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:

#defineFS_MAGIC0x011954
structfs {
structfs *fs_link;/* linked list of file systems */
structfs *fs_rlink;/* used for incore super blocks */
daddr_tfs_sblkno;/* addr of super block in filesys */
daddr_tfs_cblkno;/* offset of cyl-block in filesys */
daddr_tfs_iblkno;/* offset of inode-blocks in filesys */
daddr_tfs_dblkno;/* offset of first data after cg */
longfs_cgoffset;/* cylinder group offset in cylinder */
longfs_cgmask;/* used to calc mod fs_ntrak */
time_t fs_time;    /* last time written */
longfs_size;/* number of blocks in fs */
longfs_dsize;/* number of data blocks in fs */
longfs_ncg;/* number of cylinder groups */
longfs_bsize;/* size of basic blocks in fs */
longfs_fsize;/* size of frag blocks in fs */
longfs_frag;/* number of frags in a block in fs */
/* these are configuration parameters */
longfs_minfree;/* minimum percentage of free blocks */
longfs_rotdelay;/* num of ms for optimal next block */
longfs_rps;/* disk revolutions per second */
/* these fields can be computed from the others */
longfs_bmask;/* “blkoff” calc of blk offsets */
longfs_fmask;/* “fragoff” calc of frag offsets */
longfs_bshift;/* “lblkno” calc of logical blkno */
longfs_fshift;/* “numfrags” calc number of frags */
/* these are configuration parameters */
longfs_maxcontig;/* max number of contiguous blks */
longfs_maxbpg;/* max number of blks per cyl group */
/* these fields can be computed from the others */
longfs_fragshift;/* block to frag shift */
longfs_fsbtodb;/* fsbtodb and dbtofsb shift constant */
longfs_sbsize;/* actual size of super block */
longfs_csmask;/* csum block offset */
longfs_csshift;/* csum block number */
longfs_nindir;/* value of NINDIR */
longfs_inopb;/* value of INOPB */
longfs_nspf;/* value of NSPF */
longfs_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 */
longfs_cssize;/* size of cyl grp summary area */
longfs_cgsize;/* cylinder group size */
/* these fields should be derived from the hardware */
longfs_ntrak;/* tracks per cylinder */
longfs_nsect;/* sectors per track */
long  fs_spc;   /* sectors per cylinder */
/* this comes from the disk driver partitioning */
longfs_ncyl;   /* cylinders in file system */
/* these fields can be computed from the others */
longfs_cpg;/* cylinders per group */
longfs_ipg;/* inodes per group */
longfs_fpg;/* blocks per group * fs_frag */
/* this data must be recomputed after crashes */
structcsum 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 */
charfs_fsmnt[MAXMNTLEN];
/* name mounted on */
/* these fields retain the current block allocation info */
longfs_cgrotor;/* last cg searched */
structcsum *fs_csp[MAXCSBUFS];
/* list of fs_cs info buffers */
longfs_cpc;/* cyl per cycle in postbl */
shortfs_postbl[MAXCPG][NRPOS];
/* head of blocks for each rotation */
longfs_magic;/* magic number */
u_charfs_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. 

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 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 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 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. 

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.  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 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 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 two 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 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. sizeof (struct csum) must be a power of two 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 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).

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 information, see the include file <sys/inode.h>.