MirOS Manual: 04.quotas(SMM)

             Disc Quotas in a UNIX* Environment

                         Robert Elz

               Department of Computer Science
                  University of Melbourne,


          In most computing environments, disc space is
     not  infinite.  The  disc  quota system provides a
     mechanism to control usage of disc  space,  on  an
     individual basis.

          Quotas may be set for each  individual  user,
     on any, or all filesystems.

          The quota system will warn  users  when  they
     exceed  their allotted limit, but allow some extra
     space for current work. Repeatedly remaining  over
     quota  at  logout,  will  cause a fatal over quota
     condition eventually.

          The quota  system  is  an  optional  part  of
     VMUNIX  that  may  be  included when the system is

1. Users' view of disc quotas

     To most users, disc quotas will either be  of  no  con-
cern, or a fact of life that cannot be avoided. The quota(1)
command will provide information on any disc quotas that may
have been imposed upon a user.

     There are two individual possible quotas  that  may  be
imposed, usually if one is, both will be. A limit can be set
on the amount of space a user can occupy, and there may be a
limit on the number of files (inodes) he can own.

     Quota provides information on the quotas that have been
* UNIX is a trademark of Bell Laboratories.

SMM:4-2                    Disc Quotas in a UNIX Environment

set by the system administrators, in each  of  these  areas,
and current usage.

     There are four numbers  for  each  limit,  the  current
usage, soft limit (quota), hard limit, and number of remain-
ing login warnings. The soft  limit  is  the  number  of  1K
blocks (or files) that the user is expected to remain below.
Each time the user's usage goes past this limit, he will  be
warned. The hard limit cannot be exceeded. If a user's usage
reaches this number, further requests for space (or attempts
to  create  a  file) will fail with an EDQUOT error, and the
first time this occurs, a message will  be  written  to  the
user's  terminal.  Only  one  message  will be output, until
space occupied is reduced below the limit,  and  reaches  it
again, in order to avoid continual noise from those programs
that ignore write errors.

     Whenever a user logs in with a usage greater  than  his
soft  limit,  he will be warned, and his login warning count
decremented. When he logs in under  quota,  the  counter  is
reset  to its maximum value (which is a system configuration
parameter, that is typically 3). If the warning count should
ever  reach  zero  (caused  by  three successive logins over
quota), the particular limit that has been exceeded will  be
treated  as  if the hard limit has been reached, and no more
resources will be allocated to the user.  The  only  way  to
reset  this  condition  is to reduce usage below quota, then
log in again.

1.1. Surviving when quota limit is reached

     In most cases, the only way to recover from over  quota
conditions, is to abort whatever activity was in progress on
the filesystem that has reached its limit, remove sufficient
files  to  bring  the  limit back below quota, and retry the
failed program.

     However, if you are in the editor  and  a  write  fails
because  of  an over quota situation, that is not a suitable
course of action,  as  it  is  most  likely  that  initially
attempting  to write the file will have truncated its previ-
ous contents,  so  should  the  editor  be  aborted  without
correctly  writing the file not only will the recent changes
be lost, but possibly much, or even all, of  the  data  that
previously existed.

     There are several possible safe exits for a user caught
in this situation. He may use the editor ! shell escape com-
mand to examine his file space, and  remove  surplus  files.
Alternatively,  using csh, he may suspend the editor, remove
some files, then resume it. A third possibility, is to write
the  file  to  some  other  filesystem (perhaps to a file on
/tmp) where the user's quota has  not  been  exceeded.  Then
after  rectifying the quota situation, the file can be moved

Disc Quotas in a UNIX Environment                    SMM:4-3

back to the filesystem it belongs on.

2. Administering the quota system

     To set up and establish the disc  quota  system,  there
are  several  steps  necessary to be performed by the system

     First, the system must be  configured  to  include  the
disc quota sub-system. This is done by including the line:

        options   QUOTA

in the system configuration  file,  then  running  config(8)
followed by a system configuration*.

     Second, a decision as to what filesystems need to  have
quotas  applied  needs to be made. Usually, only filesystems
that house users' home directories,  or  other  user  files,
will need to be subjected to the quota system, though it may
also prove useful to also include /usr.  If  possible,  /tmp
should usually be free of quotas.

     Having decided on which filesystems quotas need  to  be
set  upon, the administrator should then allocate the avail-
able space amongst the competing needs. How this  should  be
done is (way) beyond the scope of this document.

