Could you elaborate a bit on your CrashPlan setup? I have a somewhat similar setup and have been trying to back up my NAS (Synology DS1812+) to CrashPlan for about half a year now. I was able to start the backup, but the transfer rate quickly dropped to about 1.2 Mbps or lower, and having backed up about 1.5 TB the CrashPlan client now won't even run a backup.

Are you using Crashplan+ or any of the Pro/Enterprise plans? What sort of transfer rates do you get? And were you able to complete a full backup of that 11x4T array?

I'm using CrashPlan+ Family Unlimited (it's a home NAS!); I also bought a separate CrashPlan+ license when I first signed up. I don't have more than 1TB backed up specifically to CrashPlan as yet though. A big chunk of that NAS is for future growth, and a lot of it is a playground for experimenting with serving up media that we either have physical copies of or is of low replacement value.

I do see CrashPlan updating at a leisurely rate, but that's OK with me, as my upload bandwidth is limited. I'd be interested to see if it completely chokes past a certain point, like you say it does for you. But it's not yet at the point where I need to worry about it.

While you might be right about the small space advantage, I would feel more safe if the position of the failing drive (and the next failing drive) does not matter at all.

If a drive develops a speed problem, I'll replace it.

You might feel more safe, but you're not as safe as you feel.

Suppose that once one drive has failed, the chance of another drive failing (at least an URE, but potentially worse) while reading it entirely (as necessary during a scrub / resilver) is 10%. I don't think this is very implausible, as RAID HDD failures are often correlated, and not only because it's inconvenient to distribute HDD purchases.

The chance of successfully recovering in a mirrored scenario is 90%. You only need to read one drive.

In raidz1 with 6 drives, your probability is 0.9^5 - you need to get lucky 5 times, because you need to read every other drive fully to recover. That brings you down to less than 60%. Raidz1 with 5 drives is 0.9^4 - a bit over 65%.

Raidz2 for 6 drives combines these two stats. The probability of one failing disk while reading is 1-0.9^5. But you can recover from that. And the probability of that recovery is 0.9^4. So your consecutive probability of failures is (1-0.9^5)(1-0.9^4). Subtract that from 1 and you get your probability of getting lucky.

Work out the numbers, and you'll find it's only 86%. It's less than mirroring.

There are a couple of simplifications in the maths above (don't forget, a second drive failure means another drive to resilver, and a restart to the procedure), and a big assumption based on reliability %. But frankly, unless you think HDD failure is already fairly rare - and it's not - mirroring is usually ahead on safety, and the lead only increases with number of disks in the array.

I used to think like you do, that I'd feel more safe if a single disk failed, knowing I could still tolerate another failure. That's why my array is currently raidz2. But I worked out the numbers, and it changed my mind. And that's why my next array is going to be raid10.

Not to mention that it's hard to get much more than 150M/sec for any realistic I/O with raidz2. I have over 1G/sec of potential HDD bandwidth. Unless I'm doing a scrub, I can't touch the potential performance. And of course all disks act like they're on the same spindle, there is zero random I/O advantage for raidz.