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1.1 root 1: .\" Copyright (c) 1983 Regents of the University of California.
2: .\" All rights reserved. The Berkeley software License Agreement
3: .\" specifies the terms and conditions for redistribution.
4: .\"
5: .\" @(#)fs.5 6.3 (Berkeley) 5/2/88
6: .\"
7: .TH FS 5 "May 2, 1988"
8: .UC 5
9: .SH NAME
10: fs, inode \- format of file system volume
11: .SH SYNOPSIS
12: .B #include <sys/types.h>
13: .br
14: .B #include <sys/fs.h>
15: .br
16: .B #include <sys/inode.h>
17: .SH DESCRIPTION
18: Every file system storage volume (disk, nine-track tape, for instance)
19: has a common format for certain vital information.
20: Every such volume is divided into a certain number of blocks.
21: The block size is a parameter of the file system.
22: Sectors beginning at BBLOCK and continuing for BBSIZE are used to
23: contain a label and for some hardware
24: primary and secondary bootstrapping programs.
25: .PP
26: The actual file system begins at sector SBLOCK with the
27: .I "super block"
28: that is of size SBSIZE.
29: The layout of the super block as defined by the include file
30: .RI < sys/fs.h >
31: is:
32: .PP
33: .nf
34: .ta \w'\ \ \ \ 'u +\w'daddr_t\ \ 'u +\w'fs_fsmnt[MAXMNTLEN];\ \ 'u
35: #define FS_MAGIC 0x011954
36: struct fs {
37: struct fs *fs_link; /* linked list of file systems */
38: struct fs *fs_rlink; /* used for incore super blocks */
39: daddr_t fs_sblkno; /* addr of super-block in filesys */
40: daddr_t fs_cblkno; /* offset of cyl-block in filesys */
41: daddr_t fs_iblkno; /* offset of inode-blocks in filesys */
42: daddr_t fs_dblkno; /* offset of first data after cg */
43: long fs_cgoffset; /* cylinder group offset in cylinder */
44: long fs_cgmask; /* used to calc mod fs_ntrak */
45: time_t fs_time; /* last time written */
46: long fs_size; /* number of blocks in fs */
47: long fs_dsize; /* number of data blocks in fs */
48: long fs_ncg; /* number of cylinder groups */
49: long fs_bsize; /* size of basic blocks in fs */
50: long fs_fsize; /* size of frag blocks in fs */
51: long fs_frag; /* number of frags in a block in fs */
52: /* these are configuration parameters */
53: long fs_minfree; /* minimum percentage of free blocks */
54: long fs_rotdelay; /* num of ms for optimal next block */
55: long fs_rps; /* disk revolutions per second */
56: /* these fields can be computed from the others */
57: long fs_bmask; /* ``blkoff'' calc of blk offsets */
58: long fs_fmask; /* ``fragoff'' calc of frag offsets */
59: long fs_bshift; /* ``lblkno'' calc of logical blkno */
60: long fs_fshift; /* ``numfrags'' calc number of frags */
61: /* these are configuration parameters */
62: long fs_maxcontig; /* max number of contiguous blks */
63: long fs_maxbpg; /* max number of blks per cyl group */
64: /* these fields can be computed from the others */
65: long fs_fragshift; /* block to frag shift */
66: long fs_fsbtodb; /* fsbtodb and dbtofsb shift constant */
67: long fs_sbsize; /* actual size of super block */
68: long fs_csmask; /* csum block offset */
69: long fs_csshift; /* csum block number */
70: long fs_nindir; /* value of NINDIR */
71: long fs_inopb; /* value of INOPB */
72: long fs_nspf; /* value of NSPF */
73: /* yet another configuration parameter */
74: long fs_optim; /* optimization preference, see below */
75: /* these fields are derived from the hardware */
76: long fs_npsect; /* # sectors/track including spares */
77: long fs_interleave; /* hardware sector interleave */
78: long fs_trackskew; /* sector 0 skew, per track */
79: long fs_headswitch; /* head switch time, usec */
80: long fs_trkseek; /* track-to-track seek, usec */
81: /* sizes determined by number of cylinder groups and their sizes */
82: daddr_t fs_csaddr; /* blk addr of cyl grp summary area */
83: long fs_cssize; /* size of cyl grp summary area */
