direntry建立了各級目錄和檔案之間的聯絡，inode建立了檔案與資料之間聯絡，但這還不夠，還需要記錄inode和資料塊從哪裡開始從哪裡結束，以及哪些是空閒的，這樣才能正確的分配檔案。

Group

塊組是一個邏輯概念，把一個分割槽分成若干個組，用來限制檔案的資料塊不要過於散落，儘量保證一個檔案的資料塊在一個組，這樣可以提高讀寫效率，

格式化一個ext4檔案系統後在磁碟上的結構：

Group 0: (Blocks 1-8192)
  Primary superblock at 1, Group descriptors at 2-2
  Reserved GDT blocks at 3-246
  Block bitmap at 247 (+246)
  Inode bitmap at 251 (+250)
  Inode table at 255-499 (+254)
  5481 free blocks, 1916 free inodes, 7 directories
  Free blocks: 1268-2657, 2866-4096, 5327-6112, 6119-8192
  Free inodes: 39, 46-1960

超級塊

超級塊記錄著一個分割槽的整體資訊，有多少inode，多少個組，多少個block資料塊，多少個空閒塊。

Group descriptors組描述符記錄了這個組中的inode數量，空閒塊數量等資訊。

Block bitmap是資料塊的點陣圖，記錄著資料塊的狀態。

Inode bitmap是inode的點陣圖，記錄著inode的狀態。

磁碟超級塊結構如下：

/*
 * Structure of the super block
 */
struct ext4_super_block {
/*00*/	__le32	s_inodes_count;		/* Inodes count */inode總數量
	__le32	s_blocks_count_lo;	/* Blocks count */block總數量
	__le32	s_r_blocks_count_lo;	/* Reserved blocks count */預留block總數量
	__le32	s_free_blocks_count_lo;	/* Free blocks count */空閒block總數量
/*10*/	__le32	s_free_inodes_count;	/* Free inodes count */空閒inode總數量
	__le32	s_first_data_block;	/* First Data Block */第一個資料塊的編號
	__le32	s_log_block_size;	/* Block size */塊大小，一般是4KB，可配置
	__le32	s_log_cluster_size;	/* Allocation cluster size */分片功能沒有啟用
/*20*/	__le32	s_blocks_per_group;	/* # Blocks per group */每個組中多個個塊
	__le32	s_clusters_per_group;	/* # Clusters per group */
	__le32	s_inodes_per_group;	/* # Inodes per group */
	__le32	s_mtime;		/* Mount time */
/*30*/	__le32	s_wtime;		/* Write time */
	__le16	s_mnt_count;		/* Mount count */被掛載的總次數
	__le16	s_max_mnt_count;	/* Maximal mount count */最大掛載次數，達到後就會進行檔案系統檢測
	__le16	s_magic;		/* Magic signature */
	__le16	s_state;		/* File system state */檔案系統狀態，為0表示被mount狀態，掛載之前會檢測此狀態防止重複掛載
	__le16	s_errors;		/* Behaviour when detecting errors */
	__le16	s_minor_rev_level;	/* minor revision level */
/*40*/	__le32	s_lastcheck;		/* time of last check */上一次檔案系統檢測的時間
	__le32	s_checkinterval;	/* max. time between checks */檢測的最大間隔
	__le32	s_creator_os;		/* OS */
	__le32	s_rev_level;		/* Revision level */
/*50*/	__le16	s_def_resuid;		/* Default uid for reserved blocks */
	__le16	s_def_resgid;		/* Default gid for reserved blocks */
	/*
	 * These fields are for EXT4_DYNAMIC_REV superblocks only.
	 *
	 * Note: the difference between the compatible feature set and
	 * the incompatible feature set is that if there is a bit set
	 * in the incompatible feature set that the kernel doesn't
	 * know about, it should refuse to mount the filesystem.
	 *
	 * e2fsck's requirements are more strict; if it doesn't know
	 * about a feature in either the compatible or incompatible
	 * feature set, it must abort and not try to meddle with
	 * things it doesn't understand...
	 */
	__le32	s_first_ino;		/* First non-reserved inode */
	__le16  s_inode_size;		/* size of inode structure */inode佔用空間大小，一般是256位元組
	__le16	s_block_group_nr;	/* block group # of this superblock */
	__le32	s_feature_compat;	/* compatible feature set */
/*60*/	__le32	s_feature_incompat;	/* incompatible feature set */
	__le32	s_feature_ro_compat;	/* readonly-compatible feature set */
/*68*/	__u8	s_uuid[16];		/* 128-bit uuid for volume */UID
/*78*/	char	s_volume_name[16];	/* volume name */分割槽名字
/*88*/	char	s_last_mounted[64];	/* directory where last mounted */
/*C8*/	__le32	s_algorithm_usage_bitmap; /* For compression */
	/*
	 * Performance hints.  Directory preallocation should only
	 * happen if the EXT4_FEATURE_COMPAT_DIR_PREALLOC flag is on.
	 */
	__u8	s_prealloc_blocks;	/* Nr of blocks to try to preallocate*/
