Recently one of our clients relocated their main office and we moved their servers for them. Upon arrival, racking and cabling the XenServer hosts all powered up immediately. The HP StoreEasy NAS with all the data on it also started up.
We noticed that our XenServer hosts had no IP addresses - no data was showing and they seemed to have lost their configuration. Uh oh.... we had a huge bet on getting this thing going by the end of the day with the managing director. There were beers involved and we were pumped to make it work.
After fiddling around with the XenServers and trying to get it working and swearing a *lot*, I turned them all off then back on. Lo and behold they came up and it took a bit to figure out what had gone wrong.
The answer was simple - the XenServer hosts are big, fast machines with a lean install on a high speed SAS drive and the NAS is slower - running Windows Storage Server 2012 and RAID'ed disks. The hosts had come up first, looked for the storage repository and then failed when they couldn't find it. Odd given that I thought the hosts retained most of their settings.
So in future - NAS up first, then start the hosts and all will be happy. The rest of the move was spectacularly successful and we had plenty of beers to drink as a result!