84: long fs_cgsize; /* cylinder group size */
85: /* these fields are derived from the hardware */
86: long fs_ntrak; /* tracks per cylinder */
87: long fs_nsect; /* sectors per track */
88: long fs_spc; /* sectors per cylinder */
89: /* this comes from the disk driver partitioning */
90: long fs_ncyl; /* cylinders in file system */
91: /* these fields can be computed from the others */
92: long fs_cpg; /* cylinders per group */
93: long fs_ipg; /* inodes per group */
94: long fs_fpg; /* blocks per group * fs_frag */
95: /* this data must be re-computed after crashes */
96: struct csum fs_cstotal; /* cylinder summary information */
97: /* these fields are cleared at mount time */
98: char fs_fmod; /* super block modified flag */
99: char fs_clean; /* file system is clean flag */
100: char fs_ronly; /* mounted read-only flag */
101: char fs_flags; /* currently unused flag */
102: char fs_fsmnt[MAXMNTLEN]; /* name mounted on */
103: /* these fields retain the current block allocation info */
104: long fs_cgrotor; /* last cg searched */
105: struct csum *fs_csp[MAXCSBUFS]; /* list of fs_cs info buffers */
106: long fs_cpc; /* cyl per cycle in postbl */
107: short fs_opostbl[16][8]; /* old rotation block list head */
108: long fs_sparecon[56]; /* reserved for future constants */
109: quad fs_qbmask; /* ~fs_bmask - for use with quad size */
110: quad fs_qfmask; /* ~fs_fmask - for use with quad size */
111: long fs_postblformat; /* format of positional layout tables */
112: long fs_nrpos; /* number of rotaional positions */
113: long fs_postbloff; /* (short) rotation block list head */
114: long fs_rotbloff; /* (u_char) blocks for each rotation */
115: long fs_magic; /* magic number */
116: u_char fs_space[1]; /* list of blocks for each rotation */
117: /* actually longer */
118: };
119: .fi
120: .LP
121: Each disk drive contains some number of file systems.
122: A file system consists of a number of cylinder groups.
123: Each cylinder group has inodes and data.
124: .LP
125: A file system is described by its super-block, which in turn
126: describes the cylinder groups. The super-block is critical
127: data and is replicated in each cylinder group to protect against
128: catastrophic loss. This is done at file system creation
129: time and the critical
130: super-block data does not change, so the copies need not be
131: referenced further unless disaster strikes.
132: .LP
133: Addresses stored in inodes are capable of addressing fragments
134: of `blocks'. File system blocks of at most size MAXBSIZE can
135: be optionally broken into 2, 4, or 8 pieces, each of which is
136: addressable; these pieces may be DEV_BSIZE, or some multiple of
137: a DEV_BSIZE unit.
138: .LP
139: Large files consist of exclusively large data blocks. To avoid
140: undue wasted disk space, the last data block of a small file is
141: allocated as only as many fragments of a large block as are
142: necessary. The file system format retains only a single pointer
143: to such a fragment, which is a piece of a single large block that
144: has been divided. The size of such a fragment is determinable from
145: information in the inode, using the ``blksize(fs, ip, lbn)'' macro.
146: .LP
147: The file system records space availability at the fragment level;
148: to determine block availability, aligned fragments are examined.
149: .LP
150: The root inode is the root of the file system.
151: Inode 0 can't be used for normal purposes and
152: historically bad blocks were linked to inode 1,
153: thus the root inode is 2 (inode 1 is no longer used for
154: this purpose, however numerous dump tapes make this
155: assumption, so we are stuck with it).
156: .LP
157: .I fs_minfree
158: gives the minimum acceptable percentage of file system
159: blocks that may be free. If the freelist drops below this level
160: only the super-user may continue to allocate blocks.
