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 due to 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 applies derived quotas to 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 and a user named jsmith creates a file in that volume, Data ONTAP applies a derived quota to jsmith to limit that user's 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 applies 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 and a file is created that is owned by a group named writers, 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 apply to 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 and 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

When a qtree is created in a volume that has a default qtree quota defined in the /etc/quotas file, and that default quota is applied as a derived quota to the qtree just created, Data ONTAP also applies derived default user and group quotas to that qtree.

If a default user quota or group quota is already defined for the volume containing the newly created qtree, Data ONTAP automatically applies that quota as the derived default user quota or group quota for that qtree.
If no default user quota or group quota is defined for the volume containing the newly created qtree, then the effective derived user or group quota for that qtree is unlimited. In theory, a single user with no explicit user quota defined can use up the newly defined qtree's entire qtree quota allotment.
You can replace these initial derived default user quotas or group quotas that Data ONTAP applies to the newly created qtree, by adding explicit or default user or group quotas for the qtree just created to the /etc/quotas file.

Example of a default user quota for a volume applied to a qtree

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, Data ONTAP applies a derived default user quota, which limits the amount of space used by each user who does not have an explicit user quota defined in the /vol/vol0/projects qtree to 10 GB.

You can 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.

Example of no default user quota for a volume applied to a qtree

Similarly, if no default user quota is defined for the /vol/vol0 volume, the default tree quota for the vol/vol0 volume specifies that all qtrees are limited to 100 GB of disk space, and if you create a qtree named projects, Data ONTAP does not apply a derived default user quota that limits the amount of disk space that users can use on the /vol/vol0/projects tree quota. In theory, a single user with no explicit user quota defined, can use as much as all 100 GB of a qtree's quota if no other user writes to disk space on the new qtree first.

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.

Even with no default user quota defined, no user with files on a qtree can use more disk space in that qtree than is allotted to that qtree as a whole.

Advantages of specifying default quotas

Specifying default quotas offers the following advantages:

You can automatically apply a limit to a large set of quota targets without typing an entry in the /etc/quotas file for each target. For example, if you want no user to use 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, all you need to do is 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.