IBM Power Systems™ servers are designed to offer very high stand-alone availability in the industry. Enterprises must occasionally restructure their infrastructure to meet new IT requirements and handle scheduled outages (such as scheduled system maintenance).
MPIO best practices have never been officially documented. There have been some documents and IBM Redbooks® that have briefly mentioned certain MPIO aspects for specific scenarios and environments, but recommendations pertaining to MPIO configurations, in general, have been lacking.
System reliability and availability are increased by a careful consideration of the user-modifiable options in each system configuration. This article outlines the best practice configuration considerations that pertain to MPIO on AIX.
Some of the features described in this article are specific to particular technology levels of AIX, or are specific to the path control module (PCM) supplied with AIX. If using Subsystem Device Driver Path Control Module (SDDPCM) or a vendor-supplied Object Data Manager (ODM) package (often referred to as a host attachment kit, or something similar), then some of these options might be unavailable, and other options can be added.
The AIX MPIO infrastructure allows IBM or third-party storage vendors to supply ODM definitions, which have unique default values for the important disk attributes. Thus, for example, the default value for attributes on an hdisk representing a logical unit number (LUN) from an IBM System Storage® SAN Volume Controller (SVC) might be different from the default values for an hdisk representing a LUN on an IBM System Storage DS8000® system. As a result, the default values for the attributes are appropriate for most situations. Generally, the hdisk attributes should be left at their default values, especially the attributes that are not mentioned in this article.
The disk attributes described in the following sections can be displayed by
lsattr command, and can be changed with the
chdev command. The path attributes, such as
path_priority, can be displayed or set by using the
chpath commands. Refer to the AIX
publications or AIX man pages for details on those commands.
This article does not address the attributes associated with the adapters
being used to attach to the MPIO devices. Some of those attributes might
also affect error detection and recovery times. In particular, the
fc_err_recov attribute for Fiber Channel adapters is an
important one to consider.
Consideration 1: MPIO algorithm and path_priority
The MPIO algorithm setting determines whether:
- The PCM can attempt to distribute I/O across all available paths to a given LUN
- The I/O will be active only on one path at a time
- The I/O flow will be weighted based on a combination of the algorithm
setting and the
path_prioritysettings per disk
A device that has multiple controllers can designate one controller as the active or preferred controller. For such a device, the PCM uses just the paths to the active or preferred controller as long as there is at least one such path that is enabled and not failed. Thus, algorithms that use all available paths might only use a subset of those paths at one time for such devices.
algorithm = fail_over
This is the default algorithm for most disks using the ODM definitions included with AIX. Some third-party ODMs use a different default value.
With this algorithm, I/O can only be routed down one path at a time. With
algorithm=fail_over, the PCM keeps track of all the enabled
paths (per disk) in an ordered list. If the path being used to send I/O
fails or is disabled, the next enabled path in the list is selected and
I/O is routed to that path. The sequence for path selection within the
list is customizable by modifying the path priority attribute on each
path, which will then sort the list by the ascending path priority
fail_over algorithm is always used for virtual SCSI
(VSCSI) disks on a Virtual I/O Server (VIOS) client, although the backing
devices on the VIOS instance might still use
Fail_over is also the only algorithm that might be
used if using SCSI-2 reserves
algorithm = round_robin
With this algorithm, I/O will be distributed and activated across all
enabled paths to a disk. The percentage of I/O routed down each path can
be weighted by setting the
path_priority attribute on each
path for each disk. If a path fails or is disabled, it is no longer used
for sending I/O. The priority of the remaining paths is then recalculated
to determine the percentage of I/O that should be sent down each path. If
all paths have the same
path_priority value, the PCM
attempts to equally distribute I/O
across all enabled paths. Optimal performance in a failed path scenario is
to ensure that the ordered path list alternate paths between separate
algorithm = shortest_queue
This algorithm is available in the latest technology levels of AIX for some
devices. The algorithm behaves very similar to
when the load is light. When the load increases, this algorithm favors the
path that has the fewest active I/O operations. Thus, if one path is slow
due to congestion in the storage area network (SAN), the other
less-congested paths are used for more of the I/O operations.
