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When multipath support is enabled, you can display the multiple paths to a device using either of the following variants of the SHOW DEVICE DCL command:
SHOW DEVICE/FULL device-name SHOW DEVICE/MULTIPATH_SET device-name |
The SHOW DEVICE/FULL device-name command displays the traditional information about the device first and then lists all the paths to a device by their path identifiers (described in Section 6.7.4).
The SHOW DEVICE/MULTIPATH_SET device-name command lists only some brief multipath information about devices that have multiple paths.
Multipath information is displayed only on nodes that are directly
connected to the multipath device.
6.7.5.1 Displaying Paths With SHOW DEVICE/FULL
The following example shows the output of a SHOW DEVICE/FULL device-name command. Note that the use of multiple paths is shown at the beginning of the display ( device has multiple I/O paths ), and the multiple path descriptions are shown toward the end of the display, beneath I/O paths to device . Note, too, that the values for Error count and Operations completed shown at the beginning of the display are the sums of the counts for each path.
$ SHOW DEVICE/FULL $1$DGA23:
Disk $1$DGA23: (WILD8), device type HSG80, is online, mounted, file-oriented
device, shareable, device has multiple I/O paths, served to cluster via MSCP
Server, error logging is enabled.
Error count 3 Operations completed 32814199
Owner process "" Owner UIC [SYSTEM]
Owner process ID 00000000 Dev Prot S:RWPL,O:RWPL,G:R,W
Reference count 9 Default buffer size 512
WWID 01000010:6000-1FE1-0000-0D10-0009-8090-0677-0034
Total blocks 17769177 Sectors per track 169
Total cylinders 5258 Tracks per cylinder 20
Host name "WILD8" Host type, avail HP AlphaServer GS160 6/731, yes
Alternate host name "W8GLX1" Alt. type, avail HP AlphaServer GS160 6/731, yes
Allocation class 1
Volume label "S5SH_V72_SSS" Relative volume number 0
Cluster size 18 Transaction count 8
Free blocks 12812004 Maximum files allowed 467609
Extend quantity 5 Mount count 8
Mount status System Cache name "_$1$DGA8:XQPCACHE"
Extent cache size 64 Maximum blocks in extent cache 1281200
File ID cache size 64 Blocks currently in extent cache 0
Quota cache size 0 Maximum buffers in FCP cache 1594
Volume owner UIC [1,1] Vol Prot S:RWCD,O:RWCD,G:RWCD,W:RWCD
Volume Status: ODS-2, subject to mount verification, file high-water marking,
write-back XQP caching enabled, write-through XFC caching enabled.
Volume is also mounted on H2OFRD, FIBRE3, NORLMN, SISKO, BOOLA, FLAM10,
W8GLX1.
I/O paths to device 5
Path PGA0.5000-1FE1-0000-0D12 (WILD8), primary path.
Error count 2 Operations completed 130666
Path PGA0.5000-1FE1-0000-0D13 (WILD8), current path.
Error count 1 Operations completed 30879310
Path PGA0.5000-1FE1-0000-0D11 (WILD8).
Error count 0 Operations completed 130521
Path PGA0.5000-1FE1-0000-0D14 (WILD8).
Error count 0 Operations completed 130539
Path MSCP (W8GLX1).
Error count 0 Operations completed 1543163
|
For each path of the multipath device, the path identifier, the host name associated with that path, the path status, the error count, and the operations count are displayed.
