4.3BSD-UWisc/man/cat5/fs.5
FS(5) UNIX Programmer's Manual FS(5)
NAME
fs, inode - format of file system volume
SYNOPSIS
#include <sys/types.h>
#include <sys/fs.h>
#include <sys/inode.h>
DESCRIPTION
Every file system storage volume (disk, nine-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 beginning at BBLOCK and continuing for BBSIZE are
used to contain primary and secondary bootstrapping pro-
grams.
The actual file system begins at sector SBLOCK with the
_s_u_p_e_r _b_l_o_c_k that is of size SBSIZE. The layout of the super
block as defined by the include file <_s_y_s/_f_s._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 */
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FS(5) UNIX Programmer's Manual FS(5)
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_optim; /* optimization preference, see below */
long fs_sparecon[5]; /* 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 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_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 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.
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FS(5) UNIX Programmer's Manual FS(5)
Addresses stored in inodes are capable of addressing frag-
ments 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 _l_o_s_t+_f_o_u_n_d directory is given the next avail-
able inode when it is initially created by _m_k_f_s.
_f_s__m_i_n_f_r_e_e 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. This 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 _f_s__m_i_n_f_r_e_e 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.
_f_s__o_p_t_i_m 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 optim-
izing for time.
_C_y_l_i_n_d_e_r _g_r_o_u_p _r_e_l_a_t_e_d _l_i_m_i_t_s: Each cylinder keeps track of
the availability of blocks at different rotational
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positions, so that sequential blocks can be laid out with
minimum rotational latency. NRPOS is the number of rota-
tional positions which are distinguished. With NRPOS 8 the
resolution of the summary information is 2ms for a typical
3600 rpm drive.
_f_s__r_o_t_d_e_l_a_y gives the minimum number of milliseconds to ini-
tiate 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 _f_s__r_o_t_d_e_l_a_y 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 conserva-
tive.
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).
N.B.: 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 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. MAXCPG is
limited only to dimension an array 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 main-
tained in _f_s__f_s_m_n_t. 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
two million cylinders.
Per cylinder group information is summarized in blocks allo-
cated from the first cylinder group's data blocks. These
blocks are read in from _f_s__c_s_a_d_d_r (size _f_s__c_s_s_i_z_e) in addi-
tion to the super block.
N.B.: sizeof (struct csum) must be a power of two in order
for the ``fs_cs'' macro to work.
_S_u_p_e_r _b_l_o_c_k _f_o_r _a _f_i_l_e _s_y_s_t_e_m: 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.
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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 ( _f_s__c_p_c).
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).
_I_n_o_d_e: 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 <_s_y_s/_i_n_o_d_e._h>.
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