[mythtv-users] thoughts on a combined backend/NAS box?

[Simon Hobson]

Joseph Fry wrote:
> There is nothing wrong with RAID 5... but you have to be aware that it cannot be used
> in place of a backup, just like any RAID solution. The odds of a two disk failure are far
> lower than a single disk... so while the potential is there, odds are it won't happen and
> in the meantime your system will keep chugging along while the array rebuilds.
I agree. If you take the objections literally, then no-one would ever, ever use a single drive for anything - after all, it may fail. mdadmin will doa  consistency check on your volumes periodically, and smartmon can keep an eye on drive state. These aren't perfect, but they do go some way to dealing with the issues that raid5 does have.
But I agree, raid 5 does work - it's saved my data a number of times over the year - but I'm aware of its limitations. I also have multiple generations of backups.
Of course, if you don't do any checks/proactive maintenance then raid5 will let you down. But then so will raid 6 if you ignore a failed drive. Yes, I've seen people just ignore a failed drive and carry on as usual - until they suffer another one that leaves them no better off than if they hadn't raided in the first place.
BTW - it helps a lot if you don't buy all your drives at the same time. It's better if you buy indivudual drives over a period of time, and put them to use as soon as you get them. Hopefully you can get them from different batches, and slightly spread the running hours. It won't make a huge difference, but it should reduce the problem of having identical drives which all fail close together due to a manufacturering issue with the one batch. Raid6 won't help you if that happens unless you can replace *AND* fully rebuild drives faster than they fail - though it will cope better than raid 5.
> Virtualization adds an unnecessary layer of complexity and another point of potential failure.

It does more than that, it adds a bucketload of compatibility issues for something like a MythTV backend. I abandoned the idea of running a virtualised backend when I found that the HVR1300 card in my HP Microserver doesn't work properly under Xen if there's more than 4G of physical RAM in the machine. It's great for a lot of things (I run many virtual hosts at work, and a few at home), but IMO it's not good for something so "hardware sensitive" as a Myth backend with tuners.