linux/Documentation/filesystems/quota.rst
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   1.. SPDX-License-Identifier: GPL-2.0
   2
   3===============
   4Quota subsystem
   5===============
   6
   7Quota subsystem allows system administrator to set limits on used space and
   8number of used inodes (inode is a filesystem structure which is associated with
   9each file or directory) for users and/or groups. For both used space and number
  10of used inodes there are actually two limits. The first one is called softlimit
  11and the second one hardlimit.  A user can never exceed a hardlimit for any
  12resource (unless he has CAP_SYS_RESOURCE capability). User is allowed to exceed
  13softlimit but only for limited period of time. This period is called "grace
  14period" or "grace time". When grace time is over, user is not able to allocate
  15more space/inodes until he frees enough of them to get below softlimit.
  16
  17Quota limits (and amount of grace time) are set independently for each
  18filesystem.
  19
  20For more details about quota design, see the documentation in quota-tools package
  21(https://sourceforge.net/projects/linuxquota).
  22
  23Quota netlink interface
  24=======================
  25When user exceeds a softlimit, runs out of grace time or reaches hardlimit,
  26quota subsystem traditionally printed a message to the controlling terminal of
  27the process which caused the excess. This method has the disadvantage that
  28when user is using a graphical desktop he usually cannot see the message.
  29Thus quota netlink interface has been designed to pass information about
  30the above events to userspace. There they can be captured by an application
  31and processed accordingly.
  32
  33The interface uses generic netlink framework (see
  34https://lwn.net/Articles/208755/ and http://www.infradead.org/~tgr/libnl/ for
  35more details about this layer). The name of the quota generic netlink interface
  36is "VFS_DQUOT". Definitions of constants below are in <linux/quota.h>.  Since
  37the quota netlink protocol is not namespace aware, quota netlink messages are
  38sent only in initial network namespace.
  39
  40Currently, the interface supports only one message type QUOTA_NL_C_WARNING.
  41This command is used to send a notification about any of the above mentioned
  42events. Each message has six attributes. These are (type of the argument is
  43in parentheses):
  44
  45        QUOTA_NL_A_QTYPE (u32)
  46          - type of quota being exceeded (one of USRQUOTA, GRPQUOTA)
  47        QUOTA_NL_A_EXCESS_ID (u64)
  48          - UID/GID (depends on quota type) of user / group whose limit
  49            is being exceeded.
  50        QUOTA_NL_A_CAUSED_ID (u64)
  51          - UID of a user who caused the event
  52        QUOTA_NL_A_WARNING (u32)
  53          - what kind of limit is exceeded:
  54
  55                QUOTA_NL_IHARDWARN
  56                    inode hardlimit
  57                QUOTA_NL_ISOFTLONGWARN
  58                    inode softlimit is exceeded longer
  59                    than given grace period
  60                QUOTA_NL_ISOFTWARN
  61                    inode softlimit
  62                QUOTA_NL_BHARDWARN
  63                    space (block) hardlimit
  64                QUOTA_NL_BSOFTLONGWARN
  65                    space (block) softlimit is exceeded
  66                    longer than given grace period.
  67                QUOTA_NL_BSOFTWARN
  68                    space (block) softlimit
  69
  70          - four warnings are also defined for the event when user stops
  71            exceeding some limit:
  72
  73                QUOTA_NL_IHARDBELOW
  74                    inode hardlimit
  75                QUOTA_NL_ISOFTBELOW
  76                    inode softlimit
  77                QUOTA_NL_BHARDBELOW
  78                    space (block) hardlimit
  79                QUOTA_NL_BSOFTBELOW
  80                    space (block) softlimit
  81
  82        QUOTA_NL_A_DEV_MAJOR (u32)
  83          - major number of a device with the affected filesystem
  84        QUOTA_NL_A_DEV_MINOR (u32)
  85          - minor number of a device with the affected filesystem
  86