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1.1 root 1: .TH FILSYS 5
2: .CT 2 sa
3: .SH NAME
4: filsys, flblk, ino \- format of a disk file system
5: .SH SYNOPSIS
6: .B #include <sys/types.h>
7: .br
8: .B #include <sys/fblk.h>
9: .br
10: .B #include <sys/filsys.h>
11: .br
12: .B #include <sys/ino.h>
13: .SH DESCRIPTION
14: Every
15: file system is divided into a certain number
16: of blocks of 1K or 4K bytes, as determined by
17: the predicate
18: .L BITFS()
19: applied to the minor device number
20: where the file system is mounted.
21: Block 0 is unused and is available to contain
22: a bootstrap program, pack label, or other information.
23: .PP
24: Block 1 is the
25: `super-block'.
26: Its layout is defined in
27: .LR <sys/filsys.h> :
28: .PP
29: .EX
30: .ta \w'#define 'u +\w's_cylsize 'u +\w'123 'u +\w'daddr_t 'u
31: struct filsys {
32: unsigned short s_isize;
33: daddr_t s_fsize;
34: short s_ninode;
35: ino_t s_inode[NICINOD];
36: char s_flock;
37: char s_ilock;
38: char s_fmod;
39: char s_ronly;
40: time_t s_time;
41: daddr_t s_tfree;
42: ino_t s_tinode;
43: short s_dinfo[2];
44: #define s_m s_dinfo[0]
45: #define s_n s_dinfo[1]
46: #define s_cylsize s_dinfo[0]
47: #define s_aspace s_dinfo[1]
48: char s_fsmnt[14];
49: ino_t s_lasti;
50: ino_t s_nbehind;
51: union {
52: struct {
53: short S_nfree;
54: daddr_t S_free[NICFREE];
55: } R;
56: struct {
57: char S_valid;
58: #define BITMAP 961
59: long S_bfree[BITMAP];
60: } B;
61: struct {
62: char S_valid;
63: char S_flag; /* 1 means bitmap not in S_bfree */
64: long S_bsize; /* size of bitmap blocks */
65: struct buf * S_blk[BITMAP-1];
66: } N;
67: } U;
68: };
69: #define s_nfree U.R.S_nfree
70: #define s_free U.R.S_free
71: #define s_valid U.B.S_valid
72: #define s_bfree U.B.S_bfree
73: .EE
74: .TF s_isize
75: .TP
76: .B s_isize
77: The address of the first block after the i-list,
78: which starts in block 2.
79: Thus the i-list is
80: .LR s_isize-2
81: blocks long.
82: .PD
83: .TP
84: .B s_fsize
85: The address of the first block not in the file system.
86: .TP
87: .B s_inode
88: Array of free inode numbers.
89: .TP
90: .B s_ninode
91: The number of free i-numbers in the
92: .L s_inode
93: array.
94: Inodes are placed in the list in LIFO order.
95: If the list underflows, it is replenished by
96: searching the i-list
97: to obtain the numbers of free inodes.
98: When the list is full,
99: freed inodes are not recorded in
100: .LR s_inode .
101: .TP
102: .B s_lasti
103: Where the last search for free inodes ended.
104: .TP
105: .B s_nbehind
106: Number of free inodes before
107: .L s_lasti
108: that are not listed in
109: .LR s_inode .
110: The system will search forward for free inodes from
111: .L s_lasti
112: for more inodes unless
113: .L s_nbehind
114: is sufficiently large, in which case it will search the
115: i-list from the beginning.
116: .TP
117: .B s_flock
118: .br
119: .ns
120: .TP
121: .B s_ilock
122: Flags maintained in the core
123: copy of the super-block while the file system
124: while it is mounted.
125: The values on disk are immaterial.
126: .TP
127: .B s_fmod
128: Flag to indicate that the super-block has
129: changed and should be copied to
130: the disk during the next periodic update of file
131: system information.
132: The value on disk is immaterial.
133: .TP
134: .B s_ronly
135: Flag for read-only file system.
136: The value on disk is immaterial.
137: .TP
138: .B s_time
139: Time of the last change to the super block.
140: .TP
141: .B s_dinfo
142: Disk interleave information:
143: .BR s_cylsize =
144: blocks per cylinder,
145: .BR s_aspace =
146: blocks to skip; see
147: .IR fsck (8).
148: .TP
149: .B s_fsmnt
150: Unused.
151: .TP
152: .B s_tfree
153: .br
154: .ns
155: .TP
156: .B s_tinode
157: Numbers of free blocks and free inodes.
158: Maintained for the benefit of
159: .IR df
160: (see
161: .IR du (1)),
162: these values are otherwise irrelevant.
