Dell EqualLogic PS6210XS EqualLogic Group Manager Administrator s Guide PS Ser - Page 135

About Space Borrowing Space borrowing includes snapshot space borrowing and replication borrowing. (For more information about these borrowing methods, see About Snapshot Space Borrowing and About Replication Borrowing.) PS Series firmware v8.0 and later provides the following borrowing capabilities: • Extends the existing snapshot and local replica borrowing capability to enable borrowing from unused delegated space • Provides borrowing to remote replicas so that they can borrow space beyond their replica reserve (similar to snapshot borrowing) Space is potentially available for borrowing from the following areas: • Unused snapshot and replication reserves • Unused delegated space • Free space You should be aware of the following restrictions regarding space borrowing: • Borrowed space is intended to help during peaks of activity when more space is needed temporarily. It does not take the place of carefully provisioning reserves. • Remote replicas can borrow beyond their associated remote replica reserve, but the total amount of reserve space must still fit within the delegated space. • When necessary, the system will automatically delete an object that is borrowing space. Any objects in borrowed space are considered to be "at risk." Benefits of Space Borrowing Space borrowing provides the following benefits: • Eases the effects of setting reserves either too high or too low, and helps alleviate the pressure of predicting reserve usage accurately, especially if this usage changes frequently. • Allows snapshots and replicas to borrow enough space to hold the copies that would otherwise be deleted. As long as the pool has enough unused space to borrow, the copy can borrow space to satisfy the max-keep policy. The borrowing capabilities added for v8.0 and later can help in the following situations: • If a volume's snapshot reserve or replica reserve is set too high or too low: - Too high - The system considers the unused snapshot and replica reserve as "borrowable" space, which is then available to other volumes whose reserves were set too low. This flexibility accommodates volumes whose IO patterns caused the volume reserves to be exceeded and better utilizes the available space. - Too low - The system allows snapshots and replicas to borrow space, which maintains the max-keep policy. Older snapshots and replicas will not need to be deleted if the system can borrow space for them. • If replica reserve is configured to be too small or too large: - Too small - The system allows the replicas to borrow from outside their replica reserve, which maintains the max-keep policy. - Too large - The system enables other snapshots and replica sets in the pool to borrow the unused space. Displaying Space-Borrowing Information The Group Manager GUI can help you monitor borrowed space on your system. To see borrowing information, either: • Click Group → Borrowed Space. PS Series Group Operations 135