hdtvguy wrote:
My gut tells me that daily snaps would occupy more than weekly, just because you have to track blocs that may change more than once during the 7 day cycle.
I think you are right here. If I use an excel sheet as an analogy of some block data with the cell contents in A1 being changed daily so that it was all 1's on Monday, all 2's on Tuesday, all 3's on Wednesday etc. then that changed data would have to be copied for each previous day's snapshots and as it is different it is not shared between them.
i.e.,
Monday evening snapshot = just metadata
Tuesday working day the cell changes, old data copied to PiT area and Monday snapshot updated to point to it and new data written to "live" VV.
Tuesday evening snapshot = just metadate
Wednesday working day the cell changes, old data copied to PiT area and Tuesday snapshot updated to point to it and new data written to "live" VV.
etc.
But a weekly snapshot taken on say a Sunday would only contain one change, i.e., the data written the next day on the Monday which would cause the old data to be copied to the PiT area. The subsequent writes on the following weekdays wouldn't cause a write to the PiT area as well as in this simplified example, it is the same blocks which are changing.
Cleanur wrote:
So multiple snaps effectively use the same space for shared blocks, only.the deltas between individual snapshots will consume additional space.
So if the cell contents alternated between 1's and 2's throughout the week then there would only be 2 blocks of changes shared between the daily snapshots. So it would be:
Monday evening snapshot = just metadata
Tuesday working day the cell changes to all 2's, old data copied to PiT area and Monday snapshot updated to point to it and new data written to "live" VV.
Tuesday evening snapshot = just metadate
Wednesday working day the cell changes back to all 1's, old data copied to PiT area and Tuesday snapshot updated to point to it and new data written to "live" VV.
Wednesday evening snapshot = just metadate
Thursday working day the cell changes back to all 2's again, old data NOT copied to PiT area as the "all 1's" data is already there so snapshot is just updated to point to that but new data written to "live" VV.
Does this sound about right? If so, I think I understand it better now.
Sorry if this is massively obvious, just trying to get my head around it as I've never really focused on the actual snapshot mechanism before. It's just something you do isn't it, tell the storage to "remember where you are right now" so you can rewind later but how that actually works I've never really given much thought to until now.