Linux Sysadmin Notes

Disk Quotas

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Disk Quotas

Concept

Disk quotas limit how much space or how many inodes a user, group, or project can consume on a filesystem. They are enforced by the filesystem (ext4, XFS) after the quota feature is turned on and the quota files are consistent.

Soft limits warn; hard limits reject the write with EDQUOT. Grace periods let a user exceed the soft limit for a while before it becomes hard.

Why it matters

  • A full filesystem that df says has free space is often a quota. The application sees Disk quota exceeded.
  • Runaway processes (logs, build caches, mail spools) are contained without filling the volume for everyone else.
  • Project quotas (XFS) or directory quotas give per-tenant limits on a shared filesystem without a separate mount.
  • Quotas that were never turned on, or whose files are corrupt, silently do nothing until the incident.

Mental Model

filesystem mounted with quota option (usrquota, grpquota, prjquota)
    → quota files (aquota.user, aquota.group) or XFS internal
         → kernel tracks usage
              → write that would exceed hard limit → EDQUOT

repquota / quota  → human view
edquota           → set limits
quotacheck        → rebuild accounting (offline or careful)

XFS project quotas are often used for containers or per-directory limits. They require the project to be defined and the directory to be marked.

Key Commands

# Is quota on for this mount?
mount | grep -E 'usrquota|grpquota|prjquota'
findmnt -o TARGET,OPTIONS /home
 
# Usage
repquota -a
repquota -u /home
quota -u username
quota -g groupname
 
# XFS
xfs_quota -x -c 'report -h' /data
xfs_quota -x -c 'report -p -h' /data
 
# Set limits (interactive or via setquota)
edquota -u username
setquota -u username 1000000 1100000 0 0 /home
 
# Turn on (ext)
quotacheck -cugm /home
quotaon -v /home
 
# Grace
edquota -t

Common Failure Modes & Symptoms

SymptomLikely causeFirst checks
Disk quota exceeded but df shows spaceUser/group/project hard limit hitquota -u, repquota
Quota “set” but never enforcedFilesystem not mounted with quota option, or quotaon not runfindmnt, quotaon -p
Usage looks wrong after crashQuota files inconsistentquotacheck (plan downtime)
XFS project quota ignoredProject not assigned to directoryxfs_quota -x -c 'project -d /path'
Soft limit exceeded, writes still workStill inside grace periodrepquota grace column
Only some users affectedLimits differ; or only user quota onrepquota -a
Container fills host despite limitNo project quota or cgroup limitboth layers

Investigation Tips

  • When an application logs EDQUOT or “quota exceeded”, run quota for that UID before you look at df. They answer different questions.
  • repquota -a on a large filesystem can be slow. Target the mount that owns the path (findmnt -T).
  • After enabling quotas, always run quotacheck or the XFS equivalent so the accounting matches reality. Otherwise the first writes are allowed and the numbers are fiction.
  • Grace periods surprise people. A user who was “fine yesterday” hits the hard limit today when the grace expires.
  • Project quotas and user quotas can both apply. The tighter one wins.
  • Quotas do not replace monitoring. Alert on soft-limit breaches so you hear about the problem before the hard limit stops the service.

Personal Lessons Learned

  • A build user filled /home according to the ticket. df showed 40 % free. quota -u build showed the hard block limit reached. Raising the quota (and cleaning the cache) fixed it; enlarging the filesystem would have been the wrong change.
  • Quotas were configured in fstab but quotaon had never been run after the last fsck. They had been decorative for months.
  • An XFS project quota on a container root was never applied because the directory was not marked with the project ID. xfs_quota report showed zero usage for that project while the directory grew.

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