Mostly replying via the Russell's post to the OP...
On Sun, 21 Apr 2013, Tony Langdon <vk3jed@gmail.com> wrote:
About to rebuild the network here after moving house. One issue I'm contemplating is performance and redundancy. One part of the network can't (easily) be reached by Cat 5/6, running cables t that part of the house would be too messy at best. I have two ways I can bridge this gap - Powerline Ethernet adapters, which have worked extremely well in the past, or WiFi, using an access point in client bridge mode.
Now the powerline adapters do work extremely well, with a rated speed of 85 Mbps. I've never had an issue, except for the switchmode supply of one laptop, which trashed the link (took a bit of detective work that one!). The biggest weakness of these devices is that they can't be battery backed up. If the mains goes down, so do they.
http://etbe.coker.com.au/2010/08/04/clusters-dont-work/
I wrote the above blog post about my experience with clusters. Some of this applies to failover of links.
One significant difference between a home network and a serious server network is that most of the functions of your home network don't matter much when you are asleep or away. Therefore a redundancy which involves you logging in as root and running a route command will work a lot better on a home network. Of course my experience is that having a sysadmin login as root and manually fail things over is better than any cluster software implementation I've seen, but that's another issue.
WiFi can be backed up, especially since a lot of the gear I have will happily run off a 12V battery, and some of the systems on the far end will be running off a battery backed DC supply. However, performance with the WiFi solution isn't as good.
A point to point 802.11n link should easily beat any powerline adapter, unless you have a solid lead wall or something?
Is there a way I can (easily and cheaply) arrange to run on the powerline devices by default and fail over to WiFi, if the power goes down?
If you know a bit about OSPF (or RIP) and can arrange a router at each end then yes, easily. If you have a proper wireless bridge at each end and can run Linux bridge at each end to bridge the two interfaces (wifi and powerline) and turn on STP then also yes, easily. My laptop has "LAN/WLAN switching" so that if I plug a network cable in the wireless switches off, which saves a heap of DHCP addresses when you consider 100 laptops on a single network. If your far end machine has that sort of capability then it would work in the "Ethernet link down" case, but not if there was some other fault that still presented a link connected without actually having end to end connectivity. If your far end machine runs linux then a simple ping script to swap over might be easiest. If it were me, I'd get a couple of openwrt based devices (netgear wndr3800's are what I've been using lately) and create separate /30 networks over each of the wireless and powerline links, then use OSPF to route over whichever one has connectivity, giving the powerline link priority if that has the better performance. Of course that doesn't work if you need the same broadcast domain, in which case I'd bridge them and use STP, but I'm less familiar with that. Also, that doesn't necessarily handle the case where powerline link is up but performing badly etc. James