Data migration between multiple tiers in a storage system using age and frequency statistics

A non-transitory computer storage medium having computer-executable instructions stored thereon which, when executed by a computer system, cause the computer system to:

• subdivide a storage capacity of a storage system into a set of territories;
• partition the set of territories into ordered tiers;
• maintain an information lifecycle management metric for each territory, wherein maintaining an information lifecycle management metric comprises combining an age statistic and a frequency statistic into a single metric, wherein respective contributions of the age statistic and the frequency statistic to the single metric are mutually exclusive, and wherein the single metric is a signed value to indicate whether the single metric represents the age statistic or the frequency statistic, respectively;
• update the information lifecycle management metric once per update period, wherein each update period has a consistent amount of time;
• promote a territory that is more eligible into a highest possible tier in response to storage capacity being available in a higher tier;
• demote a territory that is less eligible into a lower tier in response to a more eligible territory existing in the lower tier and no storage capacity being available for promotion of the more eligible territory;
• and
• demote a least eligible territory to a next lowest tier in response to a less eligible territory existing in a next highest tier and no storage capacity being available for demotion of the less eligible territory.

A data storage system comprising:

• a storage subsystem;
• a plurality of physical storage devices associated with the storage subsystem;
• and
• one or more data migration modules operable to cause the storage subsystem to:
• subdivide a storage capacity of the storage system into a set of territories;
• partition the set of territories into tiers;
• maintain an information lifecycle management metric for each territory, wherein maintaining an information lifecycle management metric comprises combining an age statistic and a frequency statistic into a single metric, wherein respective contributions of the age statistic and the frequency statistic to the single metric are mutually exclusive, and wherein the single metric is a signed value to indicate whether the single metric represents the age statistic or the frequency statistic, respectively;
• update the information lifecycle management metric once per update period, wherein each update period has a consistent amount of time;
• promote a territory that is more eligible into a highest possible tier in response to storage capacity being available in a higher tier;
• demote a territory that is less eligible into a lower tier in response to a more eligible territory existing in the lower tier and no storage capacity being available for promotion of the more eligible territory;
• and
• demote a least eligible territory to a next lowest tier in response to a less eligible territory existing in a next highest tier and no storage capacity being available for demotion of the less eligible territory.