Disk

Virtual SCSI Client Adapter Path Timeout

The virtual SCSI Client Adapter Path Timeout feature allows the client adapter to detect whether a Virtual I/O Server is not responding to I/O requests. Use this feature only in configurations in which devices are available to a client logical partition from multiple Virtual I/O Servers. These configurations could be either configurations where Multipath I/O (MPIO) is being used or where a volume group is being mirrored by devices on multiple Virtual I/O Servers.

If no I/O requests issued to the virtual SCSI server adapter have been serviced within the number of seconds specified by the virtual SCSI path timeout value, one more attempt is made to contact the virtual SCSI server adapter, waiting up to 60 seconds for a response.

If, after 60 seconds, there is still no response from the server adapter, all outstanding I/O requests to that adapter are failed and an error is written to the client logical partition error log. If MPIO is being used, the MPIO Path Control Module will retry the I/O requests down another path. Otherwise, the failed requests will be returned to the applications. If the devices on this adapter are part of a mirrored volume group, those devices will be marked as missing and the Logical Volume Manager logs errors in the client logical partition error log. If one of the failed devices is the root volume group (rootvg) for the logical partition, and the rootvg is not available via another path or is not being mirrored on another Virtual I/O Server, the client logical partition is likely to shut down. The virtual SCSI client adapter attempts to reestablish communication with the Virtual I/O Server and logs a message in the system error log when it is able to do so. Mirrored volume groups must be manually resynchronized by running the varyonvg command when the missing devices are once again available.

A configurable virtual SCSI client adapter ODM attribute, vscsi_path_to, is provided. This is a tunable attribute that is specific to an AIX® client. The path timeouts for the Linux operating system are configured differently. This attribute is used both to indicate whether the feature is enabled and to store the value of the path timeout if the feature is enabled.

The system administrator sets the ODM attribute to 0 to disable the feature, or to the time, in seconds, to wait before checking if the path to the server adapter has failed. If the feature is enabled, a minimum setting of 30 seconds is required. If a setting between 0 and 30 seconds is entered, the value will be changed to 30 seconds upon the next adapter reconfiguration or reboot.

This feature is disabled by default, thus the default value of vscsi_path_to is 0. Exercise careful consideration when setting this value, keeping in mind that when the virtual SCSI server adapter is servicing the I/O request, the storage device the request is being sent to may be either local to the VIO Server or on a SAN.

The vscsi_path_to client adapter attribute can be set by using the SMIT utility or by using the chdev -P command. The attribute setting can also be viewed by using SMIT or the lsattr command. The setting will not take affect until the adapter is reconfigured or the client partition is rebooted.

Virtual SCSI device read or write command timeout

The virtual SCSI device read or write command timeout feature facilitates the virtual SCSI device to detect a hung I/O request. You can use this feature in any virtual SCSI client configuration to detect and recover from the I/O request failures. The following configurations are supported:

• Virtual SCSI clients in which disks are exported through a single virtual SCSI server adapter.
• Same disks are available to the virtual SCSI clients from multiple virtual SCSI server adapters.

If the virtual SCSI device read or write command timeout feature is enabled, all the read or write command requests that are issued to the virtual SCSI server adapter are timed. If any read or write command is not serviced within the number of seconds that is specified by the command timeout value, then the virtual SCSI client adapter causes the command to time-out. The connection with the virtual SCSI server adapter is then closed and subsequently, a new connection is reinitialized.

A configurable virtual SCSI device ODM attribute, rw_timeout is specified. This attribute is a tunable attribute and indicates the read or write command timeout value for the device that is configured on the virtual SCSI client. You can modify the rw_timeout attribute for the virtual SCSI device by using the chdev or chdev -P command. You can use the lsattr -R -l device -a rw_timeout command that provides the range of values that can be used for the device read or write command timeout feature. You must specify the value for the read or write command timeout feature within the range of values indicated by the lsattr -R -l device -a rw_timeout command. If the specified value for the read or write command timeout feature is less than the minimum or greater than the maximum value indicated in the range of values, the chdev command returns an error.

The read or write command timeout feature is enabled by default from AIX 7.2 TL 2, AIX 7.1 TL 5, and later. This feature is disabled in the earlier AIX releases, by default.

The rw_timeout attribute is associated with every virtual SCSI device and not just the disk. With AIX 7.2 TL 5, and later, the rw_timeout attribute is not a virtual SCSI client adapter attribute.

The following table provides details about the default range and acceptable range (in seconds) of the read or write command timeout value.