Configuring the filer's storage

How to configure filer storage

You use volumes, qtrees, files, VLDs, and LUNs to configure and manage the filer's storage.

Note
When the filer is first set up and configured, Data ONTAP creates a /vol/vol0 root volume. This is used by Data ONTAP to administer the filer. Do not create any volumes or qtrees on /vol/vol0 that would be used to store LUNs.

When you create a volume, you must specify its name and the number of disks to be used to create it. The capacity and the number of disks you specify determine the size of the volume. In addition to the number of disks, you can also specify the size of the RAID group. For information about disk allocation or RAID groups and to configure them, see the Data ONTAP 6.4 System Administration Storage Management Guide. For information about disk configurations for your filer model, see the System Configuration Guide at http://now.netapp.com/NOW /knowledge/docs.

The size of the volume depends on the size of the LUN or LUNs it will contain and whether you want to enable space reservations for filling holes and overwriting files and whether you intend to create snapshots or clones of LUNs.

A qtree is a subdirectory of the root directory of a volume. You use qtrees to group LUNs to facilitate management tasks, such as backup and recovery. When you create a qtree, you specify the name of the qtree and the path of the volume where it is located. See the Data ONTAP 6.4 System Administration Storage Management Guide for details about volumes and qtrees.

How to calculate the size of a LUN

When you create a LUN, you specify its size as raw disk space, depending on the filer and the host. The usable space in the LUN is dependent on host or application overhead requirements. For example, partition tables and host file system metadata reduce usable space for applications.

The minimum size for a LUN in a Solaris environment is 4 MB. The minimum size for a LUN in a Windows environment is 31.5 MB. The maximum size for a LUN cannot exceed approximately 50% of the size of a volume.

How to calculate the volume size for LUNs

Typically, you create volumes between two and three times the size of the LUN capacity required. Creating volumes at least two times the size of the LUN capacity accounts for the creation of at least one snapshot of the LUNs along with the overhead required by the host that will be accessing the LUNs and the application that will be using it. If you want to take additional snapshots of the LUNs and store them in the same volume, multiply the size of the LUNs by that number to calculate the maximum size of the volume needed.

How to calculate the volume size for writable snapshot LUNs

If you want to create a writable snapshot LUN from a LUN in a snapshot (by using the lun create -b command), create the volume at least three times the size of the LUN, plus some amount of space to handle data writing to the writable snapshot LUN, or use the following formula to calculate the volume size needed.

(2 * LUN_size) + DataWrittenToLUN + DataWrittenToWritableSnapshotLUN = TotalUsableSpaceNeededInVolume

(2 * LUN_size)--handles the reserved space required to support overwritting all data in the LUN

DataWrittenToLUN--is the space needed to write data to the original LUN. It seems this should be the size of the LUN, but is it enforced.

DataWrittenToWritableSnapshotLUN--is the amount of data you expect to write to a LUN created using a LUN in a snapshot as backing store

Example

The following table illustrates the effect of LUN and snapshot operations upon available space in a volume for which space reservations have been enabled.

State	Process	Result
1	From the filer, create a 500-GB volume. Assume disk capacity is 72 GB and you need to round up to account for overhead. Example vol create /vol/vol18	Used space = 0 Reserved space = 0 Available space = 500 GB
2	From the filer, create a 100-GB lun0 on that volume. Example lun create -s 100g /vol/vol1/lun0	Used space = 0 Reserved space = 100 GB Available space = 400 GB
3	From the filer, map lun0 to an initiator group	No effect.
4	From the host, format lun0, create and mount the file system, and write 100 GB of data to lun0. Host and application specific.	Used space = 100 GB Reserved space = 100 GB Available space = 300 GB
5	From the filer, take a snapshot of /vol/vol1. Example snap create vol1 mysnap	Used space =100 GB Reserved space = 100 GB Available space = 300 GB
6	From the filer, create a writable snapshot of lun0, lunwsnap, from the snapshot you just created. Example `lun create -b` /vol/vol1/.snapshot/mysnap /vol/vol1/lunwsnap	Used space =100 GB Reserved space = 200 GB Available space = 200 GB
7	From the filer, map lunwsnap to an initiator group	No effect.
8	From the host, format lunwsnap, create and mount the file system, and write 10 GB of data to lunwsnap. Host and application specific.	Used space = 110 GB Reserved space = 200 GB Available space = 190 GB
9	From the filer, delete lunwsnap. Example l`un destroy -f` /vol/vol1/lunwsnap	Used space = 100 GB Reserved space = 100 GB Available space = 300 GB

How to manage space reservation in a SAN environment

Space reservation is the ability to control how space is reserved for entities such as volumes, qtree, files, VLDs, and LUNs when they are written to.