     Then, the edquota(8) command can be  used  to  actually
set  the  limits  desired  upon each user. Where a number of
users are to be given the same quotas (a common  occurrence)
the -p switch to edquota will allow this to be easily accom-

     Once the quotas are set, ready to operate,  the  system
must  be  informed to enforce quotas on the desired filesys-
tems. This is accomplished with the quotaon(8) command. Quo-
taon  will either enable quotas for a particular filesystem,
or with the -a switch, will enable quotas for each  filesys-
tem  indicated  in  /etc/fstab as using quotas. See fstab(5)
for details. Most sites using the quota system, will include
the line

                      /etc/quotaon -a

in /etc/rc.local.

     Should quotas need to be disabled, the quotaoff(8) com-
mand  will  do that, however, should the filesystem be about
to be dismounted, the umount(8) command will disable  quotas
* See also the document ``Building 4.2BSD UNIX  Systems
with Config''.

SMM:4-4                    Disc Quotas in a UNIX Environment

immediately before the  filesystem  is  unmounted.  This  is
actually  an  effect  of  the  umount(2) system call, and it
guarantees that the quota system will not be disabled if the
umount would fail because the filesystem is not idle.

     Periodically (certainly after  each  reboot,  and  when
quotas  are  first  enabled  for  a filesystem), the records
retained in the quota file should be checked for consistency
with  the actual number of blocks and files allocated to the
user. The quotacheck(8) command can be  used  to  accomplish
this.  It  is  not  necessary to dismount the filesystem, or
disable the quota system to  run  this  command,  though  on
active  filesystems  inaccurate results may occur. This does
no real harm in most cases, another run of  quotacheck  when
the  filesystem  is  idle will certainly correct any inaccu-

     The super-user may use the quota(1) command to  examine
the  usage  and quotas of any user, and the repquota(8) com-
mand may be used to check the  usages  and  limits  for  all
users on a filesystem.

3. Some implementation detail.

     Disc quota usage and information is stored in a file on
the filesystem that the quotas are to be applied to. Conven-
tionally, this file is quotas in the root of the filesystem.
While  this  name  is  not  known  to the system in any way,
several of the user level utilities "know" it, and  choosing
any other name would not be wise.

     The data in the file comprises an array of  structures,
indexed  by  uid,  one structure for each user on the system
(whether the user has a quota on this filesystem or not). If
the uid space is sparse, then the file may have holes in it,
which would be lost by copying, so it is best to avoid this.

     The system is informed of the existence  of  the  quota
file by the setquota(2) system call. It then reads the quota
entries for each user currently active, then for  any  files
open  owned by users who are not currently active. Each sub-
sequent open of a file on the  filesystem,  will  be  accom-
panied  by  a  pairing  with  its quota information. In most
cases this information will  be  retained  in  core,  either
because  the user who owns the file is running some process,
because other files are open owned  by  the  same  user,  or
because  some file (perhaps this one) was recently accessed.
In memory, the quota information is kept hashed  by  user-id
and  filesystem,  and  retained  in an LRU chain so recently
released data can be  easily  reclaimed.  Information  about
those  users  whose  last process has recently terminated is
also retained in this way.

     Each time a block is accessed  or  released,  and  each

Disc Quotas in a UNIX Environment                    SMM:4-5

time  an  inode is allocated or freed, the quota system gets
told about it, and in the  case  of  allocations,  gets  the
opportunity to object.

     Measurements have shown that the quota code uses a very
small  percentage of the system cpu time consumed in writing
a new block to disc.

4. Acknowledgments

     The current disc quota system is loosely based  upon  a
very early scheme implemented at the University of New South
Wales, and Sydney University in the mid  70's.  That  system
implemented  a  single  combined  limit  for  both files and
blocks on all filesystems.

     A later system was implemented  at  the  University  of
Melbourne by the author, but was not kept highly accurately,
eg: chown's (etc) did not affect quotas, nor did  i/o  to  a
file other than one owned by the instigator.

     The current system has been running  (with  only  minor
modifications) since January 82 at Melbourne. It is actually
just a small part of a much broader resource control scheme,
which is capable of controlling almost anything that is usu-
ally uncontrolled in unix. The rest  of  this  is,  as  yet,
still in a state where it is far too subject to change to be
considered for distribution.

     For the 4.2BSD release, much  work  has  been  done  to
clean  up  and  sanely  incorporate  the  quota  code by Sam
Leffler and Kirk McKusick at The University of California at

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