| Jan Kara | 8e89346 | 2007-10-16 23:29:31 -0700 | [diff] [blame] | 1 | |
| 2 | Quota subsystem |
| 3 | =============== |
| 4 | |
| 5 | Quota subsystem allows system administrator to set limits on used space and |
| 6 | number of used inodes (inode is a filesystem structure which is associated |
| 7 | with each file or directory) for users and/or groups. For both used space and |
| 8 | number of used inodes there are actually two limits. The first one is called |
| 9 | softlimit and the second one hardlimit. An user can never exceed a hardlimit |
| 10 | for any resource. User is allowed to exceed softlimit but only for limited |
| 11 | period of time. This period is called "grace period" or "grace time". When |
| 12 | grace time is over, user is not able to allocate more space/inodes until he |
| 13 | frees enough of them to get below softlimit. |
| 14 | |
| 15 | Quota limits (and amount of grace time) are set independently for each |
| 16 | filesystem. |
| 17 | |
| 18 | For more details about quota design, see the documentation in quota-tools package |
| 19 | (http://sourceforge.net/projects/linuxquota). |
| 20 | |
| 21 | Quota netlink interface |
| 22 | ======================= |
| 23 | When user exceeds a softlimit, runs out of grace time or reaches hardlimit, |
| 24 | quota subsystem traditionally printed a message to the controlling terminal of |
| 25 | the process which caused the excess. This method has the disadvantage that |
| 26 | when user is using a graphical desktop he usually cannot see the message. |
| 27 | Thus quota netlink interface has been designed to pass information about |
| 28 | the above events to userspace. There they can be captured by an application |
| 29 | and processed accordingly. |
| 30 | |
| 31 | The interface uses generic netlink framework (see |
| 32 | http://lwn.net/Articles/208755/ and http://people.suug.ch/~tgr/libnl/ for more |
| 33 | details about this layer). The name of the quota generic netlink interface |
| 34 | is "VFS_DQUOT". Definitions of constants below are in <linux/quota.h>. |
| 35 | Currently, the interface supports only one message type QUOTA_NL_C_WARNING. |
| 36 | This command is used to send a notification about any of the above mentioned |
| 37 | events. Each message has six attributes. These are (type of the argument is |
| 38 | in 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 | QUOTA_NL_A_DEV_MAJOR (u32) |
| 57 | - major number of a device with the affected filesystem |
| 58 | QUOTA_NL_A_DEV_MINOR (u32) |
| 59 | - minor number of a device with the affected filesystem |