Understanding derived quotas

About derived quotas

Data ONTAP derives the quota information from the default quota entry in the /etc/quotas file and applies it if a write request affects the disk space or number of files used by the quota target. A quota applied due to a default quota, not an explicit entry in the /etc/quotas file, is referred to as a derived quota.

Derived user quotas from a default user quota

When a default user quota is in effect, Data ONTAP creates derived quotas for all users in the volume or qtree to which the default quota applies, except those users who have explicit entries in the /etc/quotas file. Data ONTAP also tracks disk usage for root and BUILTIN\Administrators in that volume or qtree.

Example

If a default user quota entry specifies that users in the /vol/vol0 volume are limited to 10 GB of disk space, when a user named jsmith is created, Data ONTAP applies a derived quota to jsmith to limit his disk usage in the /vol/vol0 volume.

Derived group quotas from a default group quota

When a default group quota is in effect, Data ONTAP creates derived quotas for all UNIX groups in the volume or qtree to which the quota applies, except those groups that have explicit entries in the /etc/quotas file. Data ONTAP also tracks disk usage for the group with GID 0 in that volume or qtree.

Example

If a default group quota entry specifies that groups in the /vol/vol0 volume are limited to 10 GB of disk space, when a group named writers is created, Data ONTAP applies a derived quota to the writers group to limit its disk usage in the /vol/vol0 volume.

Derived tree quotas from a default tree quota

When a default tree quota is in effect, derived quotas are created for all qtrees in the volume to which the quota applies, except those qtrees that have explicit entries in the /etc/quotas file.

Example

If a default tree quota entry specifies that qtrees in the /vol/vol0 volume are limited to 10 GB of disk space, when a qtree named projects is created in the /vol/vol0 volume, Data ONTAP applies a derived quota to the /vol/vol0/projects qtree to limit its disk usage.

Default user or group quotas derived from default tree quotas

In addition to derived tree quotas, Data ONTAP creates derived default user or group quotas based on the default tree quota if default user or group quotas also exist in the same volume.

When a default user or group quota is created

A derived default user or group quota is created after you create a qtree in the volume where the default tree quota applies.

Limits on disk space and number of files

No limits for a default user or group quota derived from a default tree quota are specified. Data ONTAP creates the default user or group quotas automatically when the qtree is created. This enables you to create qtree-specific user or group quotas later without reinitializing quotas on the volume.

Example

Suppose the default user quota in the /vol/vol0 volume specifies that all users are limited to 10 GB of disk space and the default tree quota in the vol/vol0 volume specifies that all qtrees are limited to 100 GB of disk space. If you create a qtree named projects in the /vol/vol0 volume, a default tree quota limits the projects qtree to 100 GB. Also, a derived default user quota is created, which does not limit the amount of space or number of files used by each user in the /vol/vol0/projects qtree. This unlimited default user quota on the /vol/vol0/projects qtree causes Data ONTAP to track the amount of space and number of files being used by each user in the /vol/vol0/projects qtree. You can then change the limits on the default user quota for the /vol/vol0/projects qtree or add an explicit quota for a user in the /vol/vol0/projects qtree by using the quota resize command.

Similarly, if the default group quota in the /vol/vol0 volume specifies that all groups are limited to 10 GB of disk space, a derived default group quota results from the default tree quota. This default group quota does not limit the amount of space or number of files used by a group in the /vol/vol0/projects qtree. This unlimited default group quota causes Data ONTAP to track the amount of space and number of files being used by groups in the /vol/vol0/projects qtree. You can then change the limits on the default group quota for the /vol/vol0/projects qtree or add an explicit quota for a group in the /vol/vol0/projects qtree by using the quota resize command.

In addition, UID 0, BUILTIN\Administrators, and GID 0 have derived quotas. These derived quotas do not limit the disk space and the number of files. They only track the disk space and the number of files owned by these IDs.

Advantages of specifying default quotas

Specifying default quotas offers the following advantages:

You can automatically apply a limit to a large number of quota targets without typing an entry in the /etc/quotas file for each target. For example, if you want all users to use no more than 10 GB of disk space, you can specify a default user quota of 10 GB of disk space instead of creating an entry in the /etc/quotas file for each user.
You can be flexible in changing quota specifications. Because Data ONTAP already tracks disk and file usage for quota targets of derived quotas, you can easily change the specifications of these derived quotas.
For example, you can create a default user quota for the /vol/vol0 volume that limits all users to 10 GB of disk space. After quota initialization, this default quota and its derived quotas go into effect.

If you later decide that a user named jsmith should have a greater quota, you can add an /etc/quotas entry that specifies that jsmith is limited to 20 GB of disk space, overriding the default 10-GB limit. After making the change to the /etc/quotas file, you only need to resize the quota, which takes less time than quota reinitialization, to make the jsmith entry effective. Without a default user quota, the newly created jsmith entry requires a quota reinitialization to be effective.