If space reservation is disabled, the amount of disk space required to actually write the entity of that size is not reserved. That is, the entity contains holes and a write to that entity can fail due to a lack of disk space (resulting in an ENOSPC error). In addition, when an entity is captured in a snapshot, there is no guarantee that the original entity can be overwritten by an application. These entities are restricted from using reserved space. Writes to new or existing unreserved space in a volume fails when the total amount of free space in the volume is less than the amount needed according to the current reserve values. After free space \goes below this value, only writes to reserved space succeed and are guaranteed not to fail due to a lack of disk space.

You manage space reservation at the volume, qtree, file, VLD, or LUN level. When you create volumes, qtrees, files, and VLDs, they are space reserve disabled by default, so you must manually change the settings to enable space reservations for these entities (unless you use SnapDrive to create VLDs). At the volume level, you set a vol option (vol options vol_name create_reserved {on|off} that controls the following behaviors:

Reserves space to fill holes
Reserves space to overwrite blocks stored in snapshots

When space reservation is enabled at the volume level, all entities below it are space reserved. The same is true for qtrees and files.

At the qtree and file levels, you use the qtree and file commands to enable or disable reservation fill or reservation overwrite.

When you create LUNs, you specify whether they are space reservation enabled. Network Appliance recommend that you always enable space reservation for LUNs. This causes Data ONTAP to reserve enough space in the volume so that writes to the LUN do not fail due to a lack of disk space. This also guarantees that other operations, such as snapshots or the creation of new LUNs can occur. If you choose to disable space reservation and then later enable space reservation at the LUN level, you use the lun set reservation command. This command disables or enables both reservation fill and reservation overwrite.

Note
For CIFS files that have reserved holes or space reserved beyond eof, that space is reserved regardless of whether space reservation is enabled at the volume level.

How space reservation settings persist

Space reservation settings persist across reboots, takeovers, givebacks, and snap restores, but do not persist through reversions to an earlier software version. Single File Snap Restore restores the reserved state at the time the snap restore is taken. Space reservation remains in effect whether or not reservations are enabled at the volume level.

How space reservation settings are logged

All changes to space reservation settings are logged as system messages to the console, log files and other remote systems, as specified by its /etc/syslog.conf configuration file.

How to set space reservation settings for all entities

The following table summarizes how to enable or disable space reservations for all entities.

Entity	Command
volume	vol options volname create_reserved {on\|off} Note Space reservation settings apply only to new LUNs, or LUNs moved into the volume.
qtree	qtree reservation path_name [enable\|disable] Note Space reservation settings apply only to new LUNs, or LUNs moved into the qtree.
file	file reservation path_name [enable\|disable]
LUN	lun set reservation lun_path [enable\|disable] Note Network Appliance strongly recommends that you enable space reservation for LUNs.
VLD	Space reservation is disabled for VLDs by default. However, SnapDrive automatically enables space reservation. Caution Network Appliance strongly recommends that volumes containing VLDs are always space reservation enabled.

Monitoring reserved space

You monitor reserved space, used space, and available space with the df -r command. If available space is zero, you cannot create new LUNs in that volume, and snapshots can no longer be taken.

Reserved space for snapshots

If you have the SnapRestore® license, you can use the Data ONTAP Snapshot^TM feature to take individual or collective snapshots of LUNs when you need to store and retrieve them. You use the snap command to control snapshots at the LUN level, whether you take snapshots manually or schedule automatic snapshots. Snapshots are guaranteed when the LUNs are space reserved, so if you intend to schedule automatic snapshots, add the amount of space required to the volume size. For information about using the Snapshot feature, see Chapter 5, Using Data Protection in the SAN Environment.