[email protected]Quota subsystem allows system administrator to set limits on used space and number of used inodes (inode is a filesystem structure which is associated which 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. User can never exceed hardlimit for any resource. When user exceeds softlimit (s)he's warned that (s)he uses more space than (s)he should but space/inode is allocated (of course only if user also doesn't exceed hardlimit). If a user is exceeding softlimit for specified period of time (this period is called grace time) (s)he's not allowed to allocate more resources (so he must free some space/inodes to get under softlimit).
Quota limits are set independently for each filesystem. Currently following filesystems are supporting quota: Ext2, Ext3, ReiserFS, XFS.
In order for quota subsystem to work you have to have quota compiled in kernel and
configured appropriately. That means that filesystem on which you want to use quotas
must be mounted with option usrquota (if you want to use limits for users),
grpquota (if you want to use limits for groups) or quota (if you want
to use limits for both users and groups) -- this is equivalent to specifying
both usrquota and grpquota. These mount options are used by quota utils
to recognize filesystems they should work with. For XFS you have to specify
additional mount options ??which??. On each such filesystem you have to have
files with quota data. The name of these quota files depends on
quota format (and you can even specify the filenames by yourself by
adding =filename to usrquota or grpquota options -- eg.
usrquota=file.quota. But note that by this you prevent quota utilities to
do some useful autodetection). The quotafiles can be created by
quotacheck(8) utility. When you have successfully created quota files you
can turn quotas on (ie. system will start tracking how much each user uses and
will check set limits). This is done by quotaon(8) program.
When you already have working quota subsystem but you want to use newer quota format
you have to convert quota files. This can be done by convertquota(8) utility.
You can edit user (group) limits by edquota(8) or setquota(8) programs.
By these programs you can also set grace times.
Original quota format is used by kernels up to 2.4 Linux kernels (including). Note
that vendors such as RedHat or SuSE have already included quota format patches into
their kernels and so they use
newer quota format.
This quota format is in manpages and quota utils called vfsold.
Data for this format are usually stored in files quota.user (for user quotas) and
quota.group (for group quotas). Both files have same structure. They are just
the arrays of following structures:
struct v1_disk_dqblk {
u_int32_t dqb_bhardlimit; /* Absolute limit on disk blks alloc */
u_int32_t dqb_bsoftlimit; /* Preferred limit on disk blks */
u_int32_t dqb_curblocks; /* Current block count */
u_int32_t dqb_ihardlimit; /* Maximum # allocated inodes */
u_int32_t dqb_isoftlimit; /* Preferred limit on inodes */
u_int32_t dqb_curinodes; /* Current # allocated inodes */
time_t dqb_btime; /* Time limit for excessive disk use */
time_t dqb_itime; /* Time limit for excessive files */
};
N is stored as N-th structure in file.
In fields dqb_btime and dqb_itime of first structure (id = 0) are stored
grace times for this filesystem.
This quota format is currently used in -ac series of kernels and also in new kernels distributed
by RedHat (>= 7.1) and SuSE (>= 7.2). There are also patches for standard 2.4 kernels
which implement this format. You can download them at ftp://atrey.karlin.mff.cuni.cz/pub/local/jack/quota/v2.4/.
This format is called vfsv0 in manpages and utilities and quotafiles are usually
called aquota.user and aquota.group.
This quota format has following advantages against old quota format:
Format of quotafile is following: In the beginning of quota file there is a generic header which is intended to be present in every quota file in future. A header has following structure:
struct disk_dqheader {
__u32 dqh_magic; /* Magic number identifying file */
__u32 dqh_version; /* File version */
};
Following header might be specific for quotatype and version (currently this header is same for user and group quota and there is only one version of quotafile format).
struct disk_dqinfo {
__u32 dqi_bgrace; /* Time before block soft limit becomes hard limit - in seconds */
__u32 dqi_igrace; /* Time before inode soft limit becomes hard limit - in seconds */
__u32 dqi_flags; /* Flags for quotafile (DQF_*) (currently there are no ondisk flags) */
__u32 dqi_blocks; /* Number of blocks in file */
__u32 dqi_free_blk; /* Number of first free block in the list */
__u32 dqi_free_entry; /* Number of block with at least one free entry */
};
dqi_free_blk points to the first
element of this list). Second double direction link list is used to link all data
blocks which have at least one entry free and which also have at least one used entry.
The beginning of the list is pointed by dqi_free_entry.
The rest of file (starting at 1KB) is divided into 1KB blocks. In these blocks is stored a radix tree with quotas. The key for the radix tree is UID or GID (I'll use just ID) depending on quota file type. One node of the tree is 1KB block so there are upto 256 references to the sons. At each level we choose reference corresponding to one byte of ID so having four-level radix tree we can support 32-bit IDs. Reference from last level points to data block which contains quota structure for proper ID.
Data block has following structure: In the beginning there is header with following structure:
struct disk_dqdbheader {
__u32 dqdh_next_free; /* Number of next block with free entry */
__u32 dqdh_prev_free; /* Number of previous block with free entry */
__u16 dqdh_entries; /* Number of valid entries in block */
__u16 dqdh_pad1;
__u32 dqdh_pad2;
};
dqdh_next_free and dqdh_prev_free are used only if block has at least one
free and one used entry. If it has no free entry these references are set to 0. When block
is completely free only dqdh_next_free is used for link list of free blocks.
The rest of block is divided into 21 quota entries. Unused entry is entry that contains only zeros. Note that used entries might be freely scattered in the block. Quota entry has following structure:
struct disk_dqblk {
__u32 dqb_id; /* id this quota applies to */
__u32 dqb_ihardlimit; /* absolute limit on allocated inodes */
__u32 dqb_isoftlimit; /* preferred inode limit */
__u32 dqb_curinodes; /* current # allocated inodes */
__u32 dqb_bhardlimit; /* absolute limit on disk space (in kb) */
__u32 dqb_bsoftlimit; /* preferred limit on disk space (in kb) */
__u64 dqb_curspace; /* current space occupied (in bytes) */
__u64 dqb_btime; /* time limit for excessive disk use */
__u64 dqb_itime; /* time limit for excessive inode use */
};
As of version 3.01 quota utilities support original, vfsv0 and xfs quota format.
You can download latest version of utils from http://www.sf.net/projects/linuxquota/.
Utils try to do autodetection of currently used quota format (ie. they detect which
format is compiled into kernel and they try to use this one). Anytime you can
force utils to use different format by specifying -F <format>. More information
about quota utils can be found in appropriate manpages.