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 fs(4)								       fs(4)




 NAME
      fs - format of file system volume

 SYNOPSIS
      #include <&lt&lt&lt;sys/types.h>&gt&gt&gt;
      #include <&lt&lt&lt;sys/param.h>&gt&gt&gt;
      #include <&lt&lt&lt;sys/fs.h>&gt&gt&gt;
      #include <&lt&lt&lt;sys/inode.h>&gt&gt&gt;
      #include <&lt&lt&lt;sys/ino.h>&gt&gt&gt;
      #include <&lt&lt&lt;sys/sysmacros.h>&gt&gt&gt;

 DESCRIPTION
      Every file system storage volume has a common format for certain vital
      information.  The first 8 kbytes on a volume contain a volume header
      which identifies that volume as a Logical Interchange Format (LIF)
      volume.  Such volume may be divided into a number of sections.

      Each section can contain a file system.  The first 8 kbytes in each
      section is ignored, except where it coincides with the volume header
      discussed above.	The actual file system begins next with the "super
      block." The layout of the super block as defined by the include file
      <sys/fs.h> is:

      #define FS_MAGIC	     0x011954
      #define FS_MAGIC_LFN   0x095014
      #define FS_CLEAN	     0x17
      #define FS_OK	     0x53
      #define FS_NOTOK	     0x31
      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 */



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 fs(4)								       fs(4)




	  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 per rotation */
	  long	     fs_magic;	     /* magic number */
	  char	     fs_fname[6];    /* name of file system */
	  char	     fs_fpack[6];    /* pack name of file system */
	  u_char     fs_rotbl[1];    /* list of blocks for each rotation */
      /* actually longer */
      };

      A file system consists of a number of cylinder groups.  Each cylinder
      group has inodes and data.



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 fs(4)								       fs(4)




      A file system is described by its super-block, which in turn describes
      the cylinder groups.  The super-block is critical data and is
      replicated 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 smaller pieces, each of which is addressable;
      these pieces may be DEV_BSIZE, or some multiple of a DEV_BSIZE unit
      (DEV_BSIZE is defined in <sys/param.h>).

      Large files consist of exclusively large data blocks.  To avoid undue
      wasted disk space, the last data block of a file is allocated only as
      many fragments of a large block as are necessary, if that file is
      small enough to not require indirect data blocks.	 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.

      I-numbers begin at 0.  Inodes 0 and 1 are reserved.  Inode 2 is used
      for the root directory of the file system.  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 that can be free.	If the freelist drops below this level, only
      the super-user may continue to allocate blocks.  This can be set to 0
      if no reserve of free blocks is deemed necessary.	 However, severe
      performance degradations result if the file system is run at greater
      than 90% full; thus the default value of fs_minfree is 10%.

      The best trade-off between block fragmentation and overall disk
      utilization and performance varies for each intended use of the file
      system.  Suggested values can be found in the system administrator's
      manual for each implementation.

    Cylinder-Group-Related Limits
      Each cylinder keeps track of the availability 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.  For example, with NRPOS 8 the
      resolution of the summary information is 2ms for a typical 3600 rpm
      drive.





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 fs(4)								       fs(4)




      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.  Suggested values of fs_rotdelay
      for different disks can be found in the system administrator's manual.

      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 only a single variable
      size element (the free bit map).

      Important Note: 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 MINBSIZE must always be greater than sizeof(struct cg).
      Note that super blocks are never more than size SBSIZE.

      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.

      Important Note: sizeof (struct csum) must be a power of two in order
      for the fs_cs macro to work.

      The two possible values for fs_magic are FS_MAGIC, the default magic
      number for an HFS file system with a fixed-size directory format that
      limits file name length to DIRSIZ (14), and FS_MAGIC_LFN, the magic
      number of a file system using a variable-size directory format that
      supports file names of up to MAXNAMLEN (255) characters in length.

    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).




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 fs(4)								       fs(4)




      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 HP-UX file system.
      There is a unique inode allocated for each active file, each
      continuation inode, each current directory, each mounted-on file, text
      file, and the root.  An inode is ``named'' by its device-and-i-number
      pair.  For the format of an inode and its flags, see inode(4).

 DEPENDENCIES
    Series 700
      Series 700 systems support only one section per volume.  Thus, there
      can only be one file system on each volume and the first 8 Kbytes of a
      file system is the boot area.  This area contains the LIF volume
      header, the directory that defines the contents of the volume, and the
      bootstrapping program.

 AUTHOR
      fs was developed by HP and the University of California, Berkeley.

 SEE ALSO
      inode(4), lif(4).




























 Hewlett-Packard Company	    - 5 -   HP-UX Release 11i: November 2000