Rebalance and demotion are functions of Easy Tier in automatic mode.
Auto-rebalance
Rebalance is a function of Easy Tier automatic mode to balance the extents in the same tier based on usage. Auto-rebalance supports single managed pools as well as hybrid pools. You can use the Storage Facility Image (SFI) control to enable or disable the auto-rebalance function on all pools of an SFI. When you enable auto-rebalance, every standard and ESE volume is placed under Easy Tier
management for auto-rebalancing procedures. Using auto-rebalance gives you the advantage of these automatic functions:
v Easy Tier operates within a tier, inside a managed storage pool.
v Easy Tier automatically detects performance skew and rebalances extents within the same tier.
In any tier, placing highly active (hot) data on the same physical rank can cause the hot rank or the associated device adapter (DA) to become a performance bottleneck. Likewise, over time skews can appear within a single tier that cannot be addressed by migrating data to a faster tier alone, and require some degree of workload rebalancing within the same tier. Auto-rebalance addresses these issues within a tier in both hybrid and homogenous pools. It also helps the system respond in a more timely and appropriate manner to overloading, skews, and any under-utilization that can occur from the addition or deletion of hardware,
migration of extents between tiers, changes in the underlying volume
configurations, and variations in the workload. Auto-rebalance adjusts the system to continuously provide optimal performance by balancing the load on the ranks and on DA pairs.
The latest version of Easy Tier provides support for auto-rebalancing within homogenous pools. If you set the Easy Tier Automatic Mode Migration control to Manage All Extent Pools, extent pools with a single-tier can perform intra-tier rank rebalancing. If Easy Tier is turned off, then no volumes are managed. If Easy Tier is on, it manages all the volumes it supports, standard or ESE. TSE volumes are not supported with auto-rebalancing.
Notes:
v Standard and ESE volumes are supported.
v Merging pools are restricted to allow repository auxiliary volumes only in a single pool.
v If Easy Tier’s Automatic Mode Migration control is set to Manage All Extent Pools, then single-tier extent pools are also managed to perform intra-tier rank rebalancing.
Warm demotion
Warm demotion operation demotes warm (or mostly sequential-accessed) extents in SSD to HDD, or from Enterprise SAS to NearLine SAS drives to protect the drive performance on the system. The ranks being demoted to are selected randomly. This function is triggered when bandwidth thresholds are exceeded.
This means that extents are warm-demoted from one rank to another rank among tiers when extents have high bandwidth but low IOPS.
It is helpful to understand that warm demotion is different from auto-rebalancing.
While both warm demotion and auto-rebalancing can be event-based, rebalancing movement takes place within the same tier while warm demotion takes place among more than one tier. Auto-rebalance can initiate when the rank configuration changes. It also performs periodic checking for workload that is not balanced across ranks. Warm demotion initiates when an overloaded rank is detected.
Cold demotion
Cold demotion recognizes and demotes cold or semi-cold extents to an appropriate lower-cost tier. Cold extents are demoted in a storage pool to a lower tier as long as that storage pool is not idle.
Cold demotion occurs when Easy Tier detects any of the following scenarios:
v Extents in a storage pool become inactive over time, while other data remains active. This is the most typical use for cold demotion, where inactive data is demoted to the SATA tier. This action frees up extents on the enterprise tier before the extents on the SATA tier become hot, helping the system be more responsive to new, hot data.
v All the extents in a storage pool become inactive simultaneously due to either a planned or unplanned outage. Disabling cold demotion assists the user in scheduling extended outages or experiencing outages without effecting the extent placement.
v All extents in a storage pool are active. In addition to cold demote using the capacity in the lowest tier, an extent is selected which has close to zero activity, but with high sequential bandwidth and low random IOPS for the demotion.
Bandwidth available on the lowest tier is also used.
v All extents in a storage pool become inactive due to a planned non use event, such as an application reaching its end of life. In this situation, cold demotion is disabled and the user may select one of three options:
– Allocate new volumes in the storage pool and plan on those volumes becoming active. Over time, Easy Tier replaces the inactive extents on the enterprise tier with active extents on the SATA tier.
– Depopulate all of the enterprise HDD ranks. When all enterprise HDD ranks are depopulated, all extents in the pool are on the SATA HDD ranks. Store the extents on the SATA HDD ranks until they need to be deleted or archived to tape. Once the enterprise HDD ranks are depopulated, move them to a storage pool.
– Leave the extents in their current locations and reactivate them at a later time.
Figure 11 on page 44 illustrates all of the migration types supported by the latest Easy Tier enhancements in a three-tier configuration. The auto-performance rebalance might also include additional swap operations.