Managing fractional space reservation

The following sections explain Data ONTAP space reservation and describe how to configure SnapManager for SQL fractional space reservation policies:

Understanding fractional space reservation

This section summarizes space reservation and fractional space reservation as supported by Data ONTAP 7.1 or higher. For more detailed information about these features, see the Data ONTAP Block Access Management Guide for iSCSI and FCP for release 7.1 or higher.

Space reservation

When you create a LUN on a storage system volume, Data ONTAP reserves enough space in the traditional or flexible volume so that write operations to those LUNs do not fail due to lack of disk space on the storage system. Other operations, such as making a Snapshot copy or creating new LUNs, can occur only if there is enough available unreserved space. These operations are restricted from using reserved space.

SnapDrive creates and manages LUNs with space reservation enabled. That is, additional space on the storage system volume is automatically reserved for overwriting blocks that belong to a LUN. By default, this additional space is equal to 100 percent of the total size of all space-reserved LUNs in the storage system volume.

If space reservation is disabled, write operations to a LUN might fail due to insufficient disk space in the storage system volume; consequently, the host application might terminate, report I/O errors, or exhibit unexpected behavior.

Fractional space reservation

With fractional space reservation, the amount of space reserved for overwrites is set to less than 100 percent of the total size of all space-reserved LUNs in a traditional volume or a flexible volume that has the guarantee option set to volume rather than file. The space that is preallocated for space reservation is reduced to that percentage. Fractional space reservation is generally used for volumes with LUNs that store data with a low rate of change.

While space reservation is enabled at the LUN level, fractional space reservation overwrite reserve amounts are configured at the volume level. Fractional space reservation does not control how the total amount of space reserved for overwrites in a volume is applied to individual LUNs in that volume.

Fractional space reservation policy settings in SnapManager for SQL

In a SnapManager environment in which SQL data is stored on LUNs in a fractional space-reserved storage system volume, you need to avoid an out-of-space condition. You can do this using the SnapManager space reservation policy tool to monitor the overwrite reserve utilization on the volumes. Space reservation has two policies that you can enable:

Automatic deletion of SQL backups
Automatic dismounting of SQL databases

These policies control whether SnapManager takes action to prevent the overwrite reserve from being fully depleted when the overwrite reserve space runs low on a fractional space reserved volume.

Note
If you enable SnapManager e-mail notification, SnapManager sends an SMTP e-mail message to specified recipients after a SnapManager fractional space reservation policy event completes.

Automatic deletion of SQL backups

SnapManager provides an option for automatic deletion of backups of LUNs that store SQL data. When you enable this option, the SnapManager fractional reservation policy serves as the SQL-aware replacement for or adjunct to the Data ONTAP Snapshot copy deletion feature. If the level of overwrite reserve utilization on the volume reaches a threshold specified by the policy, automatic backup deletion is triggered and SnapManager deletes SQL backups using the following guidelines:

Delete the oldest backup Snapshot copies first.
Retain the specified number of total backup Snapshot copies on the volume.
Retain the most recent backup of any database (if it resides on the volume).
Retain backup of any databases that are no longer in existence.
Retain up-to-the-minute restore ability for the older backups in other backup management groups. This option can be enabled only if "Delete Oldest backup Snapshot first" is selected.

Select the backup retention level that is appropriate to your SnapManager backup creation and verification schedule. If Snapshot copy deletion triggers, enough backup Snapshot copies should be retained so that at least one verified backup remains on the volume. Due to these SQL-aware features, the automatic deletion of Snapshot copies does not necessarily prevent an out-of-space condition on the volume.

Automatic dismounting of SQL databases

When this option is enabled, SnapManager provides an option for the automatic dismounting of SQL databases in space-reserved LUNs. It is triggered if overwrite reserve utilization on the volume reaches the threshold specified by the fractional space reservation policy. This stops SQL write operations to LUNs in a storage system volume for which overwrite reserve space is nearly full.

This second component of the fractional space reservation policy should be always enabled. It is a last-resort action that prevents further consumption of overwrite reserve.

When both options of fractional space reservation policy are enabled, the dismounting of SQL databases must be triggered at a higher level of overwrite reserve utilization than is used to trigger the deletion of SQL backup Snapshot copies. This enables SnapManager to first use backup-set deletion to free some overwrite reserve. If this is not sufficient, dismounting the affected database prevents further consumption of overwrite reserve.

Attention
If another host or client continues to write data to the affected volume, the overwrite reserve space might still run out and the storage system volume will go offline. For this reason, it is recommended that dedicated volumes be used for SQL data.

Fractional space reservation policy settings

The following table summarizes the fractional space reservation policy by listing each setting, along with its factory default value and its configurable values.

