# Disk quota system for Linux

## Jan Kara `jack@suse.cz`

2000, 2001

This document contains documentation of Linux quota disk formats and appropriate quota utilities (currenly in version 3.01).

## 1. Introduction

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.

## 2. Configuration

## 2.1 Setting up new quota

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` (this is equivalent to `usrquota` option). 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.

## 2.2 Converting quota formats

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.

## 2.3 Setting user limits

You can edit user (group) limits by `edquota(8)` or `setquota(8)` programs. By these programs you can also set grace times.

## 3. Quota formats

## 3.1 Original quota format

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 */
    };

Structure for user (group) with id `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.

## 3.2 V0 quota format

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:

- It allows 32-bit UIDs/GIDs.
- Used space is stored in bytes and so ReiserFS can do precise accounting.
- UID/GID 0 is no longer special (grace times are stored independently).
- Header which allows quota format and version detection was added.

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 */
    };

From this header any utility or kernel code should be able to recognize whether they understand a format of file and eventually refuse to continue.

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 */
    };

There are two link lists of blocks in quotafile. First one direction link list is used to link all blocks that are completely unused (`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;
    };

Entries `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 */
    };

## 4. Utilities

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.