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