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FS(5)                     FreeBSD File Formats Manual                    FS(5)

NAME
     fs, inode -- format of file system volume

SYNOPSIS
     #include <sys/param.h>
     #include <ufs/ffs/fs.h>

     #include <sys/types.h>
     #include <sys/lock.h>
     #include <ufs/ufs/quota.h>
     #include <ufs/ufs/inode.h>

DESCRIPTION
     The files <fs.h> and <inode.h> declare several structures, defined vari-
     ables and macros which are used to create and manage the underlying for-
     mat of file system objects on random access devices (disks).

     The block size and number of blocks which comprise a file system are
     parameters of the file system.  Sectors beginning at BBLOCK and continu-
     ing for BBSIZE are used for a disklabel and for some hardware primary and
     secondary bootstrapping programs.

     The actual file system begins at sector SBLOCK with the super-block that
     is of size SBSIZE.  The following structure describes the super-block and
     is from the file <ufs/ffs/fs.h>:

     /*
      * Super block for an FFS file system.
      */
     struct fs {
             int32_t  fs_firstfield; /* historic file system linked list, */
             int32_t  fs_unused_1;   /*     used for incore super blocks */
             ufs_daddr_t fs_sblkno;  /* addr of super-block in filesys */
             ufs_daddr_t fs_cblkno;  /* offset of cyl-block in filesys */
             ufs_daddr_t fs_iblkno;  /* offset of inode-blocks in filesys */
             ufs_daddr_t fs_dblkno;  /* offset of first data after cg */
             int32_t  fs_cgoffset;   /* cylinder group offset in cylinder */
             int32_t  fs_cgmask;     /* used to calc mod fs_ntrak */
             time_t   fs_time;       /* last time written */
             int32_t  fs_size;       /* number of blocks in fs */
             int32_t  fs_dsize;      /* number of data blocks in fs */
             int32_t  fs_ncg;        /* number of cylinder groups */
             int32_t  fs_bsize;      /* size of basic blocks in fs */
             int32_t  fs_fsize;      /* size of frag blocks in fs */
             int32_t  fs_frag;       /* number of frags in a block in fs */
     /* these are configuration parameters */
             int32_t  fs_minfree;    /* minimum percentage of free blocks */
             int32_t  fs_rotdelay;   /* num of ms for optimal next block */
             int32_t  fs_rps;        /* disk revolutions per second */
     /* these fields can be computed from the others */
             int32_t  fs_bmask;      /* ``blkoff'' calc of blk offsets */
             int32_t  fs_fmask;      /* ``fragoff'' calc of frag offsets */
             int32_t  fs_bshift;     /* ``lblkno'' calc of logical blkno */
             int32_t  fs_fshift;     /* ``numfrags'' calc number of frags */
     /* these are configuration parameters */
             int32_t  fs_maxcontig;  /* max number of contiguous blks */
             int32_t  fs_maxbpg;     /* max number of blks per cyl group */
     /* these fields can be computed from the others */
             int32_t  fs_fragshift;  /* block to frag shift */
             int32_t  fs_fsbtodb;    /* fsbtodb and dbtofsb shift constant */
             int32_t  fs_sbsize;     /* actual size of super block */
             int32_t  fs_csmask;     /* csum block offset */
             int32_t  fs_csshift;    /* csum block number */
             int32_t  fs_nindir;     /* value of NINDIR */
             int32_t  fs_inopb;      /* value of INOPB */
             int32_t  fs_nspf;       /* value of NSPF */
     /* yet another configuration parameter */
             int32_t  fs_optim;      /* optimization preference, see below */
     /* these fields are derived from the hardware */
             int32_t  fs_npsect;     /* # sectors/track including spares */
             int32_t  fs_interleave; /* hardware sector interleave */
             int32_t  fs_trackskew;  /* sector 0 skew, per track */
     /* fs_id takes the space of the unused fs_headswitch and fs_trkseek fields */
             int32_t fs_id[2];       /* unique filesystem id*/
     /* sizes determined by number of cylinder groups and their sizes */
             ufs_daddr_t fs_csaddr;  /* blk addr of cyl grp summary area */
             int32_t  fs_cssize;     /* size of cyl grp summary area */
             int32_t  fs_cgsize;     /* cylinder group size */
     /* these fields are derived from the hardware */
             int32_t  fs_ntrak;      /* tracks per cylinder */
             int32_t  fs_nsect;      /* sectors per track */
             int32_t  fs_spc;        /* sectors per cylinder */
     /* this comes from the disk driver partitioning */
             int32_t  fs_ncyl;       /* cylinders in file system */
     /* these fields can be computed from the others */
             int32_t  fs_cpg;        /* cylinders per group */
             int32_t  fs_ipg;        /* inodes per group */
             int32_t  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 */
             int8_t   fs_fmod;       /* super block modified flag */
             int8_t   fs_clean;      /* file system is clean flag */
             int8_t   fs_ronly;      /* mounted read-only flag */
             int8_t   fs_flags;      /* currently unused flag */
             u_char   fs_fsmnt[MAXMNTLEN];   /* name mounted on */
     /* these fields retain the current block allocation info */
             int32_t  fs_cgrotor;    /* last cg searched */
             struct  csum *fs_csp[MAXCSBUFS];/* list of fs_cs info buffers */
             int32_t  *fs_maxcluster;/* max cluster in each cyl group */
             int32_t  fs_cpc;        /* cyl per cycle in postbl */
             int16_t  fs_opostbl[16][8];     /* old rotation block list head */
             int32_t  fs_sparecon[50];       /* reserved for future constants */
             int32_t  fs_contigsumsize;      /* size of cluster summary array */
             int32_t  fs_maxsymlinklen;/* max length of an internal symlink */
             int32_t  fs_inodefmt;   /* format of on-disk inodes */
             u_int64_t fs_maxfilesize;/* maximum representable file size */
             int64_t  fs_qbmask;     /* ~fs_bmask for use with 64-bit size */
             int64_t  fs_qfmask;     /* ~fs_fmask for use with 64-bit size */
             int32_t  fs_state;      /* validate fs_clean field */
             int32_t  fs_postblformat;/* format of positional layout tables */
             int32_t  fs_nrpos;      /* number of rotational positions */
             int32_t  fs_postbloff;  /* (u_int16) rotation block list head */
             int32_t  fs_rotbloff;   /* (u_int8) blocks for each rotation */
             int32_t  fs_magic;      /* magic number */
             u_int8_t fs_space[1];   /* list of blocks for each rotation */
     /* actually longer */
     };

