What are the best practices for tape reservations?
- ONTAP 9
- Data ONTAP 8 7-mode
- ONTAP supports several reservation methods in the SCSI command set used on directly-connected tape drives and tape libraries.
- Reservations can protect from undesirable interactions with the tape subsystem from multiple storage controllers and backup servers.
- A tape drive or medium changer will reject commands from a server without proper reservations access, leaving the tape or changer state and position intact.
- Best practice is to use the highest level of reservations the customer's backup application and infrastructure can support. In order of preference:
- Persistent reservations - hard to delete by mistake; persist through error recovery
- SCSI (classic) reservations - easier to delete by mistake and/or error recovery
- No reservations - every server for itself - data scrambling/loss a possibility
- Reservations are not required for directly attached single-port tape drives since the devices are not shared with other controllers or servers.
- Not all application packages are capable of using Persistent Reservations, and not all tape subsystem hardware can handle Persistent Reservations, so NetApp also supports using the older style SCSI reserve/release commands. Those commands are officially deprecated in later SCSI command standards, but many drives will still execute them properly.
- Knowledge of the infrastructure is key to deciding on the best reservation method. Consult vendor documentation for the backup software, the tape drives and libraries to determine which method works for the infrastructure. Some trial and error may be required.
- The storage controller
options tape.reservationscommand is used to select the reservation method that Data ONTAP employs with subsequent tape or medium changer operations. It is global and applies to all tape operations initiated after the option is set. More precisely, the option is in force when a tape drive or medium changer is opened and determines the reservation method used on that device until it is closed.
options tape.reservations - with no parameter displays the current setting
options tape.reservations persistent - enables persistent reservations
options tape.reservations scsi - enables classic SCSI reservations
options tape.reservations off - disables the use of reservations by Data ONTAP