Dell NetWorker 19.9 Administration Guide

Unintended Access to DDS device prevention

The Reserve/Release attribute has been added to the Device resource for tape devices to support Reserve/Release, including the Persistent Reserve commands.

Reserve/Release is a mechanism that uses SCSI commands to attempt to prevent unintended access to tape drives that are connected by using a shared-access technology, such as Fibre Channel, iSCSI, or SCSI multiplexers. It is a “cooperative” and host-based mechanism, which means that all applications should respect the reservations and not purposely break them. Access is granted based on the host system that reserved the device. Other applications that run on that host cannot be prevented from accessing a reserved device.

Reserve/Release cannot prevent a malicious or badly behaved application from accessing a reserved device. It also cannot prevent all problems caused by hardware issues (such as SCSI resets or FC LIPs) from interrupting data access.

The basic sequence requires that a host reserve a tape drive (using specific SCSI commands) before attempting to access the tape drive. If this “reservation” succeeds, then the host can use the drive. If the reservation fails (usually because the device is reserved by someone else), then the host attempting the reservation should not attempt to use the drive. When a host has finished using a reserved drive, that host must release the drive by using the appropriate SCSI commands.

The reservation is maintained by the drive itself. With older (called “Simple” in NetWorker software) Reserve/Release, the reservation is based on the SCSI ID of the system that issued the reserve command. For tape drives connected to Fibre Channel (FC) using FC-SCSI bridges, the mapping between FC host and reservation is done inside the bridge, since the initiator on the SCSI side is always the bridge itself, regardless which host actually issued the reserve command.

For Persistent Reserve, the reservation is associated with a 64-bit “key” that is registered by the host. Several keys can be registered with a given drive at any given time, but only one may hold the active reservation. NetWorker software uses the “exclusive” reservation method for Persistent Reserve. Only the host that holds the active reservation is allowed to access the drive.

The Reserve/Release attribute does not support file type or advanced file type devices.

The settings that relate to Reserve/Release and Persistent Reserve are found in a device’s Properties window, on the Advanced tab. They are visible only when diagnostic mode is turned on.

The default setting for Reserve/Release is None. Once any other Reserve/Release setting is selected, it works automatically, without further user intervention. The Reserve/Release attribute is supported only on Common Device Interface (CDI) platforms, so if the CDI attribute in a device’s Properties is set to Not Used, then Reserve/Release settings are ignored.

For newer hardware, once a Reserve/Release setting (other than None) has been selected, the appropriate Persistent Reserve commands are automatically issued before a device is opened for reading or writing, and before the device is closed. With older hardware, a SCSI-2 Reserve command is issued before opening the device, and a SCSI-2 Release command is issued after the device is closed.

Reserve/Release has these possible settings:

• None (the default)
• Simple
• Persistent Reserve
• Persistent Reserve + APTPL (Activate Persist Through Power Loss)

The Persistent Reserve Key attribute has also been added. It is used with Persistent Reservation calls.