The path priority values are ignored by this
Recommendation: If using SCSI-2 reserves or vSCSI disks,
fail_over must be used. For other situations,
shortest_queue (if available) or
enable maximum use of the SAN resources.
Consideration 2: Path health check settings
Path health check mode (hcheck_mode)
The path health check mode determines the paths that the MPIO's path health
checker will probe for path availability during normal business
operations. The health checker never probes paths that are in a
Disabled or Missing state. Paths in
those two states must be recovered manually with
Disabled paths) or with
cfgmgr (for Missing paths). If a
disk is not open and in use as is the case, for instance, when its
volume group is varied off, no path health checks will take place down
any path for that disk.
There are three possible modes for the MPIO path health checker.
nonactive: In this mode, the PCM
sends health check commands down paths which have no active I/O. That
includes paths with a state of failed. If the algorithm selected
is fail_over, then the health check command is also sent on each of the
paths that have a state of enabled but have no active I/O. If the
algorithm selected is
shortest_queue, then the health check command is only sent on
paths with a state of failed, because the
shortest_queue algorithms both
keep all enabled paths active with I/O when the disk is in use. If the
disk is idle, the health check command is sent on any paths that do not
have a pending I/O at the expiration of the health check interval.
enabled: In this mode, the PCM
sends health check commands down all enabled paths, even paths that have
other active I/O at the time of the health check.
failed: In this mode, the PCM only
sends path health checks down paths that are marked as
Recommendation: The default value for all devices is nonactive, and there is little reason to change this value unless business or application requirements dictate otherwise.
Path health check interval (hheck_interval)
The path health check interval is the interval, in seconds, at which MPIO
path health checks will probe and check path availability of open disks,
based on the
hcheck_interval = 0 setting disables MPIO's path health
check mechanism, which means any failed paths require manual intervention
to recover or re-enable.
Recommendation: The best practice guideline for
hcheck_interval is that it should be greater than or equal to
rw_timeout (read/write timeout) value on the disks. Also
note that it is not a good idea to lower the
rw_timeout value in order to set a lower health check
interval. The default
rw_timeout values set in ODM are based
on the recommendations of the device manufacturers for each device type.
The following section provides technical details regarding this best
It might be tempting to think that a smaller health check interval is preferable as it might lead to faster detection or recovery of failed paths. However, the cost of setting a lower health check interval far outweighs the benefits. There are several reasons for this.
- Because the health check commands can be sent on every path of every
open disk (depending on
hcheck_mode) at the expiration of the health check interval, a small health check interval can quickly use up a lot of bandwidth on the SAN if there are a large number of disks.
- The health check commands count against the disk's
queue_depth(only to be changed upon recommendation from the storage vendor), and they receive a higher priority for processing than normal user I/O. Because error scenarios typically take longer than good path scenarios, a small health check interval can negatively impact the user I/O on good paths when there are one or more failing paths. Note that because
queue_depthis a function of the disk driver,
queue_depthis on a per-LUN basis rather than a per-path basis. For example, assume that a device has a
queue_depthof 8, with eight paths. If four of those paths have failed, the health check commands on those paths might take anywhere from a few seconds up to
rw_timeoutto fail. During that time, at least four of the eight commands in the
queue_depthwill be consumed by the health check commands, leaving an effective
queue_depthof only four commands for the good paths and regular I/O for that disk.
- It is not always desirable to recover a path quickly. In a situation where a link is suffering from repeated, intermittent failures, the more quickly the link is recovered by a health check command, the more likely it is that a user I/O will be sent on that link only to fail due to the intermittent errors. A longer health check interval reduces the use of links with frequent but intermittent failures.