	__u8	s_prealloc_dir_blocks;	/* Nr to preallocate for dirs */
	__le16	s_reserved_gdt_blocks;	/* Per group desc for online growth */
	/*
	 * Journaling support valid if EXT4_FEATURE_COMPAT_HAS_JOURNAL set.
	 */
/*D0*/	__u8	s_journal_uuid[16];	/* uuid of journal superblock */
/*E0*/	__le32	s_journal_inum;		/* inode number of journal file */
	__le32	s_journal_dev;		/* device number of journal file */
	__le32	s_last_orphan;		/* start of list of inodes to delete */
	__le32	s_hash_seed[4];		/* HTREE hash seed */
	__u8	s_def_hash_version;	/* Default hash version to use */
	__u8	s_jnl_backup_type;
	__le16  s_desc_size;		/* size of group descriptor */
/*100*/	__le32	s_default_mount_opts;
	__le32	s_first_meta_bg;	/* First metablock block group */
	__le32	s_mkfs_time;		/* When the filesystem was created */
	__le32	s_jnl_blocks[17];	/* Backup of the journal inode */
	/* 64bit support valid if EXT4_FEATURE_COMPAT_64BIT */
/*150*/	__le32	s_blocks_count_hi;	/* Blocks count */
	__le32	s_r_blocks_count_hi;	/* Reserved blocks count */
	__le32	s_free_blocks_count_hi;	/* Free blocks count */
	__le16	s_min_extra_isize;	/* All inodes have at least # bytes */
	__le16	s_want_extra_isize; 	/* New inodes should reserve # bytes */
	__le32	s_flags;		/* Miscellaneous flags */
	__le16  s_raid_stride;		/* RAID stride */
	__le16  s_mmp_update_interval;  /* # seconds to wait in MMP checking */
	__le64  s_mmp_block;            /* Block for multi-mount protection */
	__le32  s_raid_stripe_width;    /* blocks on all data disks (N*stride)*/
	__u8	s_log_groups_per_flex;  /* FLEX_BG group size */
	__u8	s_checksum_type;	/* metadata checksum algorithm used */
	__u8	s_encryption_level;	/* versioning level for encryption */
	__u8	s_reserved_pad;		/* Padding to next 32bits */
	__le64	s_kbytes_written;	/* nr of lifetime kilobytes written */
	__le32	s_snapshot_inum;	/* Inode number of active snapshot */
	__le32	s_snapshot_id;		/* sequential ID of active snapshot */
	__le64	s_snapshot_r_blocks_count; /* reserved blocks for active
					      snapshot's future use */
	__le32	s_snapshot_list;	/* inode number of the head of the
					   on-disk snapshot list */
#define EXT4_S_ERR_START offsetof(struct ext4_super_block, s_error_count)
	__le32	s_error_count;		/* number of fs errors */
	__le32	s_first_error_time;	/* first time an error happened */
	__le32	s_first_error_ino;	/* inode involved in first error */
	__le64	s_first_error_block;	/* block involved of first error */
	__u8	s_first_error_func[32];	/* function where the error happened */
	__le32	s_first_error_line;	/* line number where error happened */
	__le32	s_last_error_time;	/* most recent time of an error */
	__le32	s_last_error_ino;	/* inode involved in last error */
	__le32	s_last_error_line;	/* line number where error happened */
	__le64	s_last_error_block;	/* block involved of last error */
	__u8	s_last_error_func[32];	/* function where the error happened */
#define EXT4_S_ERR_END offsetof(struct ext4_super_block, s_mount_opts)
	__u8	s_mount_opts[64];
	__le32	s_usr_quota_inum;	/* inode for tracking user quota */
	__le32	s_grp_quota_inum;	/* inode for tracking group quota */
	__le32	s_overhead_clusters;	/* overhead blocks/clusters in fs */
	__le32	s_backup_bgs[2];	/* groups with sparse_super2 SBs */
	__u8	s_encrypt_algos[4];	/* Encryption algorithms in use  */
	__u8	s_encrypt_pw_salt[16];	/* Salt used for string2key algorithm */
	__le32	s_lpf_ino;		/* Location of the lost+found inode */
	__le32	s_reserved[100];	/* Padding to the end of the block */
	__le32	s_checksum;		/* crc32c(superblock) */
};

