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The
files and 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 and continuing for are used for a disklabel and for some hardware
primary and secondary bootstrapping programs. The actual file system begins
at sector with the that is of size The following structure described
the super-block and is from the file #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 can be optionally
broken into 2, 4, or 8 pieces, each of which is addressable; these pieces
may be or some multiple of a 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 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 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 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 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 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.
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 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 is 2ms. Each file system has a statically allocated number of
inodes. An inode is allocated for each bytes of disk space. The inode allocation
strategy is extremely conservative. is the smallest allowable block size.
With a of 4096 it is possible to create files of size 2^32 with only two
levels of indirection. must be big enough to hold a cylinder group block,
thus changes to must keep its size within Note that super-blocks are never
more than size The path name on which the file system is mounted is maintained
in 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 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 (size in addition to the super-block. must be a power
of two in order for the macro to work. The The size of the rotational
layout tables is limited by the fact that the super-block is of size The
size of these tables is 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 The size of the rotational layout tables is derived from
the number of bytes remaining in 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 The 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
A super-block structure named filsys appeared
in The file system described in this manual appeared in
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