Managing Shared Disks

Subsystem Device Driver Overview

The Subsystem Device Driver (SDD) is an IBM Enterprise Storage Server (ESS) device driver that provides:

• Enhanced data availability
• Automatic path failover and recovery to an alternate path
• Dynamic load balancing of multiple paths
• Concurrent microcode upgrade.

Note that before attempting to exploit the IBM Virtual Shared Disk support for the Subsystem Device Driver, you must read IBM Subsystem Device Driver Installation and User's Guide.

When redundant paths are configured to ESS logical units, and the SDD is installed and configured, the AIX lspv command shows multiple hdisks as well as a new construct called a vpath. The hdisks and vpaths represent the same logical unit. You will need to use the lsvpcfg command to get more information. For example, after issuing lspv, you see output similar to this:

hdisk0          000047690001d59d      rootvg
hdisk1          000047694d8ce8b6      None
hdisk18         000047694caaba22      None
hdisk19         000047694caadf9a      None
hdisk20         none                  None
hdisk21         none                  None
hdisk22         000047694cab2963      None
hdisk23         none                  None
hdisk24         none                  None
vpath0          none                  None 
vpath1          none                  None
vpath2          000047694cab0b35      gpfs1scsivg
vpath3          000047694cab1d27      gpfs1scsivg

After issuing lsvpcfg, you see output similar to this:

vpath0 (Avail ) 502FCA01 = hdisk18 (Avail pv )
vpath1 (Avail ) 503FCA01 = hdisk19 (Avail pv )
vpath2 (Avail pv gpfs1scsivg) 407FCA01 = hdisk20 (Avail ) hdisk24 (Avail )

The examples above illustrate some important points:

• vpath0 consists of a single path (hdisk18) and therefore will not provide failover protection. Also, hdisk18 is defined to AIX as a physical volume (pv flag) and has a PVID, as you can see from the output of the lspv command.
• vpath2 has two paths (hdisk20 and hdisk24) and has a volume group defined on it. Notice that with the lspv command, hdisk20 and hdisk24 look like newly installed disks with no PVIDs. The lsvpcfg command had to be used to determine that hdisk20 and hdisk24 make up vpath2, which has a PVID.

IBM Virtual Shared Disk Configuration Support for Subsystem Device Driver

The IBM Virtual Shared Disk subsystem supports virtual shared disks defined in Subsystem Device Driver (SDD) volume groups, which are also referred to as vpath volume groups. To exploit the functions provided by the SDD (including automatic failover), the shared disk volume groups must be created as, or converted to, vpath volume groups.

You can configure virtual shared disks to use the SDD in the following ways:

• Use the dpovgfix volgrp or the hd2vp volgrp commands to convert an existing hdisk volume group to a vpath volume group. These commands remove the PVID from the existing hdisk paths and activate the vpath volume group.
• Use the createvsd or createhsd commands. Both hdisks and vpaths cannot be specified on the same invocation.
• Use the SDD mkvg4vp command to build volume groups. Next, use the AIX LVM commands to build logical volumes, and then use IBM VSD commands to associate the LVM constructs with IBM VSD constructs.

Note:

If you use SDD volume groups (vpaths), all nodes that access those volume groups must access them as vpath volume groups. For example, you cannot have one node accessing a normal volume group and another node accessing the same volume group as a vpath volume group.