     /*
      * Filesystem identification
      */
     #define FS_MAGIC        0x011954   /* the fast filesystem magic number */
     #define FS_OKAY         0x7c269d38 /* superblock checksum */
     #define FS_42INODEFMT   -1         /* 4.2BSD inode format */
     #define FS_44INODEFMT   2          /* 4.4BSD inode format */
     /*
      * Preference for optimization.
      */
     #define FS_OPTTIME      0       /* minimize allocation time */
     #define FS_OPTSPACE     1       /* minimize disk fragmentation */

     /*
      * Rotational layout table format types
      */
     #define FS_42POSTBLFMT          -1  /* 4.2BSD rotational table format */
     #define FS_DYNAMICPOSTBLFMT     1   /* dynamic rotational table format */

     Each disk drive contains some number of file systems.  A file system con-
     sists 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 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 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 sys-
     tem 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 fs_minfree element gives the minimum acceptable percentage of file
     system blocks that may be free.  If the freelist drops below this level
     only the super-user may continue to allocate blocks.  The fs_minfree ele-
     ment may be set to 0 if no reserve of free blocks is deemed necessary,
     however severe performance degradations will be observed if the file sys-
     tem 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 8,
     thus the default fragment size is an eighth of the block size.

     The element fs_optim specifies whether the file system should try to min-
     imize the time spent allocating blocks, or if it should attempt to mini-
     mize the space fragmentation on the disk.  If the value of fs_minfree
     (see above) is less than 10%, then the file system defaults to optimizing
     for space to avoid running out of full sized blocks.  If the value of
     minfree is greater than or equal to 10%, fragmentation is unlikely to be
     problematical, and the file system defaults to optimizing for time.

     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.  With the default
     of 8 distinguished rotational positions, the resolution of the summary
     information is 2ms for a typical 3600 rpm drive.

     The element fs_rotdelay gives the minimum number of milliseconds to ini-
     tiate another disk transfer on the same cylinder.  It is used in deter-
     mining 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.

     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 indi-
     rection.  MINBSIZE must be big enough to hold a cylinder group block,
     thus changes to (struct cg) must keep its size within MINBSIZE.  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.  For a 4096 byte block size, 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.

     The Super-block for a file system: The size of the rotational layout
     tables 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 rota-
     tional layout tables is derived from the number of bytes remaining in
     (struct fs).

     The number of blocks of data per cylinder group is limited because cylin-
     der groups are at most one block.  The inode and free block tables must
     fit into a single block after deducting space for the cylinder group
     structure (struct cg).

     The 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 <ufs/ufs/inode.h>.

HISTORY
     A super-block structure named filsys appeared in Version 6 AT&T UNIX.
     The file system described in this manual appeared in 4.2BSD.

FreeBSD 4.10                    April 19, 1994                    FreeBSD 4.10

NAME | SYNOPSIS | DESCRIPTION | HISTORY

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