fs(4) fs(4)
NAME
fs - format of file system volume
SYNOPSIS
#include <<<<sys/types.h>>>>
#include <<<<sys/param.h>>>>
#include <<<<sys/fs.h>>>>
#include <<<<sys/inode.h>>>>
#include <<<<sys/ino.h>>>>
#include <<<<sys/sysmacros.h>>>>
DESCRIPTION
Every file system storage volume has a common format for certain vital
information. The first 8 kbytes on a volume contain a volume header
which identifies that volume as a Logical Interchange Format (LIF)
volume. Such volume may be divided into a number of sections.
Each section can contain a file system. The first 8 kbytes in each
section is ignored, except where it coincides with the volume header
discussed above. The actual file system begins next with the "super
block." The layout of the super block as defined by the include file
<sys/fs.h> is:
#define FS_MAGIC 0x011954
#define FS_MAGIC_LFN 0x095014
#define FS_CLEAN 0x17
#define FS_OK 0x53
#define FS_NOTOK 0x31
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 */
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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 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 per rotation */
long fs_magic; /* magic number */
char fs_fname[6]; /* name of file system */
char fs_fpack[6]; /* pack name of file system */
u_char fs_rotbl[1]; /* list of blocks for each rotation */
/* actually longer */
};
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 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 smaller pieces, each of which is addressable;
these pieces may be DEV_BSIZE, or some multiple of a DEV_BSIZE unit
(DEV_BSIZE is defined in <sys/param.h>).
Large files consist of exclusively large data blocks. To avoid undue
wasted disk space, the last data block of a file is allocated only as
many fragments of a large block as are necessary, if that file is
small enough to not require indirect data blocks. 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.
I-numbers begin at 0. Inodes 0 and 1 are reserved. Inode 2 is used
for the root directory of the file system. 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 can be free. If the freelist drops below this level, only
the super-user may continue to allocate blocks. This can be set to 0
if no reserve of free blocks is deemed necessary. However, severe
performance degradations result if the file system is run at greater
than 90% full; thus the default value of fs_minfree is 10%.
The best trade-off between block fragmentation and overall disk
utilization and performance varies for each intended use of the file
system. Suggested values can be found in the system administrator's
manual for each implementation.
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. NRPOS is the number of rotational
positions which are distinguished. For example, with NRPOS 8 the
resolution of the summary information is 2ms for a typical 3600 rpm
drive.
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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. Suggested values of fs_rotdelay
for different disks can be found in the system administrator's manual.
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).
Important Note: 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 MINBSIZE must always be greater than sizeof(struct cg).
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 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.
Important Note: sizeof (struct csum) must be a power of two in order
for the fs_cs macro to work.
The two possible values for fs_magic are FS_MAGIC, the default magic
number for an HFS file system with a fixed-size directory format that
limits file name length to DIRSIZ (14), and FS_MAGIC_LFN, the magic
number of a file system using a variable-size directory format that
supports file names of up to MAXNAMLEN (255) characters in length.
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 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).
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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 (struct
cg).
inode:
The inode is the focus of all file activity in the HP-UX file system.
There is a unique inode allocated for each active file, each
continuation inode, each current directory, each mounted-on file, text
file, and the root. An inode is ``named'' by its device-and-i-number
pair. For the format of an inode and its flags, see inode(4).
DEPENDENCIES
Series 700
Series 700 systems support only one section per volume. Thus, there
can only be one file system on each volume and the first 8 Kbytes of a
file system is the boot area. This area contains the LIF volume
header, the directory that defines the contents of the volume, and the
bootstrapping program.
AUTHOR
fs was developed by HP and the University of California, Berkeley.
SEE ALSO
inode(4), lif(4).
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