The terms that may appear in the multiple paths portion of the display are described in Table 6-4.
| Term | Description |
|---|---|
| WWID | The worldwide ID of the SCSI logical unit. |
| Host name | The name of the system that is being used by the current path. The host name is displayed if there is an MSCP path to a multipath device. |
| Alternate host name | The name of another system that can also provide access to the device. If the current path is a direct path, this will be the host currently associated with the MSCP path. If the current path is an MSCP path, this will be the name of the local system. The alternate host name is displayed if there is an MSCP path to a multipath disk device. |
| Primary path | This was the first path to the device found by the operating system. |
| Current path | This path is currently used for I/O. |
| User disabled | The DCL command SET DEVICE/NOENABLE has been executed for this path. |
| Polling disabled | The DCL command SET DEVICE/NOPOLL has been executed for this path. |
| Not responding | This path to the device was unusable the last time it was checked. Typically, the multipath poller checks every 60 seconds if the path is good and every 30 seconds if the path is bad. |
| Unavailable | The path is unavailable because the software driver has disconnected from the path. |
You can obtain a brief listing of multiple paths for a specific device, for all the devices in an allocation class, or for all devices with the DCL command:
SHOW DEVICE/MULTIPATH_SET [device-name] |
The device name is optional; when omitted, all devices that have formed multipath sets are shown. For each multipath device found, the device name, host name, device status, error count, number of accessible paths, total number of paths, and the current path's path identifier are displayed.
The number of accessible paths can be less than the total number of paths for two reasons:
The host name displayed is the host name of the current path. For direct paths, this will be the local system's host name. For MSCP served paths, this will be the host name of the remote system which is serving access to the device.
The following example shows the output of a SHOW DEVICE/MULTIPATH command.
$ SHOW DEVICE/MULTIPATH Device Device Error Paths Current Name Status Count Avl/Tot path $1$DGA8: (H2OFRD) Mounted 3 5/ 5 PGA0.5000-1FE1-0000-0D12 $1$DGA10: (H2OFRD) ShadowSetMember 1 5/ 5 PGA0.5000-1FE1-0000-0D14 $1$DGA11: (WILD8) ShadowSetMember 3 3/ 3 MSCP $1$DGA23: (H2OFRD) Mounted 6 5/ 5 PGA0.5000-1FE1-0000-0D13 $1$DGA30: (H2OFRD) ShadowSetMember 8 5/ 5 PGA0.5000-1FE1-0000-0D13 $1$DGA31: (WILD8) ShadowMergeMbr 5 3/ 3 MSCP $1$DGA33: (H2OFRD) Online 0 5/ 5 PGA0.5000-1FE1-0000-0D12 $1$DGA40: (H2OFRD) Mounted 2 5/ 5 PGA0.5000-1FE1-0000-0D13 $1$DGA41: (H2OFRD) ShadowMergeMbr 8 5/ 5 PGA0.5000-1FE1-0000-0D12 $70$DKA100: (H2OFRD) Mounted 0 3/ 3 PKD0.1 $70$DKA104: (H2OFRD) ShadowSetMember 0 3/ 3 PKD0.1 $70$DKA200: (H2OFRD) ShadowSetMember 0 3/ 3 PKD0.2 $70$DKA300: (H2OFRD) ShadowSetMember 0 3/ 3 PKC0.3 $80$DKA1104: (H2OFRD) ShadowSetMember 0 3/ 3 PKD0.11 $80$DKA1200: (H2OFRD) ShadowSetMember 0 3/ 3 PKD0.12 $80$DKA1204: (H2OFRD) ShadowSetMember 0 3/ 3 PKC0.12 $80$DKA1207: (H2OFRD) Mounted 0 3/ 3 PKD0.12 $80$DKA1300: (H2OFRD) Mounted 0 3/ 3 PKD0.13 $80$DKA1307: (H2OFRD) ShadowSetMember 0 3/ 3 PKD0.13 $80$DKA1500: (H2OFRD) Mounted 0 3/ 3 PKD0.15 $80$DKA1502: (H2OFRD) ShadowSetMember 0 3/ 3 PKD0.15 |
If you choose to specify a partial device name, such as $70$DKA, the
display shows all devices with multiple paths whose names begin with
$70$DKA.
