Snapshot Rate Limits

All volumes in a group are associated with a volume collection. Multiple volume collections may be created for each group, and each volume collection may have multiple protection schedules. Protection schedules may have different periods, or rates, such as hourly, daily, or weekly. An hourly schedule takes a snapshot of all volumes in the volume collection once per hour. A schedule that is configured to start at midnight begins taking snapshots of all volumes in the collection at 00:00, and at each hourly interval thereafter.

Beginning with OS 4.x, the maximum expected snapshot completion rate is 250 snapshots per minute per array group. The maximum cumulative outstanding snapshot count is the total of all snapshots for all protection schedules in the group, and cannot exceed 4,000 at any point in time. The outstanding snapshot count for a schedule is the number of snapshots started at that point in time or during the prior minute, but not completed because of the 250 snapshot per minute rate limitation. Outstanding snapshots are carried forward to the next minute, and completed at a rate of 250 per minute until all are finished.

For example, a protection schedule is configured to start and repeat every five minutes (T equals 5), and contains 270 volumes. The outstanding snapshot count at minute one is 270. All snapshots in the schedule must be completed in five divided by two (T/2), or three minutes. During minute one, 250 snapshots are taken. The remaining 20 snapshots are carried forward and completed during minute two. When the schedule repeats at the next five-minute interval, 250 snapshots are taken during minute five and the remaining 20 during minute six.

Schedules that have a lower repeat interval, which is calculated as the number of minutes in the interval, have a higher priority than schedules with a higher repeat interval. For example, daily schedules are prioritized to start before hourly schedules. When multiple schedules have the same repeat interval and are configured to start at the same time, the one that was created first has the higher priority. Higher priority schedules are considered first in determining the maximum outstanding snapshot count and the total expected snapshot completion time.

The more aggressive the schedule, the fewer volumes that can be protected. In order to prioritize all snapshots, those with daily schedules are completed before those with hourly intervals. Snapshots with hourly schedules are completed before those with intervals in minutes.

If you create ten volume collections, each with a single hourly protection schedule containing 50 volumes, a total 500 snapshots are scheduled. An hourly schedule has a time (T) period of 60 minutes. All snapshots must be completed in T divided by two, or 30 minutes. If all ten schedules are set to start at the same time, a maximum of 250 snapshots are taken during the first minute of the hour, and 250 are carried forward. The remaining 250 snapshots are taken during the second minute.

When you create or edit snapshot schedules, the OS makes calculations to determine whether the changes will exceed the 250 snapshots per minute or the maximum outstanding snapshot count 4,000 limits. If either one will be exceeded, the operation fails. To avoid exceeding the outstanding snapshot count when you add a new schedule or edit an existing schedule, stagger the schedule start time.