AIX:Fast I/O Failure for Fibre Channel Devices

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Introduction

The purpose of this README is to introduce two new features in the Fibre Channel Device Driver stack, "Fast I/O Failure for Fibre Channel Devices" and "Dynamic Tracking of Fibre Channel Devices".

Fast I/O Failure for Fibre Channel Devices

This release of AIX supports Fast I/O Failure for Fibre Channel devices after link events in a switched environment. If the FC adapter driver detects a link event such as a lost link between a storage device and a switch, the FC adapter driver will wait a short period of time, on the order of 15 seconds, to allow the fabric to stabilize. At that point, if the FC adapter driver detects that the device is not on the fabric, it will begin failing all I/Os at the adapter driver. Any new I/O or future retries of the failed I/Os will be failed immediately by the adapter until the adapter driver detects that the device has rejoined the fabric.

Fast Failure of I/O is controlled by a new fscsi device attribute, 'fc_err_recov'. The default setting for this attribute is 'delayed_fail', which is the I/O failure behavior that has existed in previous versions of AIX. Setting this attribute to 'fast_fail', as shown in the example 

chdev -l fscsi0 -a fc_err_recov=fast_fail

(assuming the fscsi device instance is fscsi0), enables fast I/O failure. Fast fail logic is invoked when the adapter driver receives an indication from the switch that there has been a link event involving a remote storage device port via a Registered State Change Notification (RSCN) from the switch.

Fast I/O failure may be desirable in situations where multipathing software is being used. Setting 'fc_err_recov' to 'fast_fail' may decrease the I/O fail times due to link loss between the storage device and switch and allow faster failover to alternate paths.

In single-path configurations, especially configurations with a single-path to a paging device, the default 'delayed_fail' setting is the recommended setting.

The requirements for Fast I/O Failure support are the following:

  • Fast Fail is only supported in a switched environment. It is not supported in arbitrated loop environments, including public loop.
  • FC 6227 adapter firmware - level 3.22A1 or greater.
  • FC 6228 adapter firmware - level 3.82A1 or greater.
  • FC 6239 adapter firmware - all firmware levels


Failure to meet these requirements may cause the fscsi device to log an error log of type INFO indicating that one of these requirements was not met and that fast fail has NOT been enabled.

Dynamic Tracking of Fibre Channel Devices

This release provides support for Dynamic Tracking of Fibre Channel Devices. Previous releases of AIX required a user to unconfigure FC storage device and adapter device instances before making changes on the SAN that might result in an N_Port ID (SCSI ID) change of any remote storage ports.

If Dynamic Tracking of FC Devices is enabled, the FC adapter driver will detect when the Fibre Channel N_Port ID of a device changes and then re-route traffic destined for that device to the new address while the devices are still online. Examples of events that can cause an N_Port ID to change are moving a cable between a switch and storage device from one switch port to another, connecting two separate switches via an Inter-Switch Link (ISL), and possibly rebooting a switch.

Dynamic Tracking of FC devices is controlled by a new fscsi device attribute, 'dyntrk'. The default setting for this attribute is 'no'. Setting this attribute to 'yes', as shown in the example 

chdev -l fscsi0 -a dyntrk=yes

(assuming fscsi device instance is fscsi0), enables dynamic tracking. Dynamic Tracking logic is invoked when the adapter driver receives an indication from the switch that there has been a link event involving a remote storage device port. The requirements for Dynamic Tracking support are the same as those for "Fast I/O Failure for Fibre Channel Devices" and also include the following requirements:

- Dynamic Tracking requires that device World Wide Name (Port Name) and Node Names remain constant, and that World Wide Name be unique. Changing the World Wide Name or Node Name of an available or on-line device could result in I/O failures.

In addition, each FC storage device instance must have world_wide_name and node_name attributes. Updated filesets that contain the 'sn_location' attribute mentioned in the next bullet should also be updated to contain both of these attributes.

- The storage device must provide a reliable method to extract a unique serial number for each LUN. The AIX FC device drivers will not autodetect serial number location, so the method for serial number extraction must be explicitly provided by any storage vendor in order to support dynamic tracking for their devices. This information is conveyed to the drivers via the 'sn_location' ODM attribute for each storage device. If the disk or tape driver detects that the 'sn_location' ODM attribute is missing, an error log of type INFO will be generated and dynamic tracking will NOT be enabled. 

 Note: The 'sn_location' attribute may be non-displayable, so running the 'lsattr' command on an hdisk, for example, may not show the attribute, but it may, indeed, be present in ODM.

- The FC device drivers will be able to track devices on a SAN fabric, where a SAN fabric is defined as a fabric as seen from a single host bus adapter, if the N_Port IDs on the fabric stabilize within about 15 seconds. If cables are not reseated or N_Port IDs continue to change after the initial 15 seconds, I/O failures could result.

