Linux administration

• Quotas are a way of limiting the amount of disk space that users may take up

• Some organisations (perhaps those with untrusted external users) absolutely need to ensure that:

  • No user can prevent other users from using a reasonable amount of disk space
  • No user can impede the correct functioning of the system

• Some organisations don't need to worry about this — their users can be trusted not to abuse the system

• Quotas have hard limits and soft limits
• A user can exceed the soft limit without retribution
  • But only for a certain period of time — the grace period
  • The user is also warned that the soft limit has been exceeded
• A hard limit may never be exceeded

• If a user tries to exceed a hard limit (or an expired soft limit), the attempt fails

  • The program gets the same error message it would if the filesystem itself had run out of disk space

• Grace periods are set per-filesystem

• Most quotas are set per-user
  • Each user has his or her own soft limit and hard limit
• Quotas can also be set per-group
  • A group can be given a soft limit and hard limit
• Group quotas apply to all users in a group
• If a group hard limit has been reached, no user in the group may use more space
  • Including users who have not yet reached their individual quota

• Quotas can be set for blocks
  • Limits the amount of data space that may be used
• Quotas can also be set for inodes
  • Limits the number of files that may be created

• The quota command displays quota limits
• Specifying a username or the name of a group will show information about their quotas:

# quota fred

• The -v option will show full information about all quotas, even where there are no limits

• The options in /etc/fstab specify which filesystems should have quota enabled
  • Add the option usrquota to enable user quotas
  • Use grpquota to enable group quotas
  • Either or both can be used for each filesystem:

/dev/hda1   /   ext2    defaults
/dev/hdb1   /home   ext2    defaults,usrquota
/dev/hdb2   /work/shared    ext2    defaults,usrquota,grpquota

• The filesystems with quota enabled should have files called quota.user and quota.group in their root directories

• The following commands will create them:

# touch /partition/quota.{user,group}
# chmod 600 /partition/quota.{user,group}

• quotaon turns on quota support
  • Can only be done by root
  • Support must be compiled into the kernel, but this is done by default on all modern distributions
• quotaoff disables quota support
• For example, to turn on quota on all filesystems:

# quotaon -av

• Quota can be turned on or off for individual filesystems

• Command line program to alter quota limits for a user or group
• Specify the name of a user or group with -u username or -g groupname
• Specify the filesystem to alter after the -u or -g option
• Finally, the limits to set must be specified in the following order:
  • Soft limit for blocks
  • Hard limit for blocks
  • Soft limit for inodes
  • Hard limit for inodes
• Setting any limit to 0 will remove that limit

• edquota allows quotas to be edited interactively, in a text editor
  • The file in the text editor will be a temporary file
  • edquota will read it back in when the editor terminates
• Use the -g option to edit group quotas
• Some versions of Red Hat have a bug where you need to delete an extraneous space before the time unit when doing edquota -t

• The repquota command prints information about the quota limits assigned to each user
  • Also shows actual number of blocks and inodes used
• Use the -a option for information on all filesystems, or specify the filesystem on the command line
• Use -g to show group quotas
• Use -v for more complete information