HDLM Load Balancing

Load Balancing options

As of version 6 of the HDLM software there are some additions to the load balancing algorithm which makes HDLM the preferred piece of software for IO performance improvements on a host level.

In previous versions (before version 6) only three options were available.

1. Normal failover and failback
2. Round Robin loadbalancing
3. Extended Round Robin.

The latter was extremely useful when the host was connected to an enterprise array like the USP/NSC or USP-V/VM. HDLM checked on which paths it had the least outstanding IO's and would use that path for the following one.

As mentioned 4 new load balancing algorithms have been introduced based on either least IO or least block queues.

The new load balancing algorithm options are:
Least I/O
Least Block
Extended Least I/O
Extended Least Block.

Least I/Os: This algorithm issues I/O operations to the path with the fewest pending I/O operations.

Extended Least I/Os: Similar to the Least I/Os algorithm, this algorithm issues I/O operations to the path with the fewest pending I/O operations. However for sequential data access, several I/O operations are issued to a single path in succession in order to improve performance by increasing cache use. After the stream of I/Os is issued to the path, the algorithm once again selects the path with the fewest pending I/O operations. If a host switches from sequential to random access before a stream of I/O operations is completed, the path will immediately switch to the one with the fewest pending I/O operations.

Least Blocks: This algorithm issues I/O operations to the path with the fewest pending I/O blocks. Please note that each I/O operation can be made up of one or more I/O blocks.

Extended Least Blocks: Similar to the Least Blocks algorithm, this algorithm issues I/O operations to the path with the fewest pending I/O blocks. However for sequential access, several I/O operations are issued to a single path in succession in order to improve performance by increasing cache use. After the stream of I/Os is issued to the path, the algorithm once again selects the path with the fewest pending I/O blocks. If a host switches from sequential to random access before a stream of I/O
operations is completed, the path will immediately switch to the one with the fewest pending I/O blocks.

HDLM Advanced IO settings

Performance testing has shown excellent all around performance for the Extended Least I/O algorithm in environments exhibiting both random and sequential I/O characteristics. Due to this result, Extended Least I/O will replace Round Robin as the new default host setting for load balancing.

Read the user manual to determine which option would be best for you.

Also have a look at HDLM advanced. It includes the HGLM (Hitachi Global Link Manager) software which is extremely useful if you have multiple servers running HDLM.