86 lines
3.3 KiB
ReStructuredText
86 lines
3.3 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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===============
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Quota subsystem
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===============
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Quota subsystem allows system administrator to set limits on used space and
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number of used inodes (inode is a filesystem structure which is associated with
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each file or directory) for users and/or groups. For both used space and number
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of used inodes there are actually two limits. The first one is called softlimit
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and the second one hardlimit. A user can never exceed a hardlimit for any
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resource (unless he has CAP_SYS_RESOURCE capability). User is allowed to exceed
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softlimit but only for limited period of time. This period is called "grace
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period" or "grace time". When grace time is over, user is not able to allocate
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more space/inodes until he frees enough of them to get below softlimit.
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Quota limits (and amount of grace time) are set independently for each
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filesystem.
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For more details about quota design, see the documentation in quota-tools package
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(https://sourceforge.net/projects/linuxquota).
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Quota netlink interface
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=======================
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When user exceeds a softlimit, runs out of grace time or reaches hardlimit,
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quota subsystem traditionally printed a message to the controlling terminal of
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the process which caused the excess. This method has the disadvantage that
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when user is using a graphical desktop he usually cannot see the message.
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Thus quota netlink interface has been designed to pass information about
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the above events to userspace. There they can be captured by an application
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and processed accordingly.
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The interface uses generic netlink framework (see
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https://lwn.net/Articles/208755/ and http://www.infradead.org/~tgr/libnl/ for
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more details about this layer). The name of the quota generic netlink interface
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is "VFS_DQUOT". Definitions of constants below are in <linux/quota.h>. Since
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the quota netlink protocol is not namespace aware, quota netlink messages are
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sent only in initial network namespace.
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Currently, the interface supports only one message type QUOTA_NL_C_WARNING.
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This command is used to send a notification about any of the above mentioned
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events. Each message has six attributes. These are (type of the argument is
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in parentheses):
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QUOTA_NL_A_QTYPE (u32)
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- type of quota being exceeded (one of USRQUOTA, GRPQUOTA)
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QUOTA_NL_A_EXCESS_ID (u64)
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- UID/GID (depends on quota type) of user / group whose limit
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is being exceeded.
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QUOTA_NL_A_CAUSED_ID (u64)
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- UID of a user who caused the event
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QUOTA_NL_A_WARNING (u32)
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- what kind of limit is exceeded:
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QUOTA_NL_IHARDWARN
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inode hardlimit
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QUOTA_NL_ISOFTLONGWARN
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inode softlimit is exceeded longer
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than given grace period
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QUOTA_NL_ISOFTWARN
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inode softlimit
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QUOTA_NL_BHARDWARN
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space (block) hardlimit
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QUOTA_NL_BSOFTLONGWARN
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space (block) softlimit is exceeded
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longer than given grace period.
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QUOTA_NL_BSOFTWARN
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space (block) softlimit
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- four warnings are also defined for the event when user stops
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exceeding some limit:
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QUOTA_NL_IHARDBELOW
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inode hardlimit
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QUOTA_NL_ISOFTBELOW
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inode softlimit
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QUOTA_NL_BHARDBELOW
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space (block) hardlimit
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QUOTA_NL_BSOFTBELOW
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space (block) softlimit
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QUOTA_NL_A_DEV_MAJOR (u32)
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- major number of a device with the affected filesystem
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QUOTA_NL_A_DEV_MINOR (u32)
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- minor number of a device with the affected filesystem
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