- AIX implements an emergency last gasp health check to recover paths when needed. If a device has only one non-failed path and an error is detected on that last path, AIX sends a health check command on all of the other failed paths before retrying the I/O, regardless of the health check interval setting. This eliminates the need for a small health check interval to recover paths quickly. If there is at least one good path, AIX discovers it and uses it before failing user I/O, regardless of the health check interval setting.
Recent technology levels of AIX also make use of asynchronous events from the Fibre Channel (FC) device drivers to manipulate path states. This makes AIX less dependent on the health check commands to detect path failures or to recover paths when using Fibre Channel.
For most cases, the default value of
appropriate. There have been some storage vendors who, in older versions
of their ODM definitions, had set
hcheck_interval to a value
smaller than the
rw_timeout value. The previous recommendation from AIX
development stands in those cases: Increase
such that it is greater than or equal to
rw_timeout value. It
is much more likely to be a good idea to increase the health check
interval than to decrease it. Better performance is achieved when
hcheck_interval is slightly greater than the
rw_timeout value on the disks.
Extreme cases of the problems described in bullets 2 and 3 above can cause severe degradation of I/O performance if the health check interval is set to a small value.
Consideration 3: Time out policy
Recent technology levels of AIX include a
attribute for some devices. This attribute indicates the action that the
PCM should take when a command timeout occurs. A command timeout occurs
when an I/O operation fails to complete within the
value on the disk. There are three possible values for
retry_path: This represents the
legacy behavior, where a command may be retried on the same path that just
experienced a command timeout. This is likely to lead to delays in the I/O
recovery, as it is likely that the command will continue to fail on this
path. Only after several consecutive failures, will AIX fail the path and
try the I/O on an alternate path.
fail_path: This setting causes
AIX to fail the path after a single command timeout, assuming that the
device has at least one other path that is not in the failed
state. Failing the path forces the I/O to be retried on a different path.
This can lead to much quicker recovery from a command time out and also
much quicker detection of situations where all paths to a device have
failed. A path that is failed due to timeout policy can later be recovered
by the AIX health check commands. However, AIX avoids using the path for
user I/O for a period of time after it recovers to help ensure that the
path is not experiencing repeated failures. (Other PCMs might not
implement this grace period.)
disable_path: This setting
causes the path to be disabled. A disabled path is only recovered by
manual user intervention using the
chpath command to
re-enable the path.
Recommendation: If this attribute is available on the
device, a value of
fail_path is the recommended setting.
Consideration 4: How many paths to configure for AIX MPIO
In an MPIO configuration, more is not necessarily better. In fact, an excessive number of paths in an MPIO configuration can actually contribute to system and application performance degradation in the event of SAN, storage, or Fibre Channel fabric issues or failures.
The general recommendation for the number of paths to configure in an MPIO environment is 4 to 8 per LUN, with 16 paths being recommended as the maximum, to be used only in specialized situations. It is important to note that MPIO does support many more paths than 8 or 16, but from a design and functional perspective, four to eight paths have been proven to be the most effective.
Businesses that need to configure more than eight paths per LUN need to carefully consider the following details:
- When an error is encountered on an MPIO disk, error recovery normally
takes place down all configured paths. The most common types of disk
or SAN errors that occur will also lead to multiple retry attempts
on each path for each failed
I/O. With "N" paths, there could
easily be a situation where a disk encounters an error that would lead
to five tries on each path, multiplied by the
rw_timeoutvalue on the disks. So, total recovery per I/O could potentially be:
(N * rw_timeout value * 5)
If multiple disks were to encounter similar issues at the same time, the consequences for applications might be severe. For example, a marginal, constantly bouncing link in the SAN fabric might lead to this type of error recovery, resulting in extreme performance degradation.
This situation is somewhat ameliorated by setting the
fail_path, if that attribute is available with the device type that is being used. However, the timeout policy attribute cannot account for all possible error scenarios.
