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FS(5)                       BSD File Formats Manual                      FS(5)

NAME
     fs, inode -- format of file system volume

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

DESCRIPTION
     The files <ufs/ffs/fs.h> and <ufs/ufs/inode.h> declare several structures
     and define variables and macros which are used to create and manage the
     underlying format 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>:

     #define FS_MAGIC 0x011954
     struct fs {
             int32_t  fs_firstfield; /* historic file system linked list, */
             int32_t  fs_unused_1;   /*     used for incore super blocks */
             int32_t  fs_sblkno;     /* addr of super-block / frags */
             int32_t  fs_cblkno;     /* offset of cyl-block / frags */
             int32_t  fs_iblkno;     /* offset of inode-blocks / frags */
             int32_t  fs_dblkno;     /* offset of first data / frags */
             int32_t  fs_cgoffset;   /* cylinder group offset in cylinder */
             int32_t  fs_cgmask;     /* used to calc mod fs_ntrak */
             int32_t  fs_ffs1_time;  /* last time written */
             int32_t  fs_ffs1_size;  /* # of blocks in fs / frags */
             int32_t  fs_ffs1_dsize; /* # of data blocks in fs */
             int32_t  fs_ncg;        /* # of cylinder groups */
             int32_t  fs_bsize;      /* size of basic blocks / bytes */
             int32_t  fs_fsize;      /* size of frag blocks / bytes */
             int32_t  fs_frag;       /* # of frags in a block in fs */
     /* these are configuration parameters */
             int32_t  fs_minfree;    /* minimum percentage of free blocks */
             int32_t  fs_rotdelay;   /* # 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 # of frags */
     /* these are configuration parameters */
             int32_t  fs_maxcontig;  /* max # of contiguous blks */
             int32_t  fs_maxbpg;     /* max # 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 (now unused) */
             int32_t  fs_csshift;    /* csum block number (now unused) */
             int32_t  fs_nindir;     /* value of NINDIR */
             int32_t  fs_inopb;      /* inodes per file system block */
             int32_t  fs_nspf;       /* DEV_BSIZE sectors per frag */
     /* yet another configuration parameter */
             int32_t  fs_optim;      /* optimization preference, see below */
     /* these fields are derived from the hardware */
             int32_t  fs_npsect;     /* DEV_BSIZE sectors/track + spares */
             int32_t  fs_interleave; /* DEV_BSIZE sector interleave */
             int32_t  fs_trackskew;  /* sector 0 skew, per track */
     /* fs_id takes the space of 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 */
             int32_t  fs_ffs1_csaddr; /* blk addr of cyl grp summary area */
             int32_t  fs_cssize;     /* cyl grp summary area size / bytes */
             int32_t  fs_cgsize;     /* cyl grp block size / bytes */
     /* these fields are derived from the hardware */
             int32_t  fs_ntrak;      /* tracks per cylinder */
             int32_t  fs_nsect;      /* DEV_BSIZE sectors per track */
             int32_t  fs_spc;        /* DEV_BSIZE 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_ffs1_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_ffs1_flags; /* see FS_ below */
             u_char   fs_fsmnt[MAXMNTLEN]; /* name mounted on */
             u_char   fs_volname[MAXVOLLEN]; /* volume name */
             u_int64_t fs_swuid;     /* system-wide uid */
             int32_t  fs_pad;        /* due to alignment of fs_swuid */
     /* these fields retain the current block allocation info */
             int32_t  fs_cgrotor;     /* last cg searched */
             void    *fs_ocsp[NOCSPTRS]; /* padding; was list of fs_cs bufs */
             u_int8_t *fs_contigdirs; /* # of contiguously allocated dirs */
             struct csum *fs_csp;     /* cg summary info buffer for fs_cs */
             int32_t *fs_maxcluster;  /* max cluster in each cyl group */
             u_char  *fs_active;      /* reserved for snapshots */
             int32_t  fs_cpc;         /* cyl per cycle in postbl */
     /* this area is only allocated if fs_ffs1_flags & FS_FLAGS_UPDATED */
             int32_t  fs_maxbsize;   /* maximum blocking factor permitted */
             int64_t  fs_spareconf64[17]; /* old rotation block list head */
             int64_t  fs_sblockloc;  /* offset of standard super block */
             struct csum_total fs_cstotal; /* cylinder summary information */
             int64_t  fs_time;       /* time last written */
             int64_t  fs_size;       /* number of blocks in fs */
             int64_t  fs_dsize;      /* number of data blocks in fs */
             int64_t  fs_csaddr;     /* blk addr of cyl grp summary area */
             int64_t  fs_pendingblocks; /* blocks in process of being freed */
             int32_t  fs_pendinginodes; /* inodes in process of being freed */
             int32_t  fs_snapinum[FSMAXSNAP];/* space reserved for snapshots */
     /* back to stuff that has been around a while */
             int32_t  fs_avgfilesize; /* expected average file size */
             int32_t  fs_avgfpdir;    /* expected # of files per directory */
             int32_t  fs_sparecon[26];/* reserved for future constants */
             u_int32_t fs_flags;     /* see FS_ flags below */
             int32_t  fs_fscktime;    /* last time fsck(8)ed */
             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 quad size */
             int64_t  fs_qfmask;      /* ~fs_fmask - for use with quad 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 */
     };

     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 option-
     ally 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 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 frag-
     ment 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 (inode
     1 is no longer used for this purpose; however, numerous dump tapes make
     this assumption, so we are stuck with it).  Thus the root inode is 2.

     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 superuser may continue to allocate blocks.  The fs_minfree ele-
     ment may be set to 0 if no reserve of free blocks is deemed necessary,
     although severe performance degradations will be observed if the file
     system is run at greater than 95% full; thus the default value of
     fs_minfree is 5%.

     Empirically the best trade-off between block fragmentation and overall
     disk utilization at a loading of 95% 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 (FS_OPTTIME), or if it should
     attempt to minimize the space fragmentation on the disk (FS_OPTSPACE).
     If the value of fs_minfree (see above) is less than 5%, then the file
     system defaults to optimizing for space to avoid running out of full
     sized blocks.  If the value of fs_minfree is greater than or equal to 5%,
     fragmentation is unlikely to be problematical, and the file system
     defaults to optimizing for time.

     The fs_flags element specifies how the filesystem was mounted:

        FS_DOSOFTDEP  The filesystem was mounted using soft dependencies.
        FS_UNCLEAN    The filesystem was mounted uncleanly.

   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 min-
     imum rotational latency.  With the default of 1 distinct rotational posi-
     tion, the resolution of the summary information is 16ms for a typical
     3600 RPM drive.

     The element fs_rotdelay was once used to tweak block layout.

     Each file system has a statically allocated number of inodes, determined
     by its size and the desired number of file data bytes per inode at the
     time it was created.  See newfs(8) for details on how to set this (and
     other) filesystem parameters.  By default, 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.

     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 (of size fs_cssize) in addition to the super-block.

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

   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 rotational 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.

   Inodes
     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.

BSD                             March 23, 2017                             BSD