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FS(5)                         File Formats Manual                        FS(5)

       fs, inode - format of file system volume

       #include <&lt;sys/types.h>&gt;
       #include <&lt;sys/fs.h>&gt;
       #include <&lt;sys/inode.h>&gt;

       Every  file system storage volume (disk, nine-track tape, for instance)
       has a common format for certain vital information.  Every  such  volume
       is divided into a certain number of blocks.  The block size is a param-
       eter of the file system.  Sectors 0 to 15 on a file system are used  to
       contain primary and secondary bootstrapping programs.

       The  actual  file system begins at sector 16 with the super block.  The
       layout of the super block as defined by the include file <sys/fs.h> is:

       #define   FS_MAGIC  0x011954
       struct    fs {
            struct    fs *fs_link;        /* linked list of file systems */
            struct    fs *fs_rlink;       /*     used for incore super blocks */
            daddr_t   fs_sblkno;          /* addr of super-block in filesys */
            daddr_t   fs_cblkno;          /* offset of cyl-block in filesys */
            daddr_t   fs_iblkno;          /* offset of inode-blocks in filesys */
            daddr_t   fs_dblkno;          /* offset of first data after cg */
            long fs_cgoffset;        /* cylinder group offset in cylinder */
            long fs_cgmask;          /* used to calc mod fs_ntrak */
            time_t    fs_time;            /* last time written */
            long fs_size;       /* number of blocks in fs */
            long fs_dsize;      /* number of data blocks in fs */
            long fs_ncg;             /* number of cylinder groups */
            long fs_bsize;      /* size of basic blocks in fs */
            long fs_fsize;      /* size of frag blocks in fs */
            long fs_frag;       /* number of frags in a block in fs */
       /* these are configuration parameters */
            long fs_minfree;         /* minimum percentage of free blocks */
            long fs_rotdelay;        /* num of ms for optimal next block */
            long fs_rps;             /* disk revolutions per second */
       /* these fields can be computed from the others */
            long fs_bmask;      /* ``blkoff'' calc of blk offsets */
            long fs_fmask;      /* ``fragoff'' calc of frag offsets */
            long fs_bshift;          /* ``lblkno'' calc of logical blkno */
            long fs_fshift;          /* ``numfrags'' calc number of frags */
       /* these are configuration parameters */
            long fs_maxcontig;       /* max number of contiguous blks */
            long fs_maxbpg;          /* max number of blks per cyl group */
       /* these fields can be computed from the others */
            long fs_fragshift;       /* block to frag shift */
            long fs_fsbtodb;         /* fsbtodb and dbtofsb shift constant */
            long fs_sbsize;          /* actual size of super block */
            long fs_csmask;          /* csum block offset */
            long fs_csshift;         /* csum block number */
            long fs_nindir;          /* value of NINDIR */
            long fs_inopb;      /* value of INOPB */
            long fs_nspf;       /* value of NSPF */
            long fs_sparecon[6];          /* reserved for future constants */
       /* sizes determined by number of cylinder groups and their sizes */
            daddr_t fs_csaddr;       /* blk addr of cyl grp summary area */
            long fs_cssize;          /* size of cyl grp summary area */
            long fs_cgsize;          /* cylinder group size */
       /* these fields should be derived from the hardware */
            long fs_ntrak;      /* tracks per cylinder */
            long fs_nsect;      /* sectors per track */
            long      fs_spc;             /* sectors per cylinder */
       /* this comes from the disk driver partitioning */
            long fs_ncyl;            /* cylinders in file system */
       /* these fields can be computed from the others */
            long fs_cpg;             /* cylinders per group */
            long fs_ipg;             /* inodes per group */
            long fs_fpg;             /* blocks per group * fs_frag */
       /* this data must be re-computed after crashes */
            struct    csum fs_cstotal;    /* cylinder summary information */
       /* these fields are cleared at mount time */
            char      fs_fmod;            /* super block modified flag */
            char      fs_clean;           /* file system is clean flag */
            char      fs_ronly;           /* mounted read-only flag */
            char      fs_flags;           /* currently unused flag */
            char fs_fsmnt[MAXMNTLEN];     /* name mounted on */
       /* these fields retain the current block allocation info */
            long fs_cgrotor;         /* last cg searched */
            struct    csum *fs_csp[MAXCSBUFS];/* list of fs_cs info buffers */
            long fs_cpc;             /* cyl per cycle in postbl */
            short     fs_postbl[MAXCPG][NRPOS];/* head of blocks for each rotation */
            long fs_magic;      /* magic number */
            u_char    fs_rotbl[1];        /* list of blocks for each rotation */
       /* actually longer */

       Each disk drive contains some number of file systems.   A  file  system
       consists  of  a  number  of  cylinder  groups.  Each cylinder group has
       inodes and data.

       A file system is described by its super-block, which in turn  describes
       the  cylinder  groups.   The super-block is critical data and is repli-
       cated in each cylinder group  to  protect  against  catastrophic  loss.
       This  is done at file system creation time and the critical super-block
       data does not change, so the copies  need  not  be  referenced  further
       unless disaster strikes.

