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: 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. 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. Example 6-7 shows a failing  pcmquerypr  command to clear the reserve and the error. Use the AIX  errno.h  include file to determine what error number 16 indicates. This error  indicates a busy condition, which can indicate a legacy reserve or a persistent reserve from  another host (or that this host is from a different adapter). However, some AIX technology  levels have a diagnostic open issue that prevents the  pcmquerypr  command from opening the  device to display the status or to clear a reserve. For more information about older AIX technology levels that break the  pcmquerypr  command,  see  IBM Support , which is available at this website: IBM Support 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. 6.5.1 Clearing reserves Example 6-7 Output of the pcmquerypr command #  pcmquerypr -ph /dev/hdisk232 -V connection type: fscsi0  open dev: /dev/hdisk232  couldn't open /dev/hdisk232, errno=16  Chapter 6. Hosts  297