The Automatic Load Balancing feature provides automated I/O balancing and ensures that incoming I/O traffic from the hosts is dynamically managed and balanced across both controllers.

The Automatic Load Balancing feature provides improved I/O resource management by reacting dynamically to load changes over time and automatically adjusting volume controller ownership to correct any load imbalance issues when workloads shift across the controllers.

The workload of each controller is continually monitored and, with cooperation from the multipath drivers installed on the hosts, can be automatically brought into balance whenever necessary. When workload is automatically re-balanced across the controllers, the storage administrator is relieved of the burden of manually adjusting volume controller ownership to accommodate load changes on the storage array.

When Automatic Load Balancing is enabled, it performs the following functions:

The controller cache is a physical memory space that streamlines two types of I/O (input/output) operations: between the controllers and hosts, and between the controllers and disks.

For read and write data transfers, the hosts and controllers communicate over high-speed connections. However, communications from the back-end of the controller to the disks is slower, because disks are relatively slow devices.

When the controller cache receives data, the controller acknowledges to the host applications that it is now holding the data. This way, the host applications do not need to wait for the I/O to be written to disk. Instead, applications can continue operations. The cached data is also readily accessible by server applications, eliminating the need for extra disk reads to access the data.

The controller cache affects the overall performance of the storage array in several ways:

When the amount of unwritten data in the cache reaches a certain level, the controller periodically writes cached data to a drive. This write process is called "flushing."

The controller uses two algorithms for flushing cache: demand-based and age-based. The controller uses a demand-based algorithm until the amount of cached data drops below the cache flush threshold. By default, a flush begins when 80 percent of the cache is in use.

In System Manager, you can set the “Start demand cache flushing” threshold to best support the type of I/O used in your environment. In an environment that is primarily write operations, you should set the “Start demand cache flushing” percentage high to increase the probability that any new write requests can be processed by cache without having to go to the disk. A high percentage setting limits the number of cache flushes so that more data remains in cache, which increases the chance of more cache hits.

In an environment where the I/O is erratic (with data bursts), you can use low cache flushing so that the system flushes cache frequently between data bursts. In a diverse I/O environment that processes a variety of loads, or when the type of loads are unknown, set the threshold at 50 percent as a good middle ground. Be aware that if you choose a start percentage lower than 80 percent, you might see decreased performance because data needed for a host read might not be available. Choosing a lower percentage also increases the number of disk writes necessary to maintain the cache level, which increases system overhead.

The age-based algorithm specifies the period of time during which write data can remain in the cache before it is eligible to be flushed to the disks. The controllers use the age-based algorithm until the cache flush threshold is reached. The default is 10 seconds, but this time period is counted only during periods of inactivity. You cannot modify the flush timing in System Manager; instead, you must use the Set Storage Array command in the command-line interface (CLI).

The storage array's controller organizes its cache into "blocks," which are chunks of memory that can be 8, 16, 32 KiB in size. All volumes on the storage system share the same cache space; therefore, the volumes can have only one cache block size.

Applications use different block sizes, which can have an impact on storage performance. By default, the block size in System Manager is 32 KiB, but you can set the value to 8, 16, 32 KiBs. A smaller size is a good choice for file systems or database applications. A larger size is a good choice for applications that require large data transfer, sequential I/O, or high bandwidth, such as multimedia.

You should manually synchronize the controller clocks in the storage array if you notice that the time stamps shown in SANtricity System Manager are not aligned with time stamps shown in your management client (the computer that is accessing SANtricity System Manager through the browser). This task is only necessary if NTP (Network Time Protocol) is not enabled in SANtricity System Manager.

You can check synchronization status from the Synchronize Storage Array Clocks dialog box, which is available from the System page. If the times shown in the dialog box do not match, run a synchronization. You can periodically view this dialog box, which indicates whether the controller clocks' time displays have drifted apart and are no longer synchronized.