ffsfs(0) CLIX ffsfs(0)
NAME
ffsfs - Format of a FFS file system volume
SYNOPSIS
#include <sys/types.h>
#include <sys/fs/ffsfs.h>
#include <sys/fs/ffsinode.h>
DESCRIPTION
The Fast File System (FFS) file system storage volume (disk or 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.
The actual file system begins at sector SBLOCK with the superblock that is
of size SBSIZE. The layout of the superblock as defined by the
<sys/fs/ffsfs.h> include file is as follows:
#define FS_MAGIC 0x011954
struct fs {
struct fs *fs_link; /* linked list of file systems */
struct fs *fs_rlink; /* used for incore superblocks */
daddr_t fs_sblkno; /* addr of superblock 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 */
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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 superblock */
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 */
long fs_clean; /* file system is clean flag */
char fs_fmod; /* superblock modified flag */
char fs_clean; /* unused byte */
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 */
char fs_fname[6]; /* used by labelit */
char fs_fpack[6]; /* used by labelit */
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.
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A file system is described by its superblock, which in turn describes the
cylinder groups. The superblock is critical data and is replicated in
each cylinder group to protect against catastrophic loss. This is done at
file system creation time. The critical superblock data does not change,
so the copies need not be referenced further unless the primary superblock
becomes corrupted.
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 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. (Inode 1 is no longer used for this purpose.
However, numerous dump tapes make this assumption, so the value must
remain.) The lost+found directory is given the next available inode when
it is initially created by the ffsmkfs command.
The fs_minfree field specifies the minimum acceptable percentage of file
system blocks that may be free. If the freelist drops below this level
only the superuser 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 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.
The fs_optim parameter 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%, the file system defaults to optimizing for
space to avoid running out of full sized blocks. If the value of
fs_minfree is greater than or equal to 10%, fragmentation is unlikely to
be a problem, and the file system defaults to optimizing for time.
Cylinder group related limits
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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. NRPOS is the number of rotational positions
which are distinguished. With NRPOS 8 the resolution of the summary
information is two milliseconds for a typical 3600 rpm drive.
The fs_rotdelay parameter specifies 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 two milliseconds.
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.
MAXIPG bounds the number of inodes per cylinder group, and is needed only
to keep the structure simpler by having 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 two levels of
indirection. MINBSIZE must be big enough to hold a cylinder group block.
Thus changes to the cg structure must keep its size within MINBSIZE.
MAXCPG is limited only to dimension an array in the cg structure; it can
be made larger as long as that structure's size remains within the bounds
dictated by MINBSIZE. Note that superblocks are never more than size
SBSIZE.
The pathname 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. 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 superblock.
The size of the csum structure must be a power of two in order for the
fs_cs macro to work.
Superblock for a file system
MAXBPC bounds the size of the rotational layout tables and is limited by
the fact that the superblock 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 the fs structure.
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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 the cg structure.
Inode
The inode is the focus of all file activity in the CLIX 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.
RELATED INFORMATION
File: ffsinode(0)
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