Managing fractional space reservation

This topic summarizes Data ONTAP fractional space reservation and describes how to configure SnapManager fractional space reservation policies:

About fractional space reservation

The following paragraphs summarize space reservation and fractional space reservation as supported by Data ONTAP 7.1 or greater. For more detailed information about these features, see the Data ONTAP Block Access Management Guide for iSCSI and FCP for release 7.1 or later.

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 of a lack of disk space on the storage system. Other operations, such as taking a Snapshot copy or the creation of 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 and the host application may terminate, report I/O errors, or experience unexpected behavior.

Fractional space reservation

With fractional reserve, 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 reserve 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 overwrite reserve amounts are configured at the volume level; that is, 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.

What can happen with a fractional-space-reserved volume

When a LUN is fully space reserved, write operations to that LUN are guaranteed against failure caused by an out-of-space condition due to Snapshot copy disk space consumption. When the overwrite reserve for a volume is set to less than 100 percent, however, write operations to the LUNs on that volume are no longer guaranteed when the storage system volume runs low in free disk space due to Snapshot copy space consumption.

Attention
If a storage system volume runs out of overwrite reserve space, write operations to a LUN on that volume will fail and the host application may terminate, report I/O errors, or exhibit unexpected behavior.

Data ONTAP provides two space management tools tor ensure that a fractionally space-reserved volume does not run out of overwrite reserve: automatic FlexVol expansion and automatic Snapshot copy deletion from FlexVol volumes. These features, summarized in the following paragraphs, monitor the reserved space and take action if the free space becomes scarce. For more detailed information, see the Data ONTAP Block Access Management Guide for iSCSI and FCP for release 7.1 or later.

Automatic expansion of flexible volumes

Data ONTAP can automatically expand the volume that is used to store Snapshot copy data, provided the volume is a flexible volume with the guarantee option set to volume. When the flexible volume is nearly full, Data ONTAP automatically expands the volume into the space preallocated for it in the aggregate. The automatic Snapshot copy deletion and FlexVol expansion features can be enabled separately, or together with one policy to be applied before the other. When fractional-space-reserved volumes hold LUNs that store Exchange database files, however, only the automatic FlexVol expansion feature can be used, if needed.

Automatic deletion of Snapshot copies from flexible volumes

Data ONTAP can automatically delete one or more Snapshot copies on the volume, provided the Data ONTAP Snapshot copy autodeletion policy is enabled and set to trigger when the overwrite reserve is nearly full on the volume. If the trigger condition is detected, the oldest or newest Snapshot copies are deleted until a configured percentage of the volume is free space. Note that this Data ONTAP feature is not designed specifically to support backup and restore operations on Exchange databases:

The options for selecting Snapshot copies to be deleted do not have visibility to the automatic backup Snapshot copy deletion criteria configured in SnapManager.
Exchange administrators want to retain at least one online backup for each database at all times.

If the Data ONTAP Snapshot copy autodelete policy is enabled for a volume that stores SnapManager backup set components, either disable the policy or configure it so that it does not delete the SnapManager backup set components. For more information, see Viewing fractional space reservation status.

Fractional space reservation policies manage Exchange data

In a SnapManager environment in which Exchange data is stored on LUNs in a fractional space-reserved storage system volume, the Exchange administrator needs to avoid an out-of-space condition on the volume in a way that allows explicit or implicit Exchange-aware control over the deletion of Exchange backup set components. To address this need, SnapManager provides its own space management tool for monitoring overwrite reserve utilization on the volumes. If overwrite reserve space runs low for a fractional space-reserved volume, SnapManager can take action to prevent the overwrite reserve from becoming fully depleted. Specifically, SnapManager can delete Exchange backup sets or dismount Exchange databases (or both), triggered when the overwrite reserve utilization for the volume reaches specific thresholds specified in the fractional space reservation policy.

Note
If SnapManager e-mail notification is enabled, SnapManager sends SMTP e-mail after a SnapManager fractional space reservation policy event finishes.

Automatic deletion of Exchange backups

SnapManager provides for the automatic deletion of backups of LUNs that store Exchange data. When enabled, this component of the SnapManager fractional reservation policy serves as the Exchange-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 Exchange backups as follows:

Delete the oldest backup Snapshot copies first.
Retain the specified number total backup Snapshot copies on the volume.
Retain the most recent backup of any database (if it resides on the volume).
Retain any backups of databases no longer in existence.

Choose the backup retention level based on 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 Exchange-aware features, the automatic deletion of Snapshot copies does not necessarily prevent an out-of-space condition on the volume.

SnapManager execute based on the policy for the volume that exceeds the thresholds, not other volumes that could exist in the same backup set.

