JFS布局:日志文件系统如何处理磁盘布局(14)

/*
 *  inode allocation group page (per 4096 inodes of an AG)
 */
typedef struct {
    int64   agstart;    /* 8: starting block of ag      */
    int32   iagnum;     /* 4: inode allocation group number     */
    int32   inofreefwd; /* 4: ag inode free list forward    */
    int32   inofreeback;    /* 4: ag inode free list back       */
    int32   extfreefwd; /* 4: ag inode extent free list forward */
    int32   extfreeback;    /* 4: ag inode extent free list back    */
    int32   iagfree;    /* 4: iag free list         */

    /* summary map: 1 bit per inode extent */
    int32   inosmap[SMAPSZ];/* 16: sum map of mapwords w/ free inodes;
                 *  note: this indicates free and backed
                 *  inodes, if the extent is not backed the
                 *  value will be 1.  if the extent is
                 *  backed but all inodes are being used the
                 *  value will be 1.  if the extent is
                 *  backed but at least one of the inodes is
                 *  free the value will be 0.
                 */
    int32   extsmap[SMAPSZ];/* 16: sum map of mapwords w/ free extents */
    int32   nfreeinos;  /* 4: number of free inodes        */
    int32   nfreeexts;  /* 4: number of free extents               */
                /* (72)                                    */
    uint8   pad[1976];  /* 1976: pad to 2048 bytes                 */
    /* allocation bit map: 1 bit per inode (0 - free, 1 - allocated)   */
    uint32  wmap[EXTSPERIAG];   /* 512: working allocation map     */
    uint32  pmap[EXTSPERIAG];   /* 512: persistent allocation map  */
    pxd_t   inoext[EXTSPERIAG]; /* 1024: inode extent addresses    */
} iag_t;                    /* (4096)                          */

struct iag inode_allocation_map [ 1.. N ];