The table columns that are shown in Figure 10-2 on page 405 represent all of the possible  pool configurations; the rows report in which Easy Tier tier each MDisk tier is mapped. For  example, consider a pool with all the possible tiers configured that corresponds with the  T0+T1+T2+T3 configuration in the table in Figure 10-2 on page 405. With this configuration,  the T1 and T2 are mapped to the same Easy Tier tier. If there is no Tier 0 flash in a storage  pool, Tier 1 flash is used as the highest performance tier.  For more information about planning and configuration considerations or best practices, see  IBM System Storage SAN Volume Controller and Storwize V7000 Best Practices and  Performance Guidelines , SG24-7521.  The Easy Tier function continuously monitors volumes for host I/O activity. It collects  performance statistics for each extent, and derives averages for a rolling 24-hour period of I/O  activity. Random and sequential I/O rate and bandwidth for reads and writes are collected, as  well as I/O response time.  Different types of analytics are used to decide whether extent data migration is required.  Once per day, Easy Tier analyze the statistics to work out which data should be sent to a  higher performing tier or might be sent to a tier with lower performance. Four times per day, it  analyzes the statistics to identify if any data needs to be rebalanced between managed disks  in the same tier. Once every 5 minutes, Easy Tier checks the statistics to identify if any of the  managed disks is overloaded.  All of these analysis phases generate a list of migrations that should be executed. The system  then spends as long as needed running the migration plan.  The migration plan can consist of the following actions on volume extents (as shown in  Figure 10-3): Easy Tier actions Figure 10-3 Actions on extents 406  Implementing the IBM Storwize V7000 with IBM Spectrum Virtualize V8.2.1