2018-03-09 03:10 AM - edited 2018-03-09 03:13 AM
Does anyone know if there is any technical deep dive documentation on snapshot technology in Ontap Cluster Mode (we're running 9.1P10)?
We need a detailed understanding of what happens at the block level when snapshots are taken, exactly what changes are captured and how and when space usage for a snapshot is calculated.
We need this to assist us with some issues we are seeing that we strongly believe are bug related.
Can anyone help?
2018-03-09 04:29 AM - edited 2018-03-09 04:31 AM
Sure. We are seeing 2 things:
1) When we upgraded from 7 mode to C-Dot, the size of our daily snapshots for our biggest volumes increased by a factor of 10, a huge increase (hence an oversized snapshots issue). We've yet to understand why.
2) On other volumes, we are seeing deletions that don't tally with snapshot sizes. eg 100GB+ of data deleted, but resulting in a 100MB snapshot (so an undersized snapshot issue).
I won't go into too much detail at this point, as we are still undergoing testing, especially with the second issue that has only recently come to light.
This is why we are looking for some deepdive documentation to cement our understanding of snapshots at a technical level, so we can better engage support.
2018-03-09 05:05 AM
When you say upgrade, did you to a Copy Free transition or use TDP mirrors to migration or use a client side tool to do migrations
And is the data just file system data or Vms or databases.
Netapps snaps have been around forever, so you as for a deep dive, it's simple change blocks get put to snaps, and should be 4k increments
2018-03-09 07:40 AM
We did a copy free transisition using the 7MTT tool.
Yes this is NAS filesystem data, no VMs no databases.
Whilst snapshots may be simple "in theory", there is a lot more to snapshots than just recording changed blocks. For example how space utilization is calculated, when it is calculated, what is classed as a changed block, and system content that exists in a snapshot that you can't even see e.g. /.copy_offload/.tokens etc.
That's why we're looking for documentation to gain a better understanding of how this works at a lower level, if it exists
2018-03-09 07:46 AM
Thanks unfortunately this is just generic content. The section on "working with snapshots copies in Ontap 9.x" only covers configuring policies, managing snap reserve and snapshot recovery.
There's nothing there concerning technical "deepdive" documentation.
I'm really looking for a TR-* type document I guess. I couldn't find one on snapshots, so thought i'd ask here in case anyone can point me to one!