ext4_super_block與磁碟上面的超級塊資料基本是對應的，可直接讀出到結構體記憶體。

組描述符在磁碟上的格式如下：

/*
 * Structure of a blocks group descriptor
 */
struct ext4_group_desc
{
	__le32	bg_block_bitmap_lo;	/* Blocks bitmap block */表示bmap在這個組的那一塊
	__le32	bg_inode_bitmap_lo;	/* Inodes bitmap block */表示imap在這個組的那一塊
	__le32	bg_inode_table_lo;	/* Inodes table block */這個組的inode所在起始塊號
	__le16	bg_free_blocks_count_lo;/* Free blocks count */空閒塊數量
	__le16	bg_free_inodes_count_lo;/* Free inodes count */空閒inode數量
	__le16	bg_used_dirs_count_lo;	/* Directories count */
	__le16	bg_flags;		/* EXT4_BG_flags (INODE_UNINIT, etc) */
	__le32  bg_exclude_bitmap_lo;   /* Exclude bitmap for snapshots */
	__le16  bg_block_bitmap_csum_lo;/* crc32c(s_uuid+grp_num+bbitmap) LE */
	__le16  bg_inode_bitmap_csum_lo;/* crc32c(s_uuid+grp_num+ibitmap) LE */
	__le16  bg_itable_unused_lo;	/* Unused inodes count */
	__le16  bg_checksum;		/* crc16(sb_uuid+group+desc) */
	__le32	bg_block_bitmap_hi;	/* Blocks bitmap block MSB */
	__le32	bg_inode_bitmap_hi;	/* Inodes bitmap block MSB */
	__le32	bg_inode_table_hi;	/* Inodes table block MSB */
	__le16	bg_free_blocks_count_hi;/* Free blocks count MSB */
	__le16	bg_free_inodes_count_hi;/* Free inodes count MSB */
	__le16	bg_used_dirs_count_hi;	/* Directories count MSB */
	__le16  bg_itable_unused_hi;    /* Unused inodes count MSB */
	__le32  bg_exclude_bitmap_hi;   /* Exclude bitmap block MSB */
	__le16  bg_block_bitmap_csum_hi;/* crc32c(s_uuid+grp_num+bbitmap) BE */
	__le16  bg_inode_bitmap_csum_hi;/* crc32c(s_uuid+grp_num+ibitmap) BE */
	__u32   bg_reserved;
};

組描述符佔用64位元組，一個塊佔用4KB，所以每個組裡面的組描述符塊儲存了所有組描述符。應該是為了防止資料損壞進行修復使用。

超級塊的備份會在2n-1的組裡面，防止資料損壞進行修復。