/* crc32c(s_uuid+grp_num+bitmap) LSB */ > >> + __u16 bg_inode_bitmap_csum_lo;/* crc32c(s_uuid+grp_num+bitmap) LSB */ > >> __u16 bg_itable_unused; /* Unused inodes count */ > >> __u16 bg_checksum; /* crc16(s_uuid+grouo_num+group_desc)*/ > >> }; > >> @@ -159,7 +161,9 @@ struct ext4_group_desc > >> __u16 bg_free_inodes_count; /* Free inodes count */ > >> __u16 bg_used_dirs_count; /* Directories count */ > >> __u16 bg_flags; /* EXT4_BG_flags (INODE_UNINIT, etc) */ > >> - __u32 bg_reserved[2]; /* Likely block/inode bitmap checksum */ > >> + __u32 bg_exclude_bitmap_lo; /* Exclude bitmap for snapshots */ > >> + __u16 bg_block_bitmap_csum_lo;/* crc32c(s_uuid+grp_num+bitmap) LSB */ > >> + __u16 bg_inode_bitmap_csum_lo;/* crc32c(s_uuid+grp_num+bitmap) LSB */ > >> __u16 bg_itable_unused; /* Unused inodes count */ > >> __u16 bg_checksum; /* crc16(sb_uuid+group+desc) */ > >> __u32 bg_block_bitmap_hi; /* Blocks bitmap block MSB */ > >> @@ -169,7 +173,10 @@ struct ext4_group_desc > >> __u16 bg_free_inodes_count_hi;/* Free inodes count MSB */ > >> __u16 bg_used_dirs_count_hi; /* Directories count MSB */ > >> __u16 bg_itable_unused_hi; /* Unused inodes count MSB */ > >> - __u32 bg_reserved2[3]; > >> + __u32 bg_exclude_bitmap_hi; /* Exclude bitmap block MSB */ > >> + __u16 bg_block_bitmap_csum_hi;/* crc32c(s_uuid+grp_num+bitmap) MSB */ > >> + __u16 bg_inode_bitmap_csum_hi;/* crc32c(s_uuid+grp_num+bitmap) MSB */ > >> + __u32 bg_reserved; > > > > Should we attach a checksum to the exclude bitmap? That would, unfortunately, > > put the 64-byte block group pretty close to full, unless we're ok with making > > the bg even bigger... > > Good point. > My initial approach (which I probably expressed somewhere) was that when > exclude_bitmap feature is set, the block bitmap checksum should cover both > block bitmap and exclude bitmap. > The reason being that exclude bitmap adds little entropy over block bitmap: > - it should always be a sub set of block bitmap bits > - clearing exclude bit is done together with clearing the block used bit > - the only case of modifying exclude bitmap only is when a used block becomes > excluded (a.k.a moved to snapshot) > > so how do you feel about compressing 2 blocks into 16bits of checksum :-/? Hrm.... is it generally the case that both block and exclude bitmaps are loaded at the same time? I don't think it'd be difficult to checksum both blocks unless it's common that one of the two are not in memory. --D -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
