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diff --git a/doc/quota.html b/doc/quota.html new file mode 100644 index 0000000..2e86b5c --- /dev/null +++ b/doc/quota.html @@ -0,0 +1,288 @@ +<HTML> +<HEAD> +<TITLE>Linux DiskQuota system</TITLE> +<!-- OWNER_NAME="Marco van Wieringen, OpenWorld System Management" --> +<!-- OWNER_INFO="The OpenWorld Foundation, The Netherlands" --> +<LINK REV=MADE HREF="mailto:mvw@planets.elm.net"> +</HEAD> +<BODY> +<H1>General</H1> +<P> +In most computing environments, disc space is not infinite. The +diskquota 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 LINUX that +may be included when the system is configured. This document will describe +from what view the +<A HREF="#USERVIEW">Users</A> +will see the quota-system and the way the quota-system can be configured +by the +<A HREF="#ADMINISTERING">System Administator</A> +. +<P> +<A NAME="USERVIEW"><H1>Users' view of diskquotas</H1></A> +<H2>General</H2> +To most users, diskquotas will either be of no concern, or a fact of life +that cannot be avoided. The +<A HREF="quota(1).html">quota(1)</A> +command will provide information on +any disc quotas that may have been imposed upon a user. +<P> +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 set by the system +administrators, in each of these areas, and current usage. The inode limit and +block limit are impossed both on uid and on gid. So if there are group quotas +you can be limited because the group can't allocate any more space even if you +as user still can allocate the space. +<P> +There are four numbers for each limit, the current usage, soft limit (quota), +hard limit, and time remaining bfore the softlimit is intepretted as a hard +limit. 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. +<P> +When a use exeeds his softlimit a timer is set that normaly expires wuthin +7 days (1 week). The user can remove files in this period to make sure he is +under the softlimit again before the timer expires. When the timer expires 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 the softlimit. +<P> +<H2>Surviving when quota limit is reached</H2> +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. +<P> +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 previous +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. +<P> +There are several possible safe exits for a user caught in this situation. +He may use the editor ! shell escape command 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 back to the filesystem it belongs on. +<A NAME="ADMINISTERING"><H1>Administering the quota system</H1></A> +To set up and establish the diskquota system, there are several steps necessary +to be performed by the system administrator. The following steps must be taken: +<UL> +<LI> +<A HREF="#KERNEL">Kernel configuration</A> +<LI> +<A HREF="#ELECTING">Electing filesystems</A> +<LI> +<A HREF="#ENABLING">Enabling quotas</A> +<LI> +<A HREF="#CHECKING">Checking a filesystem for quotas</A> +<LI> +<A HREF="#SPECIFYING">Specifing a quota for a user or group</A> +<LI> +<A HREF="#CHECKING">Checking quotas for a user or group</A> +<LI> +<A HREF="#DISABLING">Disabling quota for a user or group</A> +<LI> +<A HREF="#NFS">Quotas on NFS-mounted disks</A> +</UL> +<A NAME="KERNEL"><H2>Kernel configuration</H2></A> +Before you can use the quota-system you must compile a kernel +with the quota-system enabled. This is done by answering yes +to the Disk QUOTA support question when running <make config>. Then +run <make> and install the new kernel images as the one that is booted +at boottime. +<A NAME="ELECTING"><H2>Electing filesystems</H2></A> +When you have a kernel that supports quota you need to make a decision as to what +filesystems need to have quotas applied. 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 its writable +by normal users. +<P> +To enable quotas on a certain filesystem one should edit the /etc/fstab +file and add entries for usrquota and grpquota. Mine looks like : +<PRE> +# device directory type options +/dev/hda1 / ext2 defaults +/dev/hda2 none swap sw +/dev/hda3 /usr ext2 defaults +/dev/hdb1 /usr/users ext2 defaults,usrquota,grpquota +/dev/hdb2 /usr/src ext2 defaults,usrquota +none /proc proc defaults +</PRE> +The keyword "usrquota" in the options field of each fstab-entry turns on +userquota for this device. The keyword "grpquota" in the options field turns +on groupquota for the device. When you use the usrquota and grpquota options +without the "=" option you quotafiles are located in the rootdir of each +filesystem. A file called "quota.user" is used for userquota and a file called +"quota.group" is used for groupquota. +<P> +You can also define your quotafile yourself. Something like +"usrquota=/usr/adm/quotasrc.user" puts the quotafile in /usr/adm with the +name quotasrc.user. Please be aware of the maximum lenght a line can have +in your fstab, see +<B>mntent.h</B> +for a definition. +<P> +<A NAME="ENABLING"><H2>Enabling quotas</H2></A> +Periodically (certainly after each unclean 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 +<A HREF="quotacheck(8).html">quotacheck(8)</A> +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 inaccuracy. +<P> +To check the filesystem for the actual number of blocks used by a user run +<KBD>quotacheck -avug</KBD> +to install or update all the quotafiles. +<P> +The quotacheck program takes some time on large filesystems, but whith the +new version it is quite acceptable on my machine. But when you are hacking +the kernel, I recommend not to use it because it takes some time every time +you have to reboot your machine. You also can also put it in you rc script +and run it like you run fsck on your filesystems only when the