Is there any downtime when expanding a volume on an iSCSI setup using Synology Hybrid RAID?

Solution 1:

No there wont be any down time, this really is a standard thing to do now in enterprise, where always up availability is key.

LVM does this expansion in the background and it is pretty much instant, it wont have any effect on services, I do this once or twice a week on my estate and haven't had a problem yet