ONTAP Discussions

understanding daily and weekly snapshots


We have a SLA to have a 90 day history of versions on files/folders.


I had been in the practice of retaining 14 weeklies, and 14 dailies, but we are concerned that if there was a unique change, on say, 25 days ago that was needed, would it be captured in the weeklies?  To put it more concretely, for example, if 25 days ago (on a Tuesday), I write new data, then 24 days ago (the day after, Wednesday) I delete it.  Will that file show up in any of the weeklies?


Just for confirmation, the dailies are a separate set of deltas from the weeklies right?


A separate question: In theory, doing 14 weeklies with 14 dailies would use less space than 90 dailies right? but if my scenario mentioned above doesn’t capture that deleted file, I will have no choice but to do 90 dailies.



Re: understanding daily and weekly snapshots


Any snapshot will only capture the differences between it and the previous snapshot at the moment the snapshot occurs. What does that mean?

  • If I take a daily snapshot at midnight every day, then it doesn't matter how many times during the day a file is changed, it's "state" will only be saved for the data present at midnight
  • If I take a weekly every Sunday morning at 0000, then it doesn't matter how many times during the week it changes, only the state at that time is preserved

90 daily snaps will almost certainly consume more capacity than 14 daily + 14 weekly, but that will largely depend on the amount of churn.  If you only have a few files that are constantly being changed, and the same areas of those files are constantly being overwritten, then the difference between 90 and 14+14 won't be that great.  If there's lots of files and lots of new/changed data every day, then the 90 plan will consume more space.


What you are effectively talking about here is RPO.  For the dailies, you can lose up to 24 hours of data.  For a weekly, it's 7 days of data.  For example, if I take a weekly snapshot the file is preserved at that moment in time.  Let's say 6 days and 23 hours later that file is deleted.  If there are no other snaps between the weekly and that moment, then I would lose any changes that happened since the last snapshot...nearly 7 days worth of data.


Hope that helps.



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