May be I miss something I cannot grasp how NFS and iSCSI (or LUN in general) differ w.r.t. snap reserve. You *must* reserve space for changes, be it on NFS or LUN volume; and amount of reserved space is the same. If you know that you got 20% change, just set fractional reserve to 20. If you do not know amount of change, you must reserve 100% of course for LUN - but then again, you must have 50% free on NFS volume as well.
Where exactly is the difference? Confused thank you.
Prior to the recent changes where the active volume can gain space, I saw it as the difference between how NFS clients and LUN clients operated.
When talking to a filesystem (local or NFS), you expect that it might fill. So the NFS clients (and hopefully the application) can handle a temorarily full filesystem.
With a LUN, the situation is different. The OS does not expect that a block write can fail unless there's something wrong with the storage device. So the failure mode from that is probably less defined than a failed NFS write.