       Addresses  stored  in  inodes  are  capable  of addressing fragments of
       `blocks'. File system blocks of at most size MAXBSIZE can be optionally
       broken  into  2,  4,  or  8 pieces, each of which is addressable; these
       pieces may be DEV_BSIZE, or some multiple of a DEV_BSIZE unit.

       Large files consist of exclusively large data blocks.  To  avoid  undue
       wasted  disk space, the last data block of a small file is allocated as
       only as many fragments of a large block as  are  necessary.   The  file
       system  format  retains only a single pointer to such a fragment, which
       is a piece of a single large block that has been divided.  The size  of
       such  a  fragment  is determinable from information in the inode, using
       the ``blksize(fs, ip, lbn)'' macro.

       The file system records space availability at the  fragment  level;  to
       determine block availability, aligned fragments are examined.

       The  root  inode is the root of the file system.  Inode 0 can't be used
       for normal purposes and historically bad blocks were linked to inode 1,
       thus  the  root inode is 2 (inode 1 is no longer used for this purpose,
       however numerous dump tapes make this assumption, so we are stuck  with
       it).   The  lost+found directory is given the next available inode when
       it is initially created by mkfs.

       fs_minfree gives the  minimum  acceptable  percentage  of  file  system
       blocks  which  may be free. If the freelist drops below this level only
       the super-user may continue to allocate blocks. This may be set to 0 if
       no  reserve  of free blocks is deemed necessary, however severe perfor-
       mance degradations will be observed  if  the  file  system  is  run  at
       greater than 90% full; thus the default value of fs_minfree is 10%.

       Empirically  the best trade-off between block fragmentation and overall
       disk utilization at a loading of 90% comes with a fragmentation  of  4,
       thus the default fragment size is a fourth of the block size.

       Cylinder  group related limits: Each cylinder keeps track of the avail-
       ability of blocks at different rotational positions, so that sequential
       blocks  can  be laid out with minimum rotational latency.  NRPOS is the
       number of rotational positions which are distinguished.  With  NRPOS  8
       the resolution of the summary information is 2ms for a typical 3600 rpm

       fs_rotdelay gives  the  minimum  number  of  milliseconds  to  initiate
       another  disk transfer on the same cylinder.  It is used in determining
       the rotationally optimal layout for disk  blocks  within  a  file;  the
       default value for fs_rotdelay is 2ms.

       Each file system has a statically allocated number of inodes.  An inode
       is allocated for each NBPI bytes of disk space.  The  inode  allocation
       strategy is extremely conservative.

       MAXIPG  bounds  the  number of inodes per cylinder group, and is needed
       only to keep the structure simpler by having the only a single variable
       size element (the free bit map).

       N.B.: MAXIPG must be a multiple of INOPB(fs).

       MINBSIZE is the smallest allowable block size.  With a MINBSIZE of 4096
       it is possible to create files of size 2^32 with  only  two  levels  of
       indirection.   MINBSIZE  must  be  big  enough to hold a cylinder group
       block, thus changes to (struct cg) must keep its size within  MINBSIZE.
       MAXCPG  is limited only to dimension an array in (struct cg); it can be
       made larger as long as that structure's size remains within the  bounds
       dictated  by MINBSIZE.  Note that super blocks are never more than size

       The path name on which the file system  is  mounted  is  maintained  in
       fs_fsmnt.  MAXMNTLEN defines the amount of space allocated in the super
       block for this name.  The limit on the amount  of  summary  information
       per  file system is defined by MAXCSBUFS. It is currently parameterized
       for a maximum of two million cylinders.

       Per cylinder group information is summarized in blocks  allocated  from
       the  first cylinder group's data blocks.  These blocks are read in from
       fs_csaddr (size fs_cssize) in addition to the super block.

       N.B.: sizeof (struct csum) must be a power of  two  in  order  for  the
       ``fs_cs'' macro to work.

       Super block for a file system: MAXBPC bounds the size of the rotational
       layout tables and is limited by the fact that the  super  block  is  of
       size SBSIZE.  The size of these tables is inversely proportional to the
       block size of the file system. The size of the tables is increased when
       sector  sizes  are  not  powers of two, as this increases the number of
       cylinders included before the rotational  pattern  repeats  (  fs_cpc).
       The  size of the rotational layout tables is derived from the number of
       bytes remaining in (struct fs).

       MAXBPG bounds the number of blocks of data per cylinder group,  and  is
       limited  by  the  fact that cylinder groups are at most one block.  The
       size of the free block table is derived from the size of blocks and the
       number of remaining bytes in the cylinder group structure (struct cg).

       Inode:  The  inode  is  the focus of all file activity in the UNIX file
       system.  There is a unique inode allocated for each active  file,  each
       current  directory,  each mounted-on file, text file, and the root.  An
       inode is `named' by its device/i-number pair.  For further information,
       see the include file <sys/inode.h>.

4th Berkeley Distribution        18 July 1983                            FS(5)