Managing Filesystem And Project Space - Oracle ZFS Storage Appliance Administration Manual

Snapshot Data - This represents the total amount of data currently held by all snapshots of
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the share. This is the amount of space that would be free should all snapshots be destroyed.
Quota - A quota represents a limit on the amount of space that can be consumed by any
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particular entity. This can be based on filesystem, project, user, or group, and is independent
of any current space usage.
Reservation - A reservation represents a guarantee of space for a particular project or
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filesystem. This takes available space away from the rest of the pool without increasing the
actual space consumed by the filesystem. This setting cannot be applied to users and groups.
The traditional notion of a statically sized filesystem can be created by setting a quota and
reservation to the same value.

Managing Filesystem and Project Space

The simplest way of enforcing quotas and reservations is on a per-project or per-filesystem
basis. Quotas and reservations do not apply to LUNs, though their usage is accounted for in the
total project quota or reservations.
Data Quotas - A data quota enforces a limit on the amount of space a filesystem or project
can use. By default, it will include the data in the filesystem and all snapshots. Clients
attempting to write new data will get an error when the filesystem is full, either because
of a quota or because the storage pool is out of space. As described in
Management" on page
when snapshots are present. Removing a file may cause the filesystem to write new data if the
data blocks are referenced by a snapshot, so it may be the case that the only way to decrease
space usage is to destroy existing snapshots.
If the 'include snapshots' property is unset, then the quota applies only to the immediate data
referenced by the filesystem, not any snapshots. The space used by snapshots is enforced by the
project-level quota but is otherwise not enforced. In this situation, removing a file referenced
by a snapshot will cause the filesystem's referenced data to decrease, even though the system as
a whole is using more space. If the storage pool is full (as opposed to the filesystem reaching a
preset quota), then the only way to free up space may be to destroy snapshots.
Data quotas are strictly enforced, which means that as space usage nears the limit, the amount
of data that can be written must be throttled as the precise amount of data to be written is not
known until after writes have been acknowledged. This can affect performance when operating
at or near the quota. Because of this, it is generally advisable to remain below the quota during
normal operating procedures.
Quotas are managed through the BUI under Shares > General > Space Usage > Data. They are
managed in the CLI as the quota and quota_snap properties.
439, this behavior may not be intuitive in all situations, particularly
Managing Filesystem and Project Space
"Snapshot Space
Shares and Projects 397