If a fractional-space-reserved volume runs out of overwrite reserve space, write operations to a LUN fail and the host application might terminate, report I/O errors, or exhibit unexpected behavior. Data ONTAP uses automatic expansion of flexible volumes and automatic deletion of Snapshot copies from flexible volumes to avoid this situation.
When a LUN is fully space reserved, write operations cannot fail due to an out-of-space condition. When the overwrite reserve for a volume is set to less than 100 percent, write operations to the LUNs on that volume might fail when the volume runs low in free disk space.
The automatic expansion of flexible volumes and the automatic deletion of Snapshot copies from flexible volumes 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 FC for Data ONTAP 7.1 or later.
Data ONTAP automatically expands a nearly full volume into the space preallocated for it in the aggregate. The volume must be a flexible volume with the guarantee option set to Volume. You can enable automatic deletion of Snapshot copies and FlexVol expansion features 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, you can only use only the automatic FlexVol expansion feature.
Data ONTAP automatically deletes one or more Snapshot copies on a nearly full volume, when you enable the Snapshot copy automatic deletion policy. If the trigger condition is detected, the oldest or newest Snapshot copies are deleted until a configured percentage of the volume is free space. If you do not want to automatically delete Snapshot copies on the volume, you can set the overwrite reserve to 100 percent, by setting the fractional space reserve to 100 percent on the storage system.
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. You must always retain at least one online backup copy for each database.