Release Notes

25 Dell PS Series Snapshots and Clones: Best Practices and Sizing Guidelines | BP1027
Snapshot reserve utilization from progressive snapshots and increasing % volume access
After the first snapshot, the snapshot reserve usage was about 1 GB (10% of the 10 GB base volume). Since
only 10% of the volume was accessed, all the pages making up that 10% were affected, resulting in the same
amount to be consumed by snapshot reserve space. In other words, the data in 10% of the base volume was
completely changed.
After the second snapshot, about 3 GB is consumed in snapshot reserve space. In this case, 20% of the disk
was accessed. If all pages were affected, 2GB of snapshot reserve space should be consumed. However,
one snapshot consuming 1 GB of space was already retained. Adding these two snapshots together reflects
the current usage at about 3 GB.
Adding the third snapshot results in the same cumulative effect (1GB + 2GB +3GB) returning a total of 6 GB.
After the forth snapshot, there is a similar result causing the total to approach 10 GB, or 100% of the snapshot
reserve.
At the fifth snapshot, the results begin to look different. Adding 10% + 20% + 30% + 40% + 50% (or 1GB +
2GB +3GB + 4GB +5GB) is clearly above the complete (100%) snapshot reserve which is 10 GB. The 100%
consumption of the snapshot reserve forces the default policy to delete the oldest snapshot and make room
for new snapshots. In the end, only 9 GB of the snapshot reserve is used. With this scenario, there are only
two retained snapshots (from the 40% and 50% access test cases). To retain additional snapshots, the size of
the snapshot reserve would have to be increased to more than the default setting of 100%.