Disk quotas are used to place limits on the amount of disk space available to certain users and groups. There are two types of quotas - soft and hard quotas.

1. soft quotas - allows further disk allocation but, once threshold is reached, warns users and sets a grace period time so they can decrease their disk usage below limits. After the grace period is over, the soft limit is handled as a hard limit
2. hard quotas - does not allow any more disk allocation once threshold is reached.

Currently, IRCNow's shell limits disk usage to 1GB (soft) and 1.5GB (hard) and users have 1 week to decrease disk usage.

To enable quotas, We need to mark all the desired filesystems by adding the keywords userquota and/or groupquota in fstab(5) for each filesystem We want to be able to enforce quotas.

/dev/sda.a /home ffs rw,nodev,nosuid,userquota 1 2

To set thresholds, We use edquota(8). Keep in mind that values are measured in KB (1000KB equals to 1MB).

edquota(8) invokes vi(1) unless you specify other editor.

$ edquota [-u] PiRATA
Quotas for user PiRATA:
/home: KBytes in use: 62, limits (soft = 256000, hard = 512000)
        inodes in use: 25, limits (soft = 0, hard = 0)
 
$ edquota -g staff
Quotas for group staff:
/home: KBytes in use: 62, limits (soft = 1000000, hard = 1500000)
        inodes in use: 25, limits (soft = 0, hard = 0)       

On the first above, PiRATA can use up to 512MB on his $HOME. On the latter, all staff users are limited to 15GB of disk space.

Now, what if you need to define a certain threshold for 7 or 8 users or even 50? What would you do? Probably you would use edquota(8) for each user.

Gladly, We don't need to - by invoking the -p option we can have the thresholds replicated:

$ edquota -p PiRATA jrmu jimmy_will dennis

By invoking the -t option, grate time can be defined:

$ edquota -t search_social
Time units may be: days, hours, minutes, or seconds
Grace period before enforcing soft limits for users:
/home: block grace period: 7 days, file grace period: 7 days

To turn on or off disk quotas, use quotaon(8) and/or quotaoff(8).

// Enables disk quotas on all filyesystems that have the user/diskquota flag defined on fstab(5)
$ quotaon -a
 
// Disables disk quotas for user net_wayfarer
$ quotaoff -u net_wayfarer

quota(1) outputs current disk usage and limits. By default, only the user quotas are printed but you can see group quotas by invoking -g option.

$ quota jrmu
Disk quotas for user jrmu (uid 1012): 
  Filesystem  KBytes    quota   limit   grace    files   quota   limit   grace
       /home    3704  5000000 10000000              58        0       0