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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: .\" @(#)quotas.ms 6.2 (Berkeley) 7/31/86
6: .\"
7: .EH 'SMM:4-%''Disc Quotas in a \s-2UNIX\s+2 Environment'
8: .OH 'Disc Quotas in a \s-2UNIX\s+2 Environment''SMM:4-%'
9: .ND 5th July, 1983
10: .TL
11: Disc Quotas in a \s-2UNIX\s+2\s-3\u*\d\s0 Environment
12: .FS
13: * UNIX is a trademark of Bell Laboratories.
14: .FE
15: .AU
16: Robert Elz
17: .AI
18: Department of Computer Science
19: University of Melbourne,
20: Parkville,
21: Victoria,
22: Australia.
23: .AB
24: .PP
25: In most computing environments, disc space is not
26: infinite.
27: The disc quota system provides a mechanism
28: to control usage of disc space, on an
29: individual basis.
30: .PP
31: Quotas may be set for each individual user, on any, or
32: all filesystems.
33: .PP
34: The quota system will warn users when they
35: exceed their allotted limit, but allow some
36: extra space for current work.
37: Repeatedly remaining over quota at logout,
38: will cause a fatal over quota condition eventually.
39: .PP
40: The quota system is an optional part of
41: \s-2VMUNIX\s0 that may be included when the
42: system is configured.
43: .AE
44: .NH 1
45: Users' view of disc quotas
46: .PP
47: To most users, disc quotas will either be of no concern,
48: or a fact of life that cannot be avoided.
49: The
50: \fIquota\fP\|(1)
51: command will provide information on any disc quotas
52: that may have been imposed upon a user.
53: .PP
54: There are two individual possible quotas that may be
55: imposed, usually if one is, both will be.
56: A limit can be set on the amount of space a user
57: can occupy, and there may be a limit on the number
58: of files (inodes) he can own.
59: .PP
60: .I Quota
61: provides information on the quotas that have
62: been set by the system administrators, in each
63: of these areas, and current usage.
64: .PP
65: There are four numbers for each limit, the current
66: usage, soft limit (quota), hard limit, and number
67: of remaining login warnings.
68: The soft limit is the number of 1K blocks (or files)
69: that the user is expected to remain below.
70: Each time the user's usage goes past this limit,
71: he will be warned.
72: The hard limit cannot be exceeded.
73: If a user's usage reaches this number, further
74: requests for space (or attempts to create a file)
75: will fail with an EDQUOT error, and the first time
76: this occurs, a message will be written to the user's
77: terminal.
78: Only one message will be output, until space occupied
79: is reduced below the limit, and reaches it again,
80: in order to avoid continual noise from those
81: programs that ignore write errors.
82: .PP
83: Whenever a user logs in with a usage greater than
84: his soft limit, he will be warned, and his login
85: warning count decremented.
86: When he logs in under quota, the counter is reset
87: to its maximum value (which is a system configuration
88: parameter, that is typically 3).
89: If the warning count should ever reach zero (caused
90: by three successive logins over quota), the
91: particular limit that has been exceeded will be treated
92: as if the hard limit has been reached, and no
93: more resources will be allocated to the user.
94: The \fBonly\fP way to reset this condition is
95: to reduce usage below quota, then log in again.
96: .NH 2
97: Surviving when quota limit is reached
98: .PP
99: In most cases, the only way to recover from over
100: quota conditions, is to abort whatever activity was in progress
101: on the filesystem that has reached its limit, remove
102: sufficient files to bring the limit back below quota,
103: and retry the failed program.
104: .PP
105: However, if you are in the editor and a write fails
106: because of an over quota situation, that is not
107: a suitable course of action, as it is most likely
108: that initially attempting to write the file
109: will have truncated its previous contents, so should
110: the editor be aborted without correctly writing the
111: file not only will the recent changes be lost, but
112: possibly much, or even all, of the data
113: that previously existed.
114: .PP
115: There are several possible safe exits for a user
116: caught in this situation.
117: He may use the editor \fB!\fP shell escape command to
118: examine his file space, and remove surplus files.
119: Alternatively, using \fIcsh\fP, he may suspend the
120: editor, remove some files, then resume it.
121: A third possibility, is to write the file to
122: some other filesystem (perhaps to a file on /tmp)
123: where the user's quota has not been exceeded.
124: Then after rectifying the quota situation,
125: the file can be moved back to the filesystem
126: it belongs on.
127: .NH 1
128: Administering the quota system
129: .PP
130: To set up and establish the disc quota system,
131: there are several steps necessary to be performed
132: by the system administrator.
133: .PP
134: First, the system must be configured to include
135: the disc quota sub-system.
136: This is done by including the line:
137: .DS
138: options QUOTA
139: .DE
140: in the system configuration file, then running
141: \fIconfig\fP\|(8)
142: followed by a system configuration\s-3\u*\d\s0.
143: .FS
144: * See also the document ``Building 4.2BSD UNIX Systems with Config''.
145: .FE
146: .PP
147: Second, a decision as to what filesystems need to have
148: quotas applied needs to be made.
149: Usually, only filesystems that house users' home directories,
150: or other user files, will need to be subjected to
151: the quota system, though it may also prove useful to
152: also include \fB/usr\fR.
