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DESCRIPTION

The block size and number of blocks which comprise a file system are parameters 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 <vfs/ufs/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 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 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 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 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 element may be set to 0 if no reserve of free blocks is deemed necessary, however severe performance 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 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 minimize the time spent allocating blocks, or if it should attempt to minimize 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 availability 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 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.

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. 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 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 cylinder 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 <vfs/ufs/inode.h>.