IBM DS8800 Introduction And Planning Manual page 69

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.
Three-tier migration types and their processes 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.
Chapter 3. Data management features
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