153: If possible, \fB/tmp\fP should usually be free of quotas.
154: .PP
155: Having decided on which filesystems quotas need to be
156: set upon, the administrator should then allocate the
157: available space amongst the competing needs. How this
158: should be done is (way) beyond the scope of this document.
159: .PP
160: Then, the
161: \fIedquota\fP\|(8)
162: command can be used to actually set the limits desired upon
163: each user. Where a number of users are to be given the
164: same quotas (a common occurrence) the \fB\-p\fP switch
165: to edquota will allow this to be easily accomplished.
166: .PP
167: Once the quotas are set, ready to operate, the system
168: must be informed to enforce quotas on the desired filesystems.
169: This is accomplished with the
170: \fIquotaon\fP\|(8)
171: command.
172: .I Quotaon
173: will either enable quotas for a particular filesystem, or
174: with the \fB\-a\fP switch, will enable quotas for each
175: filesystem indicated in \fB/etc/fstab\fP as using quotas.
176: See
177: \fIfstab\fP\|(5)
178: for details.
179: Most sites using the quota system, will include the
180: line
181: .DS C
182: /etc/quotaon -a
183: .DE
184: in \fB/etc/rc.local\fP.
185: .PP
186: Should quotas need to be disabled, the
187: \fIquotaoff\fP(8)
188: command will do that, however, should the filesystem be
189: about to be dismounted, the
190: \fIumount\fP\|(8)
191: command will disable quotas immediately before the
192: filesystem is unmounted.
193: This is actually an effect of the
194: \fIumount\fP\|(2)
195: system call, and it guarantees that the quota system
196: will not be disabled if the umount would fail
197: because the filesystem is not idle.
198: .PP
199: Periodically (certainly after each reboot, and when quotas
200: are first enabled for a filesystem), the records retained
201: in the quota file should be checked for consistency with
202: the actual number of blocks and files allocated to
203: the user.
204: The
205: \fIquotacheck\fP\|(8)
206: command can be used to accomplish this.
207: It is not necessary to dismount the filesystem, or disable
208: the quota system to run this command, though on
209: active filesystems inaccurate results may occur.
210: This does no real harm in most cases, another run of
211: .I quotacheck
212: when the filesystem is idle will certainly correct any inaccuracy.
213: .PP
214: The super-user may use the
215: \fIquota\fP\|(1)
216: command to examine the usage and quotas of any user, and
217: the
218: \fIrepquota\fP\|(8)
219: command may be used to check the usages and limits for
220: all users on a filesystem.
221: .NH 1
222: Some implementation detail.
223: .PP
224: Disc quota usage and information is stored in a file on the
225: filesystem that the quotas are to be applied to.
226: Conventionally, this file is \fBquotas\fR in the root of
227: the filesystem.
228: While this name is not known to the system in any way,
229: several of the user level utilities "know" it, and
230: choosing any other name would not be wise.
231: .PP
232: The data in the file comprises an array of structures, indexed
233: by uid, one structure for each user on the system (whether
234: the user has a quota on this filesystem or not).
235: If the uid space is sparse, then the file may have holes
236: in it, which would be lost by copying, so it is best to
237: avoid this.
238: .PP
239: The system is informed of the existence of the quota
240: file by the
241: \fIsetquota\fP\|(2)
242: system call.
243: It then reads the quota entries for each user currently
244: active, then for any files open owned by users who
245: are not currently active.
246: Each subsequent open of a file on the filesystem, will
247: be accompanied by a pairing with its quota information.
248: In most cases this information will be retained in core,
249: either because the user who owns the file is running some
250: process, because other files are open owned by the same
251: user, or because some file (perhaps this one) was recently
252: accessed.
253: In memory, the quota information is kept hashed by user-id
254: and filesystem, and retained in an LRU chain so recently
255: released data can be easily reclaimed.
256: Information about those users whose last process has
257: recently terminated is also retained in this way.
258: .PP
259: Each time a block is accessed or released, and each time an inode
260: is allocated or freed, the quota system gets told
261: about it, and in the case of allocations, gets the
262: opportunity to object.
263: .PP
264: Measurements have shown
265: that the quota code uses a very small percentage of the system
266: cpu time consumed in writing a new block to disc.
267: .NH 1
268: Acknowledgments
269: .PP
270: The current disc quota system is loosely based upon a very
271: early scheme implemented at the University of New South
272: Wales, and Sydney University in the mid 70's. That system
273: implemented a single combined limit for both files and blocks
274: on all filesystems.
275: .PP
276: A later system was implemented at the University of Melbourne
277: by the author, but was not kept highly accurately, eg:
278: chown's (etc) did not affect quotas, nor did i/o to a file
279: other than one owned by the instigator.
280: .PP
281: The current system has been running (with only minor modifications)
282: since January 82 at Melbourne.
283: It is actually just a small part of a much broader resource
284: control scheme, which is capable of controlling almost
285: anything that is usually uncontrolled in unix. The rest
286: of this is, as yet, still in a state where it is far too
287: subject to change to be considered for distribution.
288: .PP
289: For the 4.2BSD release, much work has been done to clean
290: up and sanely incorporate the quota code by Sam Leffler and
291: Kirk McKusick at The University of California at Berkeley.
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