Configuring disk space

Filer storage units for managing disk space

You use the following storage units to configure and manage disk space on the filer:

Volumes
Qtrees
Files
Logical Unit Numbers (LUNs)

The volume is the largest unit of storage. It can hold qtrees, files, and LUNs. A qtree is a subdirectory of the root directory of a volume. You can use qtrees to subdivide a volume to group LUNs. You use LUNs to store data in a SAN environment, and you use files to store data in a NAS environment.

Where LUNs reside

You create LUNs at the root of a volume or a qtree, with the exception of /vol/vol0. Do not create LUNs at /vol/vol0 because this is used by Data ONTAP for system administration.

The following diagram shows four LUNs that were created by using the storage from Filer_A. The four LUNs are:

/vol/vol1/LUN0--created at volume /vol/vol1
/vol/vol2/LUN1--created at /vol/vol2
/vol/vol2/qtree1/LUN2--created at the qtree of volume /vol/vol2, at /vol/vol2/qtree1
/vol/vol2/qtree1/LUN3--also created at the qtree of volume /vol/vol2, at /vol/vol2/qtree1

Why the space reservation option is important

LUNs appear as local disks attached to the host. As a result, it is imperative that LUNs behave as attached disks, which means that when you create a LUN of a specified size, the application storing data in the LUN must have guaranteed access to that amount of space. Data ONTAP achieves this with the space reservation option.

When you create a LUN of a specified size, Data ONTAP turns the space reservation on by default and reserves the number of blocks (not the physical blocks) in the WAFL® volume that are required to complete writes to the LUN. This ensures that writes to the LUN do not fail due to a lack of disk space.

How space reservation persists

Space reservation settings persist across reboots, takeovers, givebacks, and snap restores. The space reservation setting persists if you revert from Data ONTAP 6.5 to Data ONTAP 6.4, but not if you revert from Data ONTAP 6.4 to Data ONTAP 6.3. Single File SnapRestore restores the reserved state of a LUN to the reserved state at the time the snapshot was taken.

Why you need to change snapshot defaults

NetApp snapshots are required for many NetApp features, such as the SnapMirror® feature, SyncMirror^TM feature, SnapVault^TM feature, dump and restore, and ndmpcopy.

Snapshots cannot use reserved space, so snapshots can fail if there is not enough available space on the volume. When using snapshots, you need to calculate the amount of storage capacity required for the volume based on the information at Guidelines for calculating the size of a volume.

When you create a volume, Data ONTAP automatically

Enables snapshots by default
Reserves 20% of the space for snapshots (snap reserve, or snapshot reserve in FilerView)
Schedules snapshots

Because the internal scheduling mechanism for taking snapshots within Data ONTAP has no means of ensuring that the data within a LUN is in a consistent state, NetApp recommends that you change these snapshot settings by performing the following tasks:

Set the percentage of snap reserve to 0%
Turn off the automatic snap schedule

With Windows hosts, you use SnapDrive and SnapManager to ensure that applications accessing LUNs are quiesced or synchronized automatically before taking snapshots. With hosts that are not supported with SnapDrive, NetApp recommends that you ensure that the file system or application accessing the LUN is quiesced or synchronized before taking snapshots. For information about how to use snapshots, see Using snapshots.