- Devices will not be tracked across host bus adapters. Devices will only track if they remain visible from the same HBA that they were originally connected to. 

 For example, if device A were moved from one location to another on fabric A attached to host bus adapter A (i.e., its N_Port on fabric A changes), the device would seamlessly be tracked without any user intervention and I/O to this device can continue.

However, if a device A is visible from HBA A but not from HBA B, and device A is moved from the fabric attached to HBA A to the fabric attached to HBA B, device A will not be accessible on fabric A nor on 

 fabric B.  User intervention would be required to make it available on fabric B by invoking cfgmgr. The AIX device instance on fabric A would no longer be usable, and a new device instance on fabric B would be created. This device would have to manually be added to volume groups, multipath device instances, etc.   In essence, this is the same as removing a device from fabric A and adding a new device to fabric B.

- No dynamic tracking will performed for FC dump devices while an AIX system dump is in progress. In addition, dynamic tracking is not supported during boot or during cfgmgr invocations. SAN changes should not be made while any of these operations are in progress.

- Once devices are tracked, ODM will potentially contain stale information as SCSI IDs in ODM will no longer reflect actual SCSI IDs on the SAN. ODM will remain in this state until cfgmgr is run manually or the system is rebooted, provided all drivers, including any third party FC SCSI target drivers, are dynamic-tracking capable. If cfgmgr is run manually, cfgmgr must be invoked on all affected fscsi devices, which can easily be accomplished by running cfgmgr without any options, or by invoking 

 cfgmgr on each fscsi device individually.

 Note: Running cfgmgr at run time to recalibrate the SCSI IDs may not update the SCSI ID in ODM for a storage device if the storage device is currently opened, such as when volume groups are varied on.
 cfgmgr would need to be run on devices that are not opened or the system should be rebooted to recalibrate the SCSI IDs.  Note that stale SCSI IDs in ODM have no adverse affect on the FC drivers and recalibration of SCSI IDs in ODM is not necessary for the FC drivers to function properly. Any applications that communicate with the adapter driver directly via ioctl calls and use the SCSI ID values from ODM, however, need to be updated as indicated in the next bullet to avoid using potentially stale SCSI IDs.

- All applications and kernel extensions that communicate with the FC Adapter Driver, either via ioctl calls or directly to the FC driver's entry points, must support the version 1 ioctl and scsi_buf APIs of the FC Adapter Driver in order to work properly with FC dynamic tracking. Non-compliant applications or kernel extensions may not function properly and/or fail after a dynamic tracking event. If the FC adapter driver detects an application or kernel extension that is not adhering to the new version 1 ioctl and/or scsi_buf API, an error log of type INFO will be generated and dynamic tracking may not be enabled for the device that this application/ kernel extension may be trying to communicate with. 

 ISVs developing kernel extensions and/or applications that communicate with the AIX Fibre Channel Driver stack should refer to the "Fibre Channel
 Protocol for SCSI and iSCSI Subsystem" article in "AIX 5L Version 5.2 Kernel Extensions and Device Support Programming Concepts" (pay special attention to the "Required FCP and iSCSI Adapter Device Driver ioctl Commands" and "Understanding the scsi_buf Structure sections") for changes necessary to support Dynamic Tracking.

- Even with dynamic tracking enabled users are strongly encouraged to make SAN changes, such as cable moves/swaps and establishing ISL links, during 

 maintenance windows. Making SAN changes during full production runs is discouraged.  This is due to the fact that there is a short interval of
 time to perform any SAN changes.  Cables that are not reseated correctly, for example, could result in I/O failures. Performing these operations
 during a time of little/no traffic minimizes impact of I/O failures due to misplugging of cables, taking too long to recable, etc.

At the time of this release, the base AIX FC SCSI Disk and FC SCSI Tape device drivers support dynamic tracking.

For status on dynamic tracking support for the FAStT product line, refer to the following URLs:

IBM FAStT Storage Manager v7.10: 

   http://www-3.ibm.com/pc/support/site.wss/MIGR-40711.html

IBM FAStT Storage Manager v8.21: 

   http://www-3.ibm.com/pc/support/site.wss/MIGR-43839.html

IBM FAStT Storage Manager v8.3: 

   http://www-3.ibm.com/pc/support/site.wss/MIGR-50177.html

After accepting the license agreement, proceed to the link for Storage Manager for AIX. Look for the current version of this software and follow the link to the associated README. Product support and enhancement announcents will be made here. These announcements should contain information on what AIX APARs are required to support dynamic tracking on FAStT.