163: .PD
164: .PP
165: Different data are used to manage free space in 1K and 4K file systems.
166: These fields are for 1K file systems:
167: .TF s_isize
168: .TP
169: .B s_free
170: An array of free block numbers.
171: .LR s_free[0]
172: is the block address of the next
173: in a chain of blocks constituting the free list.
174: The layout of these blocks is defined in
175: .LR <sys/fblk.h> :
176: .PD
177: .LP
178: .EX
179: struct fblk {
180: int df_nfree;
181: daddr_t df_free[NICFREE];
182: }
183: .EE
184: .ns
185: .IP
186: where
187: .L df_nfree
188: and
189: .L df_free
190: are exactly like
191: .L s_nfree
192: and
193: .L s_free.
194: .TP
195: .B s_nfree
196: Blocks given in
197: .L s_free[1]
198: through
199: .L s_free[s_nfree-1]
200: are available for allocation.
201: Blocks are allocated in LIFO fashion from this list.
202: If freeing would cause
203: the array to overflow,
204: it is cleared by copying into the newly freed block,
205: which is pushed onto the free chain.
206: If allocation would cause underflow,
207: the array is replenished from the next block on the chain.
208: .PP
209: These are for 4K file systems:
210: .TF s_isize
211: .TP
212: .B s_bfree
213: a bit array specifying the free blocks of a 4K file system.
214: The bit
215: .LR (s_bfree[i/w]>>(i%w))&1 ,
216: where
217: .I w
218: is the bit size of a long,
219: is nonzero if the
220: .IR i th
221: data block is free. If the file system is too large for the bitmap
222: to fit here, then it is stored at the end of the file system, and
223: locked into memory when the file system is mounted. The
224: .B N
225: variant of the union is used by the kernel in this case.
226: .PD
227: .TP
228: .B s_valid
229: The bitmap of a mounted file system is maintained only in main memory;
230: the bitmap on the medium is marked invalid by setting
231: .L s_valid
232: to zero.
233: Unmounting
234: updates the medium copy and sets
235: .L s_valid
236: to 1.
237: A file system with invalid bitmap may be mounted
238: read-only; its bitmap can be corrected by
239: .IR chuck (8).
240: .PP
241: I-numbers begin at 1, and the storage for inodes
242: begins in block 2.
243: Inodes are 64 bytes long.
244: Inode 2 is reserved for the root directory of the file
245: system, but no other i-number has a built-in
246: meaning.
247: Each inode represents one file.
248: .PP
249: The layout of an inode is defined in
250: .LR <sys/ino.h> :
251: .PP
252: .EX
253: .ta \w'struct 'u +\w'unsigned 'u
254: struct dinode {
255: unsigned short di_mode;
256: short di_nlink;
257: short di_uid;
258: short di_gid;
259: off_t di_size;
260: char di_addr[40];
261: time_t di_atime;
262: time_t di_mtime;
263: time_t di_ctime;
264: };
265: .EE
266: .TF di_nlink
267: .TP
268: .B di_mode
269: The kind of file; it
270: is encoded as
271: .L st_mode field of
272: .IR stat (2),
273: and is 0 for a free inode.
274: .PD
275: .TP
276: .B di_nlink
277: The number of directory entries
278: (links) that refer to this inode
279: .TP
280: .B di_uid
281: Owner's userid.
282: .TP
283: .B di_gid
284: Owner's groupid.
285: .TP
286: .B di_size
287: Number of bytes in the file.
288: .TP
289: .B di_atime
290: Time of last access; see
291: .IR times (2).
292: .TP
293: .B di_mtime
294: Time of last modification.
295: .TP
296: .B di_ctime
297: Time of last change to inode or contents.
298: .TP
299: .B di_addr
300: For special files the first two bytes of
301: .L di_addr
302: contain the device code; see
303: .IR intro (4)
304: and
305: .IR types (5).
306: .PD
307: .IP
308: For plain files and directories
309: .L di_addr
310: contains block numbers packed into 3 bytes each.
311: The first 10 numbers specify device blocks directly.
312: The last 3 are singly, doubly, and triply
313: indirect and point to blocks of block pointers of type
314: .L daddr_t
315: (see
316: .IR types (5)).
317: A zero pointer indicates a `hole'
318: where no data has been written.
319: Holes read as if they contained all zeros.
320: .PP
321: A symbolic link is, aside from mode,
322: a plain file whose sole content is the name of the file linked to.
323: .SH "SEE ALSO"
324: .IR chuck (8),
325: .IR fsck (8),
326: .IR icheck (8),
327: .IR dir (5),
328: .IR mount (8),
329: .IR stat (2),
330: .IR types (5),
331: .IR l3tol (3)
332:
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