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ME4 - Questions about Snapshots and related
Hello! We are currently using the ME4024 SAN, which has been pretty nice so far. I've been playing around with the Snapshot feature and had some more technical questions I was hoping perhaps we could have sent to the engineers? Here we go:
When a snapshat is made, it looks like it essentially creates a copy of the original by keeping track of which pages are the same and which ones have changed between the two of them. When a page is updated on either the 'base' or the 'snapshot', how does system handle it? To ask more directly:
If a change is made to a currently shared page on the 'base', does it:
A.) Make a copy of the page and reassign it to the snapshot (which is still unaltered at this point) and then continue to use the original page for the base and update it, or
B.) Write a new page with the updated info for the base and reassign the page to it, leaving the original page now tied only to the snapshot.
On the reverse, if a page is updated to a currently shared page on the 'snapshot', what does it do? (it has to do one of the two options above, only opposite)
Lastly, all of this page copying could create a lot of 'holes' in the system once a Snapshot is removed, does the system consolidate/compact/move things around to help clean things up as some sort of background process?
Thanks for your time!
DELL-Sam L
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May 7th, 2019 13:00
Hello Jack,
When using snapshots on an ME4 they are a point in time copy of your data that was on the volume when the snapshot was created. All changes are tracked by the OS and not the ME4. You will need to take a new snapshot to get the changes in data that were made from the first snapshot. Here is the link to the administrator guide which has some additional information about snapshots & how they are used. https://topics-cdn.dell.com/pdf/powervault-me4012_administrator-guide_en-us.pdf
Please let us know if you have any other questions.
jack.dobiash
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May 7th, 2019 14:00
Sam, Thanks for the response, however it doesn't make sense. There is no way that the OS is keeping track of the changes between a snapshot and it's 'base'. (unless you're talking about the ME4's OS and not the 'host' OS). The Host OS doesn't know anything about it, it just sees it as a copy of the volume at the point the snapshot was taken (and i'm not even mounting my snapshots to the same physical machine anyway otherwise they would conflict with each other). What I was looking for was how the ME4 handled when changes occurred between the base and snapshot 'under the hood', just so I could get a clearer picture. Also I was looking for some answers about possible 'cleanup/consolidation/compact' operations as well. I was hoping perhaps these questions could be sent to an engineer level person? Thanks for your time!