In addition, the IBM ESS, EMC Symmetrix and HDS storage devices support dynamic tracking provided that the vendor provides the ODM filesets with the necessary 'sn_location' and 'node_name' attributes. Contact the storage vendor if you are not sure if your current level of ODM fileset supports dynamic tracking.

If vendor-specific ODM entries are not being used for the storage device, but the ESS, Symmetrix or HDS storage subsystem is configured with the displayable message of "MPIO Other FC SCSI Disk", then dynamic tracking is supported for these devices in this configuration.

The STK tape device using the standard AIX device driver also supports dynamic tracking provided the STK fileset contains the necessary 'sn_location' and 'node_name' attributes.

Note: It is strongly recommended that SAN changes involving tape devices be made with no active I/O. Due to the serial nature of tape devices, a single I/O failure can cause an application, such as a tape backup, to fail.

Devices that configure with the displayable messages of "Other FC SCSI Disk" or "Other FC SCSI Tape" will not support dynamic tracking.

Fast Fail and Dynamic Tracking Interaction

Although Fast Fail and Dynamic Tracking of FC Devices are technically separate features, the enabling of one may change the interpretation of the other in certain situations. The following table shows the behavior exhibited by the FC drivers with the various permutations of these settings.

dyntrk fc_err_recov FC Driver Behavior
no delayed_fail This is the default setting. This is legacy behavior existing in previous versions of AIX. The FC drivers will not recover if the scsi_id of a device changes, and I/Os will take longer to fail when a link loss occurs between a remote storage port and switch. This may be desirable in single-path situations if dynamic tracking support is not a requirement.
no fast_fail If the driver receives a Registered State Change Notification (RSCN) from the switch, this could indicate a link loss between a remote storage port and switch. After an initial 15 second delay, the FC drivers will query to see if the device is on the fabric. If not, I/Os will be flushed back by the adapter. Future retries or new I/Os will fail immediately if the device is still not on the fabric.

If the FC drivers detects the device is on the fabric but the scsi_id has changed, the FC device drivers will not recover, i.e., I/Os will be failed with PERM errors.

yes delayed_fail If the driver receives a Registered State Change Notification (RSCN) from the switch, this could indicate a link loss between a remote storage port and switch. After an initial 15 second delay, the FC drivers will query to see if the device is on the fabric. If not, I/Os will be flushed back by the adapter. Future retries or new I/Os will fail immediately if the device is still not on the fabric, although the storage driver (disk, tape, FAStT) drivers may inject a small delay (2-5 seconds) between I/O retries.

If the FC drivers detects the device is on the fabric but the scsi_id has changed, the FC device drivers will reroute traffic to the new scsi_id.

yes fast_fail If the driver receives a Registered State Change Notification (RSCN) from the switch, this could indicate a link loss between a remote storage port and switch. After an initial 15 second delay, the FC drivers will query to see if the device is onthe fabric. If not, I/Os will be flushed back by the adapter. Future retries or new I/Os will fail immediately if the device is still not on the fabric. The storage driver (disk, tape, FAStT) will likely not delay between retries.

If the FC drivers detects the device is on the fabric but the scsi_id has changed, the FC device drivers will reroute traffic to the new scsi_id.

Note that with dynamic tracking disabled, there is a marked difference between the 'delayed_fail' and 'fast_fail' settings of the 'fc_err_recov' attribute. However, with dynamic tracking enabled, the setting of the 'fc_err_recov' attribute is less significant. This is because there is some overlap in the dynamic tracking and fast fail error recovery policies. As such, enabling dynamic tracking inherently enables some of the fast fail logic.

The general error recovery procedure when a device is no longer reachable on the fabric is the same for both 'fc_err_recov' settings with dynamic tracking enabled, with the minor difference that the storage drivers may choose to inject delays between I/O retries if 'fc_err_recov' is set to 'delayed_fail'. This will increase the I/O failure time by some additional amount depending on the delay value and number of retries before permanently failing the I/O. With high I/O traffic, however, the difference between 'delayed_fail' and 'fast_fail' may be more noticeable.

SAN administrators may want to experiment with these settings to find the correct combination of settings for their environment.

Known Hardware Issues

SWITCHES

In general, if problems are encountered due to cable swaps or other operations involving a switch, the switch firmware should always be updated to the latest supported levels regardless of the type of switch being used.

IBM 2109 FC Switch

Some problems were experienced when using these features and performing multiple cable swaps with the IBM FC 2109 F16 switch at GA firmware level 3.0. These problems are solved by updating the switch firmware to the latest levels. These features were tested using v3.0.2k firmware for this switch.

Inrange Switch

If using an Inrange FC/9000 FC Director, the Inrange switch must be at a minimum level of "FC 4.1.2.7" firmware in order to support Dynamic Tracking and Fast Fail.

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