Operating Systems

Since POWER6, IBM has provided a way to move a logical partition (LPAR) between servers while it’s running, allowing you to migrate an LPAR or LPARs without any downtime. This feature is called Live Partition Mobility (LPM) and is provided by the Enterprise Edition of PowerVM, which must be installed on all servers involved.

LPM is used for multiple reasons, ranging from server consolidation, server migration and workload balancing to completely evacuating servers so they can be shut down for planned maintenance or other purposes. LPM can be used to move Linux and AIX (5.3 and up) workloads between POWER6 and POWER7 servers and back, provided certain prerequisites are met. With certain limitations, iOS LPARs can also be made mobile. However, LPM isn’t disaster recovery nor is it a replacement for PowerHA or other high-availability solutions. It’s designed for planned migrations only. Crashed kernels can’t be migrated and partitions can’t be moved from failed machines.

There are three options for partition mobility:

• Active partition mobility is the actual movement of a running LPAR from one server to another without disrupting the applications currently running in the LPAR. At the end of the migration, network applications may see a brief (less than 1 second) suspension, but connectivity isn’t lost. The amount of time for the migration depends on the amount of memory in the LPAR and how busy the LPAR is.
• Inactive partition mobility is when an LPAR is transferred when it’s shut down. This kind of migration only takes seconds.
• Suspended partition mobility refers to an LPAR that has been suspended and is then migrated from one server to another.

Migration Prerequisites

Several prerequisites must be met in order for LPM to work smoothly. Details on all prerequisite firmware, virtual I/O server (VIOS), etc. levels can be found online (see the “References” section).

Server: First, the servers must be compatible. Both the source and target need to be POWER6 or above and PowerVM Enterprise Edition must be installed on both servers. There are also minimum firmware requirements. Additionally, the target server must have sufficient available memory, CPU and virtual adapter slots to accommodate the LPAR being moved. Finally, the logical memory block (LMB) or memory region size for the servers must be the same. This can be checked on the server via the hardware management console (HMC). If they aren’t the same, they will need to be reset, which requires powering the server off and on.

Management console: LPM servers need to be managed by a management console. An HMC can be used to manage current POWER blades as well as POWER servers and Flex servers. Integrated virtualization manager (IVM) can be used to manage POWER blades only. Flex system manager (FSM) can be used to manage Flex servers only. Since an HMC can manage them all, it’s recommended that HMCs be used where possible to provide one simple interface. There are minimum software requirements for each console and these affect the options available for migration, including the number of concurrent migrations possible. As an example, remote migration (between servers controlled by different HMCs) became available at v7.3.4 of the HMC, and mobility between the HMC and an FSM is only available after v7.7.1.0. It’s recommended that the HMC be kept at the current level that’s available.

VIO Server: LPM was first introduced at VIOS v1.5.2.1-FP11 and v2.1. The current HMC version is v2.2.3.1, which became available in November 2013. A VIOS on each server must be set as a mover server partition (MSP), as these are used to control the migration.

LPAR: The mobile LPAR itself also has some requirements. There are minimum operating system levels as well as configuration requirements. The first requirement is that all resources be virtualized through a VIO server. This means all storage is provided via the storage area network (SAN) either using virtual SCSI (vSCSI) or N_Port ID virtualization (NPIV), and all networking is provided through the VIO server using a shared Ethernet adapter (SEA). Adapters should also be defined as desired, not required and must not be marked as for “Any client.” While dedicated adapters can be used, they must be removed prior to the move. This also applies to any virtual CDs (vtopt) defined to take advantage of file backed optical (FBO). The LPAR must also have a name that’s unique to both the source and target servers. Additionally, the LPAR can’t be marked as the service partition, nor can it have any open consoles at the time of the move.

Inactive migration has the same requirements as active migration with a couple of exceptions. An LPAR can use inactive migration if it uses huge memory pages, has barrier synchronization register (BSR) arrays, uses redundant error reporting or has physical I/O attached. Active migrations can’t be done in those cases. It should be noted that an LPAR must have been activated at least once (even if only in system managed services [SMS] mode) in order for any migration to occur.

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