SnapManager fractional space reservation policy setting	Factory default value	Configurable values
Deletion of SQL backup Snapshot copies
Status	Enabled	Enabled or disabled¹
Trigger on overwrite reserve utilization	70%	1% - 99%²
Number of Snapshot copies to retain	5	1 - 256
Dismounting of SQL databases
Status	Always enabled¹
Trigger on overwrite reserve utilization	90%	1% - 99%²

¹ Enabling automatic deletion of SQL backup Snapshot copies does not necessarily prevent an out-of-space condition on the volume. Therefore, database dismounting is always enabled.

² Enabling automatic deletion of SQL backup Snapshot copies does not necessarily prevent an out-of-space condition on the volume. Therefore, if Snapshot copy deletion is enabled, it must be configured to trigger prior to database dismounting.

About the default fractional space reservation policy

The default fractional space reservation policy is automatically enabled for any traditional or flexible storage system volume that has overwrite reservation set to less than 100 percent and contains LUNs that store SQL database files, SQL transaction log files, or SnapManager SnapInfo directories.

Default policy with default settings

You can use the default policy as-is, allowing the factory default values to be applied to every storage system volume that contains fractional-space-reserved LUNs.

Default policy with customized settings

You can customize the default policy that is applied to all storage system volumes that contain fractional-space-reserved LUNs. You can configure the default policy settings to required values. This will be applied to the storage system volume if the "Apply default policy to selected object in the left window" check box is selected in the Fractional Space Reservation Settings screen.

Volume-specific policies

You can override the default policy for any particular volume that contains fractional-space-reserved LUNs by applying a custom policy.

For more information on how to configure volume-specific policies, see Configuring fractional space reservation policies

Viewing fractional space reservation status

In the Fractional Space Reservation Settings dialog box, use the Current Status tab to view the current space consumption in the storage system volumes that contain LUNs that store SQL data or SnapInfo directories. You will see the following information:

Drive Letter or Mount Point

A SnapManager configuration setting for the LUN. The drive letter or NTFS mount point on which the LUN is mounted.

Fractional Overwrite Reserve (%)

The amount of space reserved for overwrites on the storage system volume that contains the LUN. It is expressed as a percentage of the total size of all space-reserved LUNs in the volume.

Backup AutoDelete Trigger (%)

A SnapManager fractional space reservation policy setting for the storage system volume that contains the LUN. It is expressed as a percentage of overwrite reserve utilization that triggers automatic deletion of SQL backup sets.

Disable Database Trigger (%)

Used Overwrite Reserve (%)

The amount of overwrite reserve in use for the storage system volume that contains the LUN. It can be expressed in two ways:

As a percentage of the total size of all space-reserved LUNs in the volume
In megabytes representing the total size of all space-reserved LUNs in the volume

Available Reserve (MB)

This is the amount of overwrite reserve available for the storage system volume that contains the LUN.

Storage System Snapshot Autodelete

This state of the Data ONTAP Snapshot copy autodeletion feature: enabled or disabled for the storage system volume that contains the LUN. If this LUN stores SQL data files and is contained in a storage system volume for which the Data ONTAP Snapshot copy autodeletion feature is enabled, disable this feature on that volume or ensure that it is configured so that it will not delete SnapManager backup set components.

To view the current space consumption information about each LUN, complete the following steps.

Step	Action
1	In the SnapManager main window, select Options > Fractional Space Reservation Settings.
2	In the Current Status tab, note the space consumption status for each LUN that stores SQL data or SnapInfo directories. The following columns displays SnapManager configuration information: Drive Letter or Mount Point Backup Autodelete Trigger (%) Disable Database Trigger (%) Note The SnapManager fractional space reservation policy triggers (listed above) are not applicable to fully space-reserved LUNs. The following columns display the fractional overwrite reserve settings and status: Fractional Overwrite Reserve (%) Used Overwrite Reserve (%) Used Overwrite Reserve (MB) Available Reserve (MB) Storage System Snapshot Autodelete Note If Fractional Overwrite Reserve (%) is 100, the LUN is contained in a fully space-reserved volume rather than a fractionally space-reserved volume. The information displayed in this tab automatically refreshes every 60 seconds. Note Only the Drive Letter or Mount Point column displays LUN-specific information. All other columns in this tab display information that applies across the storage system volume that contains the LUN.
3	If the Snapshot Autodelete column is `Enabled`, investigate the cause and take preventive action if necessary. Attention If the Storage Snapshot Autodelete column is `Enabled`, the LUN is contained in a FlexVol^TM volume that has overwrite reserve set to less than 100 percent and that also has the Data ONTAP automatic Snapshot copy deletion feature enabled and configured to trigger when the overwrite reserve is nearly full. If SQL data or SnapManager SnapInfo directories are stored on LUNs contained in a volume with these features, the Data ONTAP Snapshot copy autodeletion policy might delete SQL backup set components. If the Storage Snapshot Autodelete column is `Enabled`, take one of the following actions on the volume: Disable the Data ONTAP Snapshot copy autodelete feature. Ensure that the Data ONTAP Snapshot copy autodelete feature is configured in such a way that it will not delete SQL backup set components. For details about the `snap autodelete` storage system command, see the Data ONTAP® Block Access Management Guide for iSCSI and FCP for release 7.1 or later. Note The SnapManager fractional space reservation policy includes a separate, SQL-aware autodeletion feature. For details, see Automatic deletion of SQL backups and Configuring fractional space reservation policies. The SnapManager autodeletion feature can be used in place of or in conjunction with the Data ONTAP autodeletion feature; you can also choose to disable the SnapManager autodeletion feature.
4	To close the dialog box, click OK.

