  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).
  ______________________________________________________________________

  Table of Contents


  1. Introduction

  2. Configuration

     2.1 Setting up new quota
     2.2 Converting quota formats
     2.3 Setting user limits

  3. Quota formats

     3.1 Original quota format
     3.2 V0 quota format

  4. Utilities



  ______________________________________________________________________

  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:

  o  It allows 32-bit UIDs/GIDs.

  o  Used space is stored in bytes and so ReiserFS can do precise
     accounting.


  o  UID/GID 0 is no longer special (grace times are stored
     independently).

  o  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 recog-
  nize 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.






























