fs(5) File Formats Manual fs(5)
Name
fs, inode - format of file system volume
Syntax
#include <sys/types.h>
#include <sys/fs.h>
#include <sys/inode.h>
Description
Every file system storage volume (disk, 9-track tape, for instance) has a common format for certain vital information. Every such volume
is divided into a certain number of blocks. The block size is a parameter of the file system. Sectors 0 to 15 on a file system are used
to contain primary and secondary bootstrapping programs.
The actual file system begins at sector 16 with the super block. The layout of the super block as defined by the include file <sys/fs.h>
is:
#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 */
long fs_sparecon[6]; /* reserved for future constants */
/* 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 should be 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 recomputed 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_postbl[MAXCPG][NRPOS];
/* head of blocks for each rotation */
long fs_magic; /* magic number */
u_char fs_rotbl[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 repli-
cated 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 can 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 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 cannot be used for normal purposes and historically bad blocks were linked to inode
1; thus the root inode is 2. (Although inode 1 is no longer used for this purpose, numerous dump tapes make this assumption.) The
lost+found directory is given the next available inode when it is initially created by mkfs.
fs_minfree gives the minimum acceptable percentage of file system blocks that may be free. If the freelist drops below this level, only the
superuser can continue to allocate blocks. This can be set to 0 if no reserve of free blocks is deemed necessary; however, severe perfor-
mance 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
4. Thus, the default fragment size is a fourth of the block size.
Cylinder group related limits: Each cylinder keeps track of the availability of blocks at different rotational positions, so that sequen-
tial blocks can be laid out with minimum rotational latency. NRPOS is the number of rotational positions which are distinguished. With
NRPOS 8, the resolution of the summary information is 2ms for a typical 3600 rpm drive.
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 alloca-
tion strategy is extremely conservative.
MAXIPG bounds the number of inodes per cylinder group and is needed only to keep the structure simpler by having the only a single variable
size element (the free bit map). MAXIPG must be a multiple of INOPB(fs).
MINBSIZE is the smallest allowable block size. With a MINBSIZE of 4096, it is possible to create files of size 2^32 with only 2 levels of
indirection. MINBSIZE must be big enough to hold a cylinder group block, so changes to (struct cg) must keep its size within MINBSIZE.
MAXCPG is limited only to the dimension of an array given in (struct cg); it can be made larger as long as that structure's size remains
within the bounds dictated by 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. It is currently parameterized
for a maximum of 2,000,000 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. sizeof (struct csum) must be a power of 2 in order for the ``fs_cs'' macro
to work.
Super block for a file system: MAXBPC bounds the size of the rotational layout tables and 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 2, 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).
MAXBPG bounds the number of blocks of data per cylinder group, and is limited by the fact that cylinder groups are at most one block. The
size of the free block table is derived from the size of blocks and the number of remaining bytes in the cylinder group structure (struct
cg).
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 informa-
tion, see the include file <sys/inode.h>.
fs(5)