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Mac OS X / Darwin man pages : fs (5)
fs (5)

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Name

fs, inode - format of file system volume

Synopsis

#include <sys/types.h>
#include <ufs/fs.h>
#include <ufs/inode.h>

Description

The files <fs.h> and <inode.h> declare several structures, defined 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 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 described the super-block and is from the file <ufs/fs.h>:

#define FS_MAGIC 0x011954
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 */
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 */ /* yet another configuration parameter */
long
fs_optim; /* optimization preference, see below */ /* these fields are derived from the hardware */
long
fs_npsect; /* # sectors/track including spares */
long
fs_interleave; /* hardware sector interleave */
long
fs_trackskew; /* sector 0 skew, per track */
long
fs_headswitch; /* head switch time, usec */
long
fs_trkseek; /* track-to-track seek, usec */ /* 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 are 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_opostbl[16][8]; /* old rotation block list head */ long fs_sparecon[56]; /* reserved for future constants */
quad
fs_qbmask; /* ~fs_bmask - for use with quad size */
quad
fs_qfmask; /* ~fs_fmask - for use with quad size */
long
fs_postblformat; /* format of positional layout tables */
long
fs_nrpos; /* number of rotational positions */
long
fs_postbloff; /* (short) rotation block list head */
long
fs_rotbloff; /* (u_char) blocks for each rotation */ long fs_magic; /* magic number */ u_char fs_space[1]; /* list of blocks for each rotation */ /* actually longer */ };

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 superblock 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 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 <sys/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.

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