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Possible License(s): GPL-2.0, LGPL-2.0, AGPL-1.0
 2Quota subsystem
 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.
15Quota limits (and amount of grace time) are set independently for each
18For more details about quota design, see the documentation in quota-tools package
21Quota netlink interface
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.
31The interface uses generic netlink framework (see
32 and 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