quota.rst (3408B)
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