NAME
filsys, flblk, ino - format of file system volume
SYNOPSIS
#include <sys/types.h>
#include <sys/flbk.h>
#include <sys/filsys.h>
#include <sys/ino.h>
DESCRIPTION
Every file system storage volume (e.g. RF disk, RK disk, RP disk, DECtape reel) has a common format for certain vital information. Every such volume is divided into a certain number of 512-byte blocks. Block 0 is unused and is available to contain a bootstrap program, pack label, or other information.
Block 1 is the super block. The layout of the super block as defined by the include file <sys/filsys.h> is:
/*
* Structure of the super-block
*/ struct filsys { unsigned short s_isize; /* size in blocks of i-list */ daddr_t s_fsize; /* size in blocks of entire volume */ short s_nfree; /* number of addresses in s_free */ daddr_t s_free[NICFREE];/* free block list */ short s_ninode; /* number of i-nodes in s_inode */ ino_t s_inode[NICINOD];/* free i-node list */ char s_flock; /* lock during free list manipulation */ char s_ilock; /* lock during i-list manipulation */ char s_fmod; /* super block modified flag */ char s_ronly; /* mounted read-only flag */ time_t s_time; /* last super block update */ /* remainder not maintained by this version of the system */ daddr_t s_tfree; /* total free blocks*/ ino_t s_tinode; /* total free inodes */ short s_m; /* interleave factor */ short s_n; /* " " */ char s_fname[6]; /* file system name */ char s_fpack[6]; /* file system pack name */ };
S_isize is the address of the first block after the i-list, which starts just after the super-block, in block 2. Thus is i-list is s_isize-2 blocks long. S_fsize is the address of the first block not potentially available for allocation to a file. These numbers are used by the system to check for bad block addresses; if an `impossible' block address is allocated from the free list or is freed, a diagnostic is written on the on-line console. Moreover, the free array is cleared, so as to prevent further allocation from a presumably corrupted free list.
The free list for each volume is maintained as follows. The s_free array contains, in s_free[1], ... , s_free[s_nfree-1], up to NICFREE free block numbers. NICFREE is a configuration constant. S_free[0] is the block address of the head of a chain of blocks constituting the free list. The layout of each block of the free chain as defined in the include file <sys/fblk.h> is:
struct fblk
{
int df_nfree;
daddr_t df_free[NICFREE];
};
The fields df_nfree and df_free in a free block are used exactly like s_nfree and s_free in the super block. To allocate a block: decrement s_nfree, and the new block number is s_free[s_nfree]. If the new block address is 0, there are no blocks left, so give an error. If s_nfree became 0, read the new block into s_nfree and s_free. To free a block, check if s_nfree is NICFREE; if so, copy s_nfree and the s_free array into it, write it out, and set s_nfree to 0. In any event set s_free[s_nfree] to the freed block's address and increment s_nfree.
S_ninode is the number of free i-numbers in the s_inode array. To allocate an i-node: if s_ninode is greater than 0, decrement it and return s_inode[s_ninode]. If it was 0, read the i-list and place the numbers of all free inodes (up to NICINOD) into the s_inode array, then try again. To free an i-node, provided s_ninode is less than NICINODE, place its number into s_inode[s_ninode] and increment s_ninode. If s_ninode is already NICINODE, don't bother to enter the freed i-node into any table. This list of i-nodes is only to speed up the allocation process; the information as to whether the inode is really free or not is maintained in the inode itself.
S_flock and s_ilock are flags maintained in the core copy of the file system while it is mounted and their values on disk are immaterial. The value of s_fmod on disk is likewise immaterial; it is used as a flag to indicate that the super-block has changed and should be copied to the disk during the next periodic update of file system information. S_ronly is a write-protection indicator; its disk value is also immaterial.
S_time is the last time the super-block of the file system was changed. During a reboot, s_time of the super-block for the root file system is used to set the system's idea of the time.
The fields s_tfree, s_tinode, s_fname and s_fpack are not currently maintained.
I-numbers begin at 1, and the storage for i-nodes begins in block 2. I-nodes are 64 bytes long, so 8 of them fit into a block. I-node 2 is reserved for the root directory of the file system, but no other i-number has a built-in meaning. Each i-node represents one file. The format of an i-node as given in the include file <sys/ino.h> is:
/*
* Inode structure as it appears on
* a disk block.
*/ struct dinode { unsigned short di_mode; /* mode and type of file */ short di_nlink; /* number of links to file */ short di_uid; /* owner's user id */ short di_gid; /* owner's group id */ off_t di_size; /* number of bytes in file */ char di_addr[40]; /* disk block addresses */ time_t di_atime; /* time last accessed */ time_t di_mtime; /* time last modified */ time_t di_ctime; /* time created */ }; #define INOPB 8 /* 8 inodes per block */ /*
* the 40 address bytes:
* 39 used; 13 addresses
* of 3 bytes each.
*/
Di_mode tells the kind of file; it is encoded identically to the st_mode field of stat(2). Di_nlink is the number of directory entries (links) that refer to this i-node. Di_uid and di_gid are the owner's user and group IDs. Size is the number of bytes in the file. Di_atime and di_mtime are the times of last access and modification of the file contents (read, write or create) (see times(2)); Di_ctime records the time of last modification to the inode or to the file, and is used to determine whether it should be dumped.
Special files are recognized by their modes and not by i-number. A block-type special file is one which can potentially be mounted as a file system; a character-type special file cannot, though it is not necessarily character-oriented. For special files, the di_addr field is occupied by the device code (see types(5)). The device codes of block and character special files overlap.
Disk addresses of plain files and directories are kept in the array di_addr packed into 3 bytes each. The first 10 addresses specify device blocks directly. The last 3 addresses are singly, doubly, and triply indirect and point to blocks of 128 block pointers. Pointers in indirect blocks have the type daddr_t (see types(5)).
For block b in a file to exist, it is not necessary that all blocks less than b exist. A zero block number either in the address words of the i-node or in an indirect block indicates that the corresponding block has never been allocated. Such a missing block reads as if it contained all zero words.
SEE ALSO
icheck(1), dcheck(1), dir(5), mount(1), stat(2), types(5)