Understanding quotas

Reasons for specifying quotas

You specify a quota for the following reasons:

To limit the amount of disk space or the number of files that can be used by a quota target
To track the amount of disk space or the number of files used by a quota target, without imposing a limit
To warn users when their disk space or file usage is high

Quota targets

A quota target can be

A user, as represented by a UNIX ID or a Windows ID.
A group, as represented by a UNIX group name or GID. Data ONTAP does not apply group quotas based on Windows IDs.
A qtree, as represented by the path name to the qtree.

The quota target determines the quota type. For example, a quota whose target is a user is called a user quota, and a quota whose target is a group is called a group quota.

Exception

A quota for a qtree is called a tree quota, not a qtree quota.

Tree quotas

If you apply a tree quota to a qtree, the qtree is similar to a disk partition, except that you can change its size at any time. When applying a tree quota, Data ONTAP limits the disk space and number of files regardless of the owner of the disk space or files in the qtree. No users, including the root and members of the BUILTIN\Administrators group, can write to the qtree if the write exceeds the tree quota.

Quota specifications

Quota specifications are stored in the /etc/quotas file, which you can edit at any time.

User and group quotas are applied on a per-volume or per-qtree basis. You cannot specify a user or group quota for the entire filer or for multiple volumes.

Example

You can specify that a user named jsmith can use up to 10 GB of disk space in the /vol/vol0 volume, or that a group named engineering can use up to 50 GB of disk space in the /vol/vol0/projects qtree.

Explicit quotas

If the quota specification references the name or ID of the quota target, the quota is an explicit quota. For example, if you specify a user name, jsmith, as the quota target, the quota is an explicit user quota. If you specify the path name of a qtree, /vol/vol0/engineering, as the quota target, the quota is an explicit tree quota.

Default quotas and derived quotas

The disk space used by a quota target can be restricted or tracked even if you do not specify an explicit quota for it in the /etc/quotas file. The target might be under the effect of a quota that is derived from a default quota. If a quota is applied to a target but the name or ID of the target does not appear in an /etc/quotas entry, the quota is called a derived quota. For more information about default quotas, see Understanding default quotas. For more information about derived quotas, see Understanding derived quotas.

Hard quotas, soft quotas, and threshold quotas

A hard quota is a limit that cannot be exceeded. If an operation, such as a write, causes a quota target to exceed a hard quota, the operation will fail. When this happens, a warning message is logged to the filer console and an SNMP trap is issued.

A soft quota is a limit that can be exceeded. When a soft quota is exceeded, a warning message is logged to the filer console and an SNMP trap is issued. When the soft quota returns to normal, another syslog message and SNMP trap are generated. In Data ONTAP, you can specify both hard and soft quota limits for the amount of disk space used (in blocks) and the number of files created.

A threshold quota is similar to a soft quota restricted to specifying quotas in blocks. When a threshold quota is exceeded, a warning message is logged to the filer console and an SNMP trap is issued.

A single type of SNMP trap is generated for all types of quota events. You can find details on SNMP traps in the filer's /etc/mib/netapp.mib file.

Syslog messages about quotas contain qtree ID numbers rather than qtree names. You can correlate qtree names to the qtree ID numbers in syslog messages by using the qtree status -i command. For more information, see Displaying qtree status.

Notification about threshold crossings

SNMP traps can be used to arrange e-mail notification when quotas are exceeded. You can access and adapt a sample quota notification script on the NOW site at now.netapp.com in the Tools and Utilities section.