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- Quota subsystem
- ===============
- Quota subsystem allows system administrator to set limits on used space and
- number of used inodes (inode is a filesystem structure which is associated with
- each file or directory) for users and/or groups. For both used space and number
- of used inodes there are actually two limits. The first one is called softlimit
- and the second one hardlimit. An user can never exceed a hardlimit for any
- resource (unless he has CAP_SYS_RESOURCE capability). User is allowed to exceed
- softlimit but only for limited period of time. This period is called "grace
- period" or "grace time". When grace time is over, user is not able to allocate
- more space/inodes until he frees enough of them to get below softlimit.
- Quota limits (and amount of grace time) are set independently for each
- filesystem.
- For more details about quota design, see the documentation in quota-tools package
- (http://sourceforge.net/projects/linuxquota).
- Quota netlink interface
- =======================
- When user exceeds a softlimit, runs out of grace time or reaches hardlimit,
- quota subsystem traditionally printed a message to the controlling terminal of
- the process which caused the excess. This method has the disadvantage that
- when user is using a graphical desktop he usually cannot see the message.
- Thus quota netlink interface has been designed to pass information about
- the above events to userspace. There they can be captured by an application
- and processed accordingly.
- The interface uses generic netlink framework (see
- http://lwn.net/Articles/208755/ and http://people.suug.ch/~tgr/libnl/ for more
- details about this layer). The name of the quota generic netlink interface
- is "VFS_DQUOT". Definitions of constants below are in <linux/quota.h>.
- Since the quota netlink protocol is not namespace aware, quota netlink messages
- are sent only in initial network namespace.
- Currently, the interface supports only one message type QUOTA_NL_C_WARNING.
- This command is used to send a notification about any of the above mentioned
- events. Each message has six attributes. These are (type of the argument is
- in parentheses):
- QUOTA_NL_A_QTYPE (u32)
- - type of quota being exceeded (one of USRQUOTA, GRPQUOTA)
- QUOTA_NL_A_EXCESS_ID (u64)
- - UID/GID (depends on quota type) of user / group whose limit
- is being exceeded.
- QUOTA_NL_A_CAUSED_ID (u64)
- - UID of a user who caused the event
- QUOTA_NL_A_WARNING (u32)
- - what kind of limit is exceeded:
- QUOTA_NL_IHARDWARN - inode hardlimit
- QUOTA_NL_ISOFTLONGWARN - inode softlimit is exceeded longer
- than given grace period
- QUOTA_NL_ISOFTWARN - inode softlimit
- QUOTA_NL_BHARDWARN - space (block) hardlimit
- QUOTA_NL_BSOFTLONGWARN - space (block) softlimit is exceeded
- longer than given grace period.
- QUOTA_NL_BSOFTWARN - space (block) softlimit
- - four warnings are also defined for the event when user stops
- exceeding some limit:
- QUOTA_NL_IHARDBELOW - inode hardlimit
- QUOTA_NL_ISOFTBELOW - inode softlimit
- QUOTA_NL_BHARDBELOW - space (block) hardlimit
- QUOTA_NL_BSOFTBELOW - space (block) softlimit
- QUOTA_NL_A_DEV_MAJOR (u32)
- - major number of a device with the affected filesystem
- QUOTA_NL_A_DEV_MINOR (u32)
- - minor number of a device with the affected filesystem
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