The host software uses several methods to implement host clusters. These methods require sharing the volumes on IBM Spectrum Virtualize between hosts. To share storage between hosts, maintain control over accessing the volumes. Some clustering software use software locking methods. 

You can choose other methods of control by the clustering software or by the device drivers to use the SCSI architecture reserve or release mechanisms. The multipathing software can change the type of reserve that is used from an earlier reserve to persistent reserve, or remove the reserve. 

Persistent reserve  refers to a set of SCSI-3 standard commands and command options that provide SCSI initiators with the ability to establish, preempt, query, and reset a reservation policy with a specified target device. The functions that are provided by the persistent reserve commands are a superset of the original reserve or release commands. 

The persistent reserve commands are incompatible with the earlier reserve or release mechanism. Also, target devices can support only reservations from the earlier mechanism or the new mechanism. Attempting to mix persistent reserve commands with earlier reserve or release commands results in the target device returning a reservation conflict error. 

Earlier reserve and release mechanisms (SCSI-2) reserved the entire LUN (volume) for exclusive use down a single path. This approach prevents access from any other host or even access from the same host that uses a different host adapter. The persistent reserve design establishes a method and interface through a reserve policy attribute for SCSI disks. This design specifies the type of reservation (if any) that the operating system device driver establishes before it accesses data on the disk. 

The following possible values are supported for the reserve policy: 

GLYPH<SM590000> No_reserve: No reservations are used on the disk. 

GLYPH<SM590000> Single_path: Earlier reserve or release commands are used on the disk. 

GLYPH<SM590000> PR_exclusive: Persistent reservation is used to establish exclusive host access  to the disk. 

GLYPH<SM590000> PR_shared: Persistent reservation is used to establish shared host access  to the disk. 

When a device is opened (for example, when the AIX varyonvg  command opens the underlying hdisks), the device driver checks the object data manager (ODM) for a reserve_policy and a PR_key_value. The driver then opens the device. For persistent reserve, each host that is attached to the shared disk must use a unique registration key value. 

## 7.5.1 Clearing reserves 

It is possible to accidentally leave a reserve on the IBM Spectrum Virtualize volume or on the IBM Spectrum Virtualize MDisk during migration into IBM Spectrum Virtualize or when disks are reused for another purpose. Several tools are available from the hosts to clear these reserves. The easiest tools to use are the pcmquerypr  (AIX SDDPCM host) commands. Another tool is a menu-driven Windows SDDDSM tool. 

The Windows Persistent Reserve Tool is called PRTool.exe  and is installed automatically when SDDDSM is installed in the C:\Program Files\IBM\Subsystem Device Driver\PRTool.exe  directory. You can clear the IBM Spectrum Virtualize volume reserves by removing all the host mappings. 

