SnapRaid Question, please help...

[Elpee]

I know some of you guys here now running SnapRaid on your home servers. Hope you know it.
One of the best feature that I love Flexraid is the ability to span DRU (data) drive. It allows a user to assemble several drives to make 1 DRU especially when your system is made of drives of very different sizes. Why do I need that? It's
- better RAID performance (the lower the DRU count, the better the performance)
- better protection level
For more info. see THIS

I believe SnapRaid also has this feature but how to set it up? I don't know. That's why I'm here and ask.
Maybe something like
disk d1 {C:\mount\disk01;C:\mount\disk02;...}
Arrrggggg.... I don't know...

 

[JoeComp]

No, snapraid cannot span data drives together.

I think your link significantly overstates the advantages of doing so. Yes, in the case that you lose all of the data drives in a spanned DRU, it is equivalent to only losing a single drive as far as redundancy. But the chances of that happening are rather small, and if you are really concerned about losing multiple disks simultaneously, the more important thing to do is to use dual parity (since with single parity, if you lose any of your other data drives during a rebuild you're out of luck). As for increasing performance, I don't see it. I think the performance will be the same (unless you have an anemic HBA that limits your aggregate throughput to less than that of all your drives reading simultaneously).

That said, if you really want to span data drives together, you can do it outside of snapraid so that snapraid just sees it as a single drive / filesystem. On linux, you could use LVM or mdadm. On Windows, I think there is drive spanning capability, or Windows software RAID 0, something like that.

 

[dalekphalm]

As JoeComp said, there are no drive spanning features of SnapRAID. It's essentially just the parity snapshot functions. Because of that, I doubt you can span the parity drives into one "parity unit" either, without 3rd party software.

If you do wish to add spanning features to SnapRAID, I suggest using StableBit DrivePool (considered by some to be the most stable/reliable 3rd party drive spanning software).

I personally use FlexRAID, and it's fantastic. I have 5 drives (2x 3TB, 1x 2TB, 2x 500GB) which are arranged in 3TB units. The PPU (Parity Protection Unit - or your Parity Drive) is the 2TB combined with the 2 500GB drives. This gives me 3 virtual 3TB drives in RAID 5 basically. Eventually I will be adding more 3TB HDD's and adding a second PPU, once I get enough HDD's.

I prefer that FlexRAID has all the features I want built into it, without having to rely on more and more third party software. Why use multiple software when one can do it all?

 

[JoeComp]

If you do wish to add spanning features to SnapRAID, I suggest using StableBit DrivePool (considered by some to be the most stable/reliable 3rd party drive spanning software).

You are using the word "spanning" to mean something other than what I was talking about. I use the word "pooling" to refer to what StableBit DrivePool does.

By spanning, I meant the merging capability that FlexRAID has to span (merge) two or more drives together into a DRU or PPU.

 

[dalekphalm]

You are using the word "spanning" to mean something other than what I was talking about. I use the word "pooling" to refer to what StableBit DrivePool does.

By spanning, I meant the merging capability that FlexRAID has to span (merge) two or more drives together into a DRU or PPU.

Fair enough, though I would argue that "pooling" and "spanning" are largely interchangeable in this context.

Spanning multiple drives into a larger single DRU or PPU is also extremely helpful, and is one of the things that allows you to use random drive sizes in a useful RAID-like environment.

 

[Elpee]

...
Spanning multiple drives into a larger single DRU or PPU is also extremely helpful, and is one of the things that allows you to use random drive sizes in a useful RAID-like environment.

Yes, that's a big plus for FlexRaid, IMHO.

 

[JoeComp]

Spanning multiple drives into a larger single DRU or PPU is also extremely helpful, and is one of the things that allows you to use random drive sizes in a useful RAID-like environment.

Certainly not "extremely helpful". It is only a benefit in the rare case where you have multiple simultaneous drive failures AND two or more of the drive failures are in the same merged unit.

As for being "one of the things that allows you to use random drive sizes"...that is incorrect. Snapshot RAID can mix drive sizes without merging drives into larger units. SnapRAID and FlexRAID can both use any combination of drive sizes you have without doing any merging.

 

[dalekphalm]

Certainly not "extremely helpful". It is only a benefit in the rare case where you have multiple simultaneous drive failures AND two or more of the drive failures are in the same merged unit.

As for being "one of the things that allows you to use random drive sizes"...that is incorrect. Snapshot RAID can mix drive sizes without merging drives into larger units. SnapRAID and FlexRAID can both use any combination of drive sizes you have without doing any merging.

While it's true that you can use whatever you want in terms of drive sizes, from my understanding, the reason FlexRAID has the DRU/PPU spanning function was that "supposedly" having a parity drive (or unit in this case) that has the same storage capacity (or larger if you can't match it up properly) as your other drives was the optimal way to arrange your drives in terms of the parity calculations working, and in terms of maximizing performance.

Now I could be mistaken here - Or what is more likely, the developers could be OVERSTATING the benefits of what I've described.