6.7.6 Path Polling
When SCSI multipath support is in effect, the system periodically polls all the I/O paths from each host adapter to each HSZ or HSG controller or to an MDR to determine the status of each I/O path. If the system detects any changes to a path, it outputs a message, similar to the following messages, to the console and to the operator's log:
All multipath devices on path PKB0.5 are either disabled or not reachable. |
or
At least one multipath device on path PKB0.5 is enabled and reachable. |
If all the devices on a path are removed, a path failure is reported. The path from the host to the HSx controller may still function, but this cannot be determined when there are no devices to poll.
You can turn polling on or off with the following command:
SET DEVICE device/[NO]POLL/PATH=path-identifier |
Turning off polling for a path that will be out of service for a
prolonged period is useful because it can reduce system overhead.
6.7.7 Switching Current Paths Manually
You can switch a device's current path manually using the SET DEVICE command with the /SWITCH qualifier. The most common reason for doing this is to balance the aggregate I/O load across multiple HSx controller modules, MDRs, and buses.
The command syntax for switching the current path is:
SET DEVICE device-name/SWITCH/PATH=path-identifier |
This command requires the OPER privilege. Additionally, if the device is currently allocated by another process, as tape devices often are, the SHARE privilege is needed.
The following command switches the path of device $2$DKA502 to an MSCP served path.
$ SET DEVICE $2$DKA502/SWITCH/PATH=MSCP |
Note that this command initiates the process of switching the path and then returns to the DCL prompt immediately. A delay may occur between when the DCL prompt reappears and when the path switch is complete.
A manual path switch of a mounted device takes place within mount verification, which is triggered by the path switch command. It is accompanied by the usual mount verification messages and a path switch message, as shown in Example 6-1.
| Example 6-1 Messages Resulting from Manual Path Switch |
|---|
%%%%%%%%%%% OPCOM 15-JUN-2001 09:04:23.05 %%%%%%%%%%% Device $1$DGA23: (H2OFRD PGA) is offline. Mount verification is in progress. %%%%%%%%%%% OPCOM 15-JUN-2001 09:04:25.76 %%%%%%%%%%% 09:04:25.76 Multipath access to device $1$DGA23: has been manually switched from path PGA0.5000-1FE1-0000-0D11 to path PGA0.5000-1FE1-0000-0D14 %%%%%%%%%%% OPCOM 15-JUN-2001 09:04:25.79 %%%%%%%%%%% Mount verification has completed for device $1$DGA23: (H2OFRD PGA) |
You can check for completion of a path switch by issuing the SHOW DEVICE/FULL command, or the SHOW DEVICE/MULTIPATH command.
Note that if the path that is designated in a manual path switch fails during the switch operation, then automatic path switching takes over. This can result in a switch to a path different from the one designated in the command.
If a manual path switch causes a logical unit to switch from one HSG80 controller to another controller, then the command can affect other nodes in the cluster. These nodes will experience a mount verification on their current path, causing an automatic switch to a path on the other HSG80 controller. Example 6-2 shows the messages that indicate this event.
| Example 6-2 Messages Displayed When Other Nodes Detect a Path Switch |
|---|
%%%%%%%%%%% OPCOM 15-JUN-2001 09:04:26.48 %%%%%%%%%%% Device $1$DGA23: (WILD8 PGA, H20FRD) is offline. Mount verification is in progress. %%%%%%%%%%% OPCOM 15-JUN-2001 09:04:26.91 %%%%%%%%%%% 09:04:29.91 Multipath access to device $1$DGA23: has been auto switched from path PGA0.5000-1FE1-0000-0D12 (WILD8) to path PGA0.5000-1FE1-0000-0D13 (WILD8) %%%%%%%%%%% OPCOM 15-JUN-2001 09:04:27.12 %%%%%%%%%%% Mount verification has completed for device $1$DGA23: (WILD8 PGA, H20FRD) |
The WILD8 node name is displayed for each path because each path is a
direct path on node WILD8. The node name field in the mount verification
in progress
and
completed
messages shows both a local path and an MSCP alternative. In this
example, the
WILD8 PGA, H20FRD
name shows that a local PGA path on WILD8 is being used and that an
MSCP path via node H20FRD is an alternative.