Configuring fractional space reservation policies

In the Fractional Space Reservation Settings dialog box, use the Policy Settings tab to view or customize the default policy and to configure custom policies for individual fractional-space-reserved LUNs.

The default fractional space reservation policy and its factory default settings are described in Appendix C, About the default fractional space reservation policy.

Note
SnapManager automatically applies the default policy to every storage system volume that contains fractional-space-reserved LUNs that store SQL database files or SnapInfo directories. Therefore, your storage is protected from an out-of-space condition, without requiring you to explicitly enable or configure any fractional space reservation policies.

To configure the fractional space reservation policy, complete the following steps.

Step	Action
1	In the SnapManager main window, select Options > Fractional Space Reservation Settings.
2	Select the Policy Settings tab.
Choose to specify either the default policy or a volume-specific policy
3	In the left navigation tree, select the scope of the policy you want to view or change in the main panel on the right-hand side of the tab:
	If you want to view or change...	Then do this...
	The default policy	In the navigation tree, select Default Policy.
	A volume-specific policy	In the navigation tree, select the storage system and then the volume.
Disable or configure automatic deletion of SQL backup Snapshot copies
4	Use the "Automatic deletion of backups" panel to disable, enable, or configure automatic deletion of SQL backup Snapshot copies in fractional-space-reserved LUNs on the volume. Note Although automatic deletion of SQL backup Snapshot copies does not necessarily prevent an out-of-space condition on the volume, it is recommended that this feature be enabled for every volume that contains fractional-space-reserved LUNs that store SQL data. For more information, see Automatic deletion of SQL backups.
	If you want to...	Then do this...
	Enable automatic deletion of SQL backup Snapshot copies	Select the "Delete backups that include LUNs which have less than 100% overwrite reservation" option, and then skip to Step 7.
	Disable automatic deletion of SQL backup Snapshot copies	De-select the "Delete backups that include LUNs which have less than 100% overwrite reservation" option, and then proceed to Step 5.
	Note Data ONTAP includes a separate Snapshot copy autodeletion feature. For details, see Viewing fractional space reservation status. The SnapManager autodeletion feature can be used in place of or in conjunction with the Data ONTAP autodeletion feature.
5	In the "Trigger point for overwrite reserve utilization" field, enter the level of overwrite reserve utilization (in percentage of total reserve) that is to trigger deletion of SQL backup Snapshot copies. The value must be a non-negative integer that is less than the "Trigger point for overwrite reserve utilization" value in the "Automatically dismount databases" panel.
6	In the "Number of most recent backup sets to retain" field, enter the number of backup set to be retained, if automatic backup set deletion is triggered. The value must be an integer from 1 through 256 and should be based on the backup creation and verification schedule. For more information, see Automatic deletion of SQL backups".
Configure automatic dismounting of SQL databases
7	Use the "Automatically dismount databases" panel to configure automatic dismounting of SQL databases in fractional-space-reserved LUNs on the volume. Note The automatic deletion of SQL backup Snapshot copies does not necessarily prevent an out-of-space condition on the volume. SnapManager does not allow you to disable dismounting of databases for any fractional space reservation policy. In the "Trigger point for overwrite reserve utilization" field, enter the level of overwrite reserve utilization (in percentage of total reserve) that is to trigger dismounting of SQL databases. The value must be an integer from 0 through 99. Note If Snapshot copy autodeletion is enabled, SnapManager requires that this threshold be set to a higher level than the threshold that triggers automatic Snapshot copy deletion. This ensures that Snapshot copy autodeletion is triggered first. For more information, see Automatic dismounting of SQL databases.
Apply the changes to the default or volume-specific policy
8	To apply your changes and close the dialog box, click OK.