fastreboot +flag is not set. There is no support for parallel checking of filesystems. +<P> +Ok now one should have all the quotafiles one needs. +Now you can add a line like: +<KBD>/usr/etc/quotaon -avug</KBD> +<P> +to your /etc/rc. This is to turn the quotas on when you boot your machine. +This is they way to go and not turn it on yourself any time when you boot +your machine. +<A NAME="CHECKING"><H2>Checking a filesystem for quotas</H2></A> +The super-user may use the +<A HREF="quota(1).html">quota (1)</A> +command to examine the usage and quotas +of any user, and the +<A HREF="repquota(8).html">repquota (8)</A> +command may be used to check the usages and +limits for all users on a filesystem. Just run +<KBD>quotacheck -avug</KBD> +and the quotafiles are updated automagicaly and also the tables that are +currently used by the kernel. Watch for the "updating in core quotas" message +of the +<A HREF="quotacheck(8).html">quotacheck (8)</A> +program this says if it updates the in core quotas in the kernel. +<P> +I cannot state this enough the quotafile is build as (uid || gid * +sizeof(struct dquot)) so when you have nobody as uid 65535 and nobody owns a +file you get big quota files, lets say of about 2 Mb all filed with zero's +for users that don't have quota. So please be aware of that and don't mail +me about that. It isn't much of a problem because the file isn't that big +really all 0 blocks are not allocated on the disk. +<A NAME="SPECIFYING"><H2>Specifing a quota for a user or group</H2></A> +To edit the quotas for various users we use the +<A HREF="edquota(8).html">edquota (8)</A> program. Now use +<KBD>edquota -u <username | uid></KBD> +to edit user quotas and +<KBD>edquota -g <groupname | gid></KBD> +to edit group quotas. +<P> +Edit only the numbers behind the soft and hard keywords. There are two lines +for each filesystem that has quota turned on. Soft means the softlimit, if +people or groups go over there softlimit they have some grace period to make +sure they go under there softlimit. +<P> +The graceperiod can be changed with +<KBD>edquota -t</KBD> +and enter the number of days there. If they don't remove it within there graceperiod +it is counted as a hardlimit. The hardlimit is the absolute maximum they can allocate, +if they want more the files are truncated. +<P> +The one line that says blocks is the number of blocks one can allocate, +the line that says inodes is the number of inodes +(files/named pipes/devices/etc.) one can allocate. +<P> +Most of the time you have groups of users with the same quota. A quick way of editing +the quota for all those users is change to the dir where there homedirs reside. Do a +edquota for one of the users and change the quotas to the approriate values. This user +becomes the so called prototype user or group for all the others. Then execute +<KBD>edquota -p prototypeusername *</KBD> +this should do the trick, all users now have the quota they need, now +you could edit the ones that still need other values. +<A NAME="CHECKING"><H2>Checking quotas for a user or group</H2></A> +Run the quota program. The syntax for this program is : +<PRE> +quota [-guqv] +quota [-qv] -u username ... +quota [-qv] -g groupname ... +</PRE> +Use -v to see the quotas on +<UL> +<LI> +filesystems you don't have quotas on +<LI> +filesystems on which you do have quota but haven't allocated any blocks yet +</UL> +Use -q to only see filesystems on which you are over your softlimit or +have reached your hardlimit. +<P> +The -g flags give you all quotas for the groups you are in (also +additional groups). +<A NAME="DISABLING"><H2>Disabling quota for a user or group</H2></A> +When you want to disable quotas for a certain user use the quota editor +edquota. Type +<KBD>edquota username | uid</KBD> +or +<KBD>edquota -g groupname | gid</KBD> +and set block softlimit and hardlimit, and the inode soft- and hardlimit to 0. +This should disable the quota for that user an the user can allocate as many +blocks and inodes as he/she wants. +<A NAME="NFS"><H2>Quotas on NFS-mounted disks</H2></A> +To have quotas on NFS filesystems, you need to install quotas on the +fileserver and not on the client. Clients can obtain quota information +with the quota command which does a query to the +<A HREF="rquotad(8).html">rquotad (8)</A> +running on the fileserver from which you mount your NFS disks. So don't put any +usrquota or grpquota flags in the flags for mounting NFS disks. Instead install +quotas on your fileserver. And start the rpc.rquotad from your network rc-file. +<H1>Some implementation details</H2> +Diskquota usage and information is stored in a file on the filesystem that the +quotas are to be applied to. Conventionally, this file is quota.user or quota.group +in the root of the filesystem. +<P> +The data in the file comprises an array of structures, indexed by uid or gid, one +structure for each user or group on the system (whether the user or group has a +quota on this filesystem or not). If the uid or gid space is sparse, then the +file may have holes in it, which would be lost by copying, so it is best to avoid +this. +<P> +The system is informed of the existence of the quota file by the +<A HREF="quotactl(2).html">quotactl (2)</A> +system call. It then reads the quota entries for each user or group currently +active, then for any files open owned by users who are not currently active. +Each subsequent open of a file on the filesystem, will be accompanied 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 uid or group 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. +<P> +Each time a block is accessed or released, and each 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 disk. +</BODY> +</HTML> |