XRAID2 vs RAID6, etc.

Stephen, you really know your stuff on this. I'm really impressed with this community forum--everyone seems very helpful. Thank you :D

OK, my RN104 just arrived and I've set it to chasing rabbits!

Strange, though... it automatically selected a RAID5 under XRAID for me. I thought RAID5/RAID6 were FLEXRAID-only stuff? And how did I miss the option whether or not to choose XRAID2? The printed Systems Installation Guide from the box promises XRAID2, but the browser-based admin tool shows only XRAID.

So I assume this means that I'll have to do a factory default when I slide in that 4th drive, yes? If so I guess I'd better go ahead and order it up pronto.

StephenB wrote:
You can add a 4 TB red, but you will waste 2 TB of its capacity until you have two of them installed. BTW, with RAID-6 you will waste 2 TB of capacity on the 4 TB drives until all 4 reds are installed.

Ah, good tip, thanks. That's OK about the capacity loss though--I don't have that much data to begin with. But I am concerned about the auto-resizing. As I swap in the larger drives over time, will I have to do anything besides sit and wait for the volume to magically get bigger?

StephenB wrote:
CoW and Bit Rot protection are available in both xraid2 and RAID-6.

As noted above, things seem to pointing to the two as being synonymous; I must be misunderstanding something somewhere. So XRAID2 has CoW and BitRot--how 'bout double parity (in case of failure during a rebuild)?

StephenB wrote:
RAID-6 works out better on the RN300 and RN500 series. There is a performance hit for disk writes because the two parity blocks need to be computed, and the slower CPU in the RN100 makes that more significant.

I'm using the device for backup storage only, so write speed performance isn't all that important to me. All the same, I appreciate the heads-up.

StephenB wrote:
If you want to use RAID6 anyway, then change to flexraid before you install the fourth drive.

But doesn't FLEXRAID preclude auto-resizable volumes? And I'm still cornfused about how I managed to get a RAID5 going under XRAID. (Should be XRAID2?)

From reading some FAQs at the website, I was under the impression that it went something like this:

                    BTRFS
            ---------------------
             FLEX-RAID   X-RAID2
            ----------- ---------
RAID5/6          X
Vol Resize                  X

No?

StephenB wrote:
There will then be an option to use the fourth disk for redundancy instead of additional space. After you get the RAID-6 volume synced, you can convert back to XRAID2 (and you won't lose the dual redundancy).

Oh, OK--I think I get what you're saying.

1. Switch to FLEXRAID until I have the 4th drive in hand (didn't know you could switch on the fly--thought it required a reset)
2. Install the drive, let it sync
3. Switch back to XRAID (should be XRAID2?)
4. Live happily ever after

Am I reading you right?




StephenB wrote:
Expansion with RAID-6 requires all 4 drives to be upgraded (switching back to xraid2 doesn't change that).


So, just to clarify... I can have a RAID6 with the 3x2TB/1x4TB mismatched sizes and everything will work, but the volume won't get expanded to the drives' full combined capacity until I get all the 4x4TBs installed? (That was also another auto-expand question, snuck in there sideways.)




StephenB wrote:
You need backups anyway, even with RAID-6 protection.


This seems to be a recurring theme I'm seeing here on the boards. I thought the device itself was the backup. That's why I bought it, anyway, with that in mind. Am I missing something?




StephenB wrote:
So personally I'd go with single redundancy.


I get a really nice warm fuzzy from the idea of protection against failure during a rebuild. That's a biggie for me.




StephenB wrote:
BTW, if you are planning to use the green drives in the NAS for the long term, you might want to set the head parking threshold


Probably not... I want to get that volume size up asap, even if I'm not using a lot of it at first.




StephenB wrote:
Opinions on whether this really matters vary.


I'm presently a man without opinions. ReadyNAS opinions, that is ;)




StephenB wrote:
On paper it voids the WDC warranty. I've had some green drives in my NV+ for almost 5 years, and chose not to set the parking threshold. They continue to perform with no problems with load cycle counts over 500K.


Well that's good enough for me then. They're staying where they're at.

Looks like I wrote a book here. As usual, one answer spawns forty more questions. Your patience with my ignorance in these matters is appreciated.

Thanks,
Jeff Bowman
Fairbanks, Alaska