161: .I Fs_minfree
162: may be set to 0 if no reserve of free blocks is deemed necessary,
163: however severe performance degradations will be observed if the
164: file system is run at greater than 90% full; thus the default
165: value of
166: .I fs_minfree
167: is 10%.
168: .LP
169: Empirically the best trade-off between block fragmentation and
170: overall disk utilization at a loading of 90% comes with a
171: fragmentation of 8, thus the default fragment size is an eighth
172: of the block size.
173: .LP
174: .I fs_optim
175: specifies whether the file system should try to minimize the time spent
176: allocating blocks, or if it should attempt to minimize the space
177: fragmentation on the disk.
178: If the value of fs_minfree (see above) is less than 10%,
179: then the file system defaults to optimizing for space to avoid
180: running out of full sized blocks.
181: If the value of minfree is greater than or equal to 10%,
182: fragmentation is unlikely to be problematical, and
183: the file system defaults to optimizing for time.
184: .LP
185: .I Cylinder group related
186: .IR limits :
187: Each cylinder keeps track of the availability of blocks at different
188: rotational positions, so that sequential blocks can be laid out
189: with minimum rotational latency. With the default of 8 distinguished
190: rotational positions, the resolution of the
191: summary information is 2ms for a typical 3600 rpm drive.
192: .LP
193: .I fs_rotdelay
194: gives the minimum number of milliseconds to initiate
195: another disk transfer on the same cylinder. It is used in
196: determining the rotationally optimal layout for disk blocks
197: within a file; the default value for
198: .I fs_rotdelay
199: is 2ms.
200: .LP
201: Each file system has a statically allocated number of inodes.
202: An inode is allocated for each NBPI bytes of disk space.
203: The inode allocation strategy is extremely conservative.
204: .LP
205: MINBSIZE is the smallest allowable block size.
206: With a MINBSIZE of 4096
207: it is possible to create files of size
208: 2^32 with only two levels of indirection.
209: MINBSIZE must be big enough to hold a cylinder group block,
210: thus changes to (struct cg) must keep its size within MINBSIZE.
211: Note that super blocks are never more than size SBSIZE.
212: .LP
213: The path name on which the file system is mounted is maintained in
214: .IR fs_fsmnt .
215: MAXMNTLEN defines the amount of space allocated in
216: the super block for this name.
217: The limit on the amount of summary information per file system
218: is defined by MAXCSBUFS.
219: For a 4096 byte block size, it is currently parameterized for a
220: maximum of two million cylinders.
221: .LP
222: Per cylinder group information is summarized in blocks allocated
223: from the first cylinder group's data blocks.
224: These blocks are read in from
225: .I fs_csaddr
226: (size
227: .IR fs_cssize )
228: in addition to the super block.
229: .LP
230: .B N.B.:
231: sizeof (struct csum) must be a power of two in order for
232: the ``fs_cs'' macro to work.
233: .LP
234: .I Super block for a file
235: .IR system :
236: The size of the rotational layout tables
237: is limited by the fact that the super block is of size SBSIZE.
238: The size of these tables is
239: .B inversely
240: proportional to the block
241: size of the file system. The size of the tables is
242: increased when sector sizes are not powers of two,
243: as this increases the number of cylinders
244: included before the rotational pattern repeats (
245: .IR fs_cpc ).
246: The size of the rotational layout
247: tables is derived from the number of bytes remaining in (struct fs).
248: .LP
249: The number of blocks of data per cylinder group
250: is limited because cylinder groups are at most one block.
251: The inode and free block tables
252: must fit into a single block after deducting space for
253: the cylinder group structure (struct cg).
254: .LP
255: .IR Inode :
256: The inode is the focus of all file activity in the
257: UNIX file system. There is a unique inode allocated
258: for each active file,
259: each current directory, each mounted-on file,
260: text file, and the root.
261: An inode is `named' by its device/i-number pair.
262: For further information, see the include file
263: .RI < sys/inode.h >.
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