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

     fs, inode -- format of file system	volume

     #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>

     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 pa-
     rameters of the file system.  Sectors beginning at	BBLOCK and continuing
     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

     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

     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 de-
     termine 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 ta-
     bles 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 be-
     fore the rotational pattern repeats (fs_cpc).  The	size of	the rotational
     layout tables is derived from the number of bytes remaining in (struct

     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 in-
     ode is `named' by its device/i-number pair.  For further information, see
     the include file <ufs/ufs/inode.h>.

     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				April 19, 1994				   BSD


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