Migration columns, a subcategory of the ResUsageSpdsk table statistics, identify the number of cylinders that migrated to a different location on a device as well as the time, in centiseconds, of all migration I/Os used, incurred, or saved during the log period.
|Column Name||Mode||Description||Data Type|
|ExtMigrateFaster||count||Number of cylinders migrated to a faster location on a device. This count is for cylinders that were allocated on this device and migrated to a different location within the same device or migrated to a completely different device.
The following formula calculates a ExtMigrateSlower value, which is the number of cylinders migrated to slower locations:
ExtMigrateSlower = ExMigrateTotal - ExMigrateFaster.
|ExtMigrateIOTimeBenefit||count||Estimates the total I/O time savings achieved by migrations completing in the log period. The I/O time savings include the improvement in response time caused by the new data arrangement up to the time horizon. ExtMigrateIOTimeBenefit does not include the cost of the migration I/Os and is a gross benefit, not a net benefit.||FLOAT|
|ExtMigrateIOTimeCost||count||Estimates the total cost, in centiseconds, incurred by migration I/Os completing during the log period, where cost is the extra time waited by all non-migration I/Os as a result of the migration I/O.||FLOAT|
|ExtMigrateIOTimeImprove||count||Estimates the percent improvement in average I/O response time due to migrations completing in the log interval. For example, if, right before a particular log interval, the average I/O response time was 10 milliseconds (ms), then the migration logs an ExtMigrateIOTimeImprove value of 10% in this interval. The average I/O response time after the log interval should be (100%-10%)*10ms = 9ms. Migration then logs an ExtMigrateIOTimeImprove of 1% in the next interval. The average I/O response time in the new log interval is (100%-1%)*9ms = 8.91ms.
ExtMigrateIOTimeImprove is only an estimate. Its permanent improvement remains in effect as long as the workload does not change and newer migrations do not significantly alter the data arrangement.
When the workload changes or new migrations affect data arrangement, response time changes in an unquantifiable way.
Despite this, ExtMigrateIOTimeImprove is useful because it predicts actual system performance at least for short periods of time and can be used to understand why the migration algorithm is doing what it is doing.
|ExtMigrateReadRespTot||count||Migration read I/O response time.||FLOAT|
|ExtMigrateWriteRespTot||count||Migration write I/O response time.||FLOAT|
|ExtMigrateTotal||count||Total number of cylinders migrated to a different physical location. For more information, see the ExtMigrateFaster field.||FLOAT|