6.7.8 Path Selection by OpenVMS
The selection of the current path to a multipath device is determined by the device type as well as by the event that triggered path selection.
Path Selection for Initial Configuration at System Startup
When a new path to a multipath disk (DG, DK) or tape device (MG) is configured, the path chosen automatically as the current path is the direct path with the fewest devices. No operator messages are displayed when this occurs. (This type of path selection is introduced in OpenVMS Alpha Version 7.3-1.) A DG, DK, or MG device is eligible for this type of path selection until the device's first use after a system boot or until a manual path switch is performed by means of the SET DEVICE/SWITCH command.
When a new path to a generic multipath SCSI device (GG, GK) is configured, the path chosen automatically as the current path is the first path discovered, which is also known as the primary path. For GG and GK devices, the primary path remains the current path even as new paths are configured. GG and GK devices are typically the console LUNs for HSG or HSV controller LUNs or for tape media robots.
Path Selection When Mounting a Disk Device
The current path to a multipath disk device can change as a result of a MOUNT command. The I/O performed by the MOUNT command triggers a search for a direct path that does not require the disk device to fail over from one HSx controller to another.
The path selection initiated by a MOUNT command on a DG or DK disk device proceeds as follows:
The MOUNT utility might trigger this path selection algorithm a number of times until a working path is found. The exact number of retries depends on both the time elapsed for the prior attempts and the qualifiers specified with the MOUNT command.
This path selection process, introduced in OpenVMS Alpha Version 7.3-1, has the following benefits:
Note that this selection process allows devices to be distributed between the two HSx controllers. You can accomplish this by using HSx console commands, such as the following:
HSG> SET UNIT PREFERRED_PATH=THIS_CONTROLLER HSG> SET UNIT PREFERRED_PATH=OTHER_CONTROLLER |
In addition, you can use the DCL commands for manual path switching described in Section 6.7.7, to select a different host bus adapter or a different port on the same HSx controller, or to force the device to fail over to a different HSx controller.
Path Selection When Mounting Tape Drive Device
Support for multipath tape drives and this type of path selection was introduced in OpenVMS Alpha Version 7.3-1. Path selection when the MOUNT command is issued differs somewhat between multipath tape drives and disk devices for several reasons:
The path selection initiated by a MOUNT command on an MG tape drive device proceeds as follows:
When an I/O operation fails on a device that is subject to mount verification, and the failure status suggests that retries are warranted, mount verification is invoked. If the device is a multipath device or a shadow set that includes multipath devices, alternate paths to the device are automatically tried during mount verification. This allows the system to recover transparently from cases in which the device has failed over from one HSx controller or MDR to another, and to recover transparently from failures in the path to the device.
The following devices are subject to mount verification:
Note that foreign mounted disk volumes and generic SCSI devices (GG and GK) are not subject to mount verification and, therefore, are not eligible for automatic multipath failover.
Path selection during mount verification proceeds as follows:
Steps 1 through 6 are repeated until either a working path is found or mount verification times out. If a working path is found and the path is different, the current path is automatically switched to the new path and an OPCOM message is issued. Mount verification then completes, the failed I/O is restarted, and new I/O is allowed to proceed. This path selection procedure attempts to avoid unnecessary failover of devices from one HSx controller to another because:
This path selection procedure prefers direct paths over MSCP served paths because the use of an MSCP served path imposes additional CPU and I/O overhead on the server system. In contrast, the use of a direct path imposes no additional CPU or I/O overhead on the MSCP-server system. This procedure selects an MSCP served path only if none of the available direct paths are working. Furthermore, this path selection procedure tends to balance the use of available direct paths, subject to the constraint of avoiding unnecessary controller failovers.
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