- With the
round_robinalgorithm, having too many paths results in overhead as the PCM attempts to load balance I/O among the many paths.
- With the
fail_overalgorithm, the PCM encounters additional overhead in determining the paths to use for failover in a failed path scenario.
- Each configured path requires additional memory in AIX, as each path is represented by data structures in the MPIO device drivers. Having too many paths to a large number of disks can reduce the amount of memory available to the rest of the system for running applications.
- As noted above, the health check commands count against the queue depth for the device. So, health check processing has a greater effect on devices with a large number of paths, especially with devices that have smaller queue depths, and especially when there are paths in the failed state.
The optimal configuration for a device having four paths on AIX is to use four physical paths to the storage subsystem with a 1:1 relationship between the host-side host bus adapter (HBA) port and the remote storage ports. If using multiport adapters on the AIX host, split at least half the paths among separate physical adapters for optimum redundancy. The AIX and device ports can be connected to the same FC switch or to two different switches in the same fabric. If using two switches, there is no single point of failure. However, certain switch or port failures might affect an entire SAN, thus impacting all four paths.
One possible eight-path configuration that provides full redundancy uses two distinct SAN fabrics. The AIX node and the storage device each have two ports connected to each of the two SAN fabrics, using a total of four ports on AIX and four ports on the storage device. There are four paths between AIX and the storage device for each of the two distinct SAN fabrics, for a total of eight paths. Thus, there is no single point of failure for either SAN fabric, and there are redundant SAN fabrics. (Note: This is just an example. It is completely possible to have full redundancy with four paths per LUN using dual fabrics.)
The only case for more than eight paths is for specialized storage devices that configure a cluster of controllers, or for devices using Peer-to-Peer Remote Copy (PPRC). For example, an hdisk representing an IBM HyperSwap® pair of LUNs on two DS8000 devices could have 16 paths if each of the DS8000 systems used to form the HyperSwap pair are configured in the 8-path configuration described above. After the two 8-path hdisks are configured as a single HyperSwap enabled hdisk, it will have 16 paths.
There are other possible configurations beyond what is described here that can be considered. However, as noted above, going beyond eight paths can be more problematic than helpful, and should be carefully considered.
Recommendation: Configure 4 or 8 paths per disk, or up to 16 paths for rare situations. Carefully consider the impacts of extra, unnecessary redundancy before using more paths.
Consideration 5: Operational considerations
Scheduled maintenance: AIX MPIO is capable of robust error
detection and recovery. However this error detection and recovery might
take some time, and that delay might impact applications. If scheduled
maintenance is planned for a SAN or for a storage device, it is best to
identify the disk paths that will be impacted by that maintenance and use
rmpath command to manually disable those paths before
starting the maintenance. AIX MPIO stops using any disabled or
Defined paths, and therefore, no error detection or
recovery will be done as a result of the scheduled maintenance. This
ensures that the AIX host does not go into extended error recovery for a
scheduled maintenance activity. After the maintenance is complete, the
paths can be re-enabled with
cfgmgr. (Note: When disabling
multiple paths for multiple LUNs,
rmpath is simpler than
chpath, as it does not have to be run on a per-disk basis.)
lspath command (or in newer technology levels, the
lsmpio command) can be used to determine the MPIO paths that
are associated with specific SAN ports.
Changing attributes: For most attributes and most levels
of AIX, attributes could historically only be changed on devices that were
not in use. For disks, this meant that the disk must be closed (for
example, volume group varied off) in order to change attributes. If the
disk could not be closed, such as the disks containing rootvg, then the
user had to include the
-P flag in the
command to write the attribute change to ODM and then restart AIX in order
for the attribute to take effect.
For the newest technology levels of AIX (at the time of publishing this
article), some disk attributes on some devices support the
flag on the
chdev command. This flag instructs
chdev to attempt a dynamic update of the attribute value.
With this flag, the attribute value can be changed without closing the
disk and the change takes effect immediately.
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