For example, suppose you have an Exchange server the has backup that span multiple volumes and with the following automatic deletion thresholds configured:

Volume 1: Delete all but 2 Snapshot copies if 20% overwrite reserve utilization is exceeded.
Volume 2: Delete all but 5 Snapshot copies if 20% overwrite reserve utilization is exceeded.
Volume 3: Delete all but 10 Snapshot copies if 20% overwrite reserve utilization R is exceeded.

If the 20% overwrite reserve utilization threshold for Volume 1 is exceeded, SnapManager deletes all but two Snapshot copies, regardless of the policies for Volumes 2 and 3. If the 20% overwrite reserve utilization threshold for Volume 2 is exceeded, SnapManager deletes all but five Snapshot copies., regardless of the policies for Volumes 1 and 3.

Set the same number of backup sets to delete on exchange database and logs LUN residing on storage system volumes. If there is a mismatch in this number, SnapManager attempts to delete backup sets based on the the fractional reserve policy settings.

Automatic dismounting of Exchange databases

SnapManager provides for the automatic dismounting of Exchange databases in space-reserved LUNs, triggered if overwrite reserve utilization on the volume reaches the threshold specified by the fractional space reservation policy. This effectively stops Exchange write operations to LUNs in a storage system volume where overwrite reserve space is nearly full. This second component of the fractional space reservation policy is a last resort action that prevents further consumption of overwrite reserve. Therefore, it is always enabled.

When both components of a fractional space reservation policy are enabled, the dismounting of Exchange databases must be triggered at a higher level of overwrite reserve utilization than is used to trigger the deletion of Exchange backup Snapshot copies. This causes SnapManager to first use backup set deletion to free up 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 may still run out and the storage system volume will go offline. For this reason, it is recommended that dedicated volumes are used for Exchange 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 Exchange 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 Exchange databases
Status:	Always enabled¹
Trigger on overwrite reserve utilization:	90%	1% - 99%²

¹ Enabling automatic deletion of Exchange 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 Exchange 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 reserve set to less than 100 percent and contains LUNs that store Exchange database files, Exchange 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

Optionally, you can customize the default policy that is applied to all storage system volume that contains fractional-space-reserved LUNs.

Volume-specific policies

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

Viewing fractional space reservation status

In the Fractional Space Reservation Settings dialog box, use the Current Settings tab to view the current space consumption in the storage system volumes that contain LUNs that store Exchange data or SnapInfo directories.

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 Reserve (%)

The amount of space reserved for overwrites on the storage system volume that contains this LUN. 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. The percentage of overwrite reserve utilization that triggers automatic deletion of Exchange backup sets.

Disable Database Trigger (%)

Used Reserve

For the storage system volume that contains this LUN, the amount of overwrite reserve in use. Expressed in two ways: as a percentage of the total size of all space-reserved LUNs in the volume and in megabytes.

Available Reserve (MB)

For the storage system volume that contains this LUN, the amount of overwrite reserve available.

Snapshot Autodelete

For the storage system volume that contains this LUN, the state of the Data ONTAP Snapshot copy autodeletion feature: enabled or disabled. If this LUN stores Exchange 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 Settings tab, note the space consumption status for each LUN that stores Exchange 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 displays the fractional overwrite reserve settings and status: Fractional Reserve (%) Used Reserve (%) Used Reserve (MB) Available Reserve (MB) 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 Exchange data or SnapManager SnapInfo directories are stored on LUNs contained in a volume with these characteristics, the Data ONTAP Snapshot copy autodeletion policy might delete Exchange 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 Exchange backup set components. For details about the `snap autodelete` storage system command, see the Data ONTAP^TM Block Access Management Guide for iSCSI and FCP for release 7.1 or later. Note The SnapManager fractional space reservation policy includes a separate, Exchange-aware autodeletion feature. For details, see Automatic deletion of Exchange 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 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 Exchange 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 Exchange backup Snapshot copies
4	Use the "Automatically delete backup sets" panel to disable, enable, or configure automatic deletion of Exchange backup Snapshot copies in fractional-space-reserved LUNs on the volume. Note Although automatic deletion of Exchange 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 Exchange data. For more information, see Automatic deletion of Exchange backups.
	If you want to...	Then do this...
	Enable automatic deletion of Exchange backup Snapshot copies	Select the "Delete backups that include LUNs which have less than 100% overwrite reservation" option, and then skip ahead to Step 7.
	Disable automatic deletion of Exchange 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 Exchange 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 "Automatic dismount of 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 Exchange backups.
Configure automatic dismounting of Exchange databases
7	Use the "Automatically dismount databases" panel to configure automatic dismounting of Exchange databases in fraction-space-reserved LUNs on the volume. Note Because automatic deletion of Exchange 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 Exchange 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 Exchange databases.
Apply the changes to the default or volume-specific policy
8	To apply your changes and close the dialog box, click OK.