MPIO device attributes
The following attributes are supported only by multipath devices. The attributes can be displayed or changed using the SMIT, or commands (in particular, the lsattr and the chdev commands).
Some of the Multiple Path I/O (MPIO) device attributes are enabled for concurrent update of the attribute. You can update the attribute values while the disk is open and is in use, and the new values take effect immediately. For some attributes, particularly the reserve_policy attribute, there might be restrictions on when the attribute can be changed or what new values the attribute can accept. For example, if a disk is open and is currently in use as a cluster repository disk, the AIX® operating system blocks any attempt to set a reserve policy on the disk because it causes other cluster nodes to lose access to the repository.
The required device attributes that all MPIO devices must support is reserve_policy. Typically, a multipath device also has the PR_key_value device attribute. A multipath device can have additional device-specific attributes. Other device-specific attributes are as follows:
- FC3_REC
- Indicates whether the device must turn on error recovery that uses Fibre Channel class 3 or not.
Enabling this feature improves error detection and error recovery for certain types of fabric errors
that are related to Fibre Channel. This attribute is available only on a limited set of devices.
This attribute can have the following values:
- true
- Enables error recovery that uses Fibre Channel class 3.
- false
- Disables error recovery that uses Fibre Channel class 3 error recovery.
- reserve_policy
- Defines whether a reservation methodology is employed when the
device is opened. The values are as follows:
- no_reserve
- Does not apply a reservation methodology for the device. The device might be accessed by other initiators, and these initiators might be on other host systems.
- single_path
- Applies a SCSI2 reserve methodology for the device, which means the device can be accessed only
by the initiator that issued the reserve. This policy prevents other initiators in the same host or
on other hosts from accessing the device. This policy uses the SCSI2 reserve to lock the device to a
single initiator (path), and commands routed through any other path result in a reservation
conflict.
Path selection algorithms that alternate commands among multiple paths can result in thrashing when the single_path value is selected. As an example, assume a device-specific PCM has a required attribute that is set to a value that distributes I/O across multiple paths. When single_path is in effect, the disk driver must issue a bus device reset (BDR) and then issue a reserve using a new path for sending the next command to break the previous reservation. Each time a different path is selected, thrashing occurs and performance degrades because of the overhead of sending a BDR and issuing a reserve to the target device. (The AIX PCM does not allow you to select an algorithm that could cause thrashing.)
- PR_exclusive
- Applies a SCSI3 persistent-reserve, exclusive-host methodology when the device is opened. The PR_key_value attribute value must be unique for each host system. The PR_key_value attribute is used to prevent access to the device by initiators from other host systems.
- PR_shared
- Applies a SCSI3 persistent-reserve, shared-host methodology when the device is opened. The PR_key_value value must be a unique value for each host system. Initiators from other host systems are required to register before they can access the device.
- PR_key_value
- Required only if the device supports any of the persistent reserve policies (PR_exclusive or PR_shared).