Mirrored Pool Usable Space

[robsonj]

Hi There,

I currently have 3 drobo 4bay boxes and am gonna be migrating to a freeNAS solution in the next few weeks. I am new to FreeNAS so figured I would play around with it in VirtualBox first to get a feel for how it works and maintenance. My drive setup which will come from the drobos is a bit of a hodge podge, but basically...

2 x 4Tb WD Greens, 2 x 3Tb Seagate Barracudas, 2 x 2Tb Hitachi

My plan was to setup a mirrored vpool...

tank
|_4TB mirrored with 4Tb
|_3Tb mirrored with 3Tb
(may add the mirrored 2Tb drives later)

I figured this would give me ~7Tb of usable space, faster reads, reasonable failure protection and the ability to upgrade the drives in pairs.

In virtual box I created a bunch of virtual disks to play with, instead of 4Tb I created 4Gb drives, then 3Gb for the 3Tb etc etc.

I started in virtual box by creating...

tank
|_4Gb mirrored with 4Gb

I expected that would give me ~4Gb of usable space, but I ended up with 2Gb!

Did I do something wrong, or is my understanding of mirroring just wrong? Maybe its just overhead of ZFS, hence I may try a mirrored 40Gb virtual drive to see if its just ZFS overhead which is more noticeable on such small test drives.

Any help appreciated.

Cheers
Jonathan

 

[pirateghost]

Edit: wrong URL.

Freenas creates a 2gb swap

 

[robsonj]

Thanks. I have created my FreeNAS box install, shared it, can access it remotely etc. Works well. My question was about the usable space, 2 x 4Gb drives mirrored should be ~4Gb usable I assumed, but I am getting 2Gb.

 

[pirateghost]

Thanks. I have created my FreeNAS box install, shared it, can access it remotely etc. Works well. My question was about the usable space, 2 x 4Gb drives mirrored should be ~4Gb usable I assumed, but I am getting 2Gb.

Read my edit and this link

http://doc.freenas.org/9.3/freenas_intro.html?highlight=swap#storage-disks-and-controllers

 

[robsonj]

Aaaa, thank you. Yea, I also created 2 x 40Gb virtual disks in the meantime and see 38Gb usable from a 40Gb mirror, it is just a little overhead. Thanks again for the help!

 

[Robert Trevellyan]

2 x 4Tb WD Greens, 2 x 3Tb Seagate Barracudas, 2 x 2Tb Hitachi

My plan was to setup a mirrored vpool...

Nothing wrong with a mirrored pool, but another option is to create a 6-drive RAIDZ2 pool with all your drives. The capacity would be roughly 8TB less overhead, since all drives would be treated as 2TB, but you could plan to replace the smaller drives over time, eventually yielding roughly 16TB less overhead.

 

[robsonj]

My understanding with that setup though is that if I add larger drives, their additional space wouldn't be realized until all the drives had been increased in size? I'll try it on my virtual box.

My remaining problem is that I had thought I could put a eSata device on the back of the freeness box, then sync the pool to it - it thus serving as a backup. I haven't discovered if thats possible though. Plan B is to setup a Raspberry Pi as an rsync target with a USB drive connected to it.

 

[Robert Trevellyan]

if I add larger drives, their additional space wouldn't be realized until all the drives had been increased in size?

Right, once you replace the 2TB drives, your pool would behave as if all drives were 3TB. Then after replacing the 3TB drives, it would behave as though all drives were 4TB.

 

[robsonj]

Yea. I'm just trying to setup a raidz2 with my virtual box to experiment with it, but the volume manager doesn't seem to allow a raidz2 with differing sized drives. Maybe I have to drop to the cli to do that? It only seems to allow mirrored pools.

 

[jgreco]

There's an option (used to be an option?) for manual config in the volume manager.

 

[Robert Trevellyan]

You have to use Manual mode when the drives aren't all the same size.

 

[Robert Trevellyan]

My remaining problem is that I had thought I could put a eSata device on the back of the freeness box, then sync the pool to it - it thus serving as a backup. I haven't discovered if thats possible though.

It is possible. Either use an eSATA card with a supported chipset (e.g. Sil3132), or use a bracket to expose internal SATA ports as eSATA ports, if you have enough.

 

[Arwen]

My understanding with that setup though is that if I add larger drives, their additional space wouldn't be realized until all the drives had been increased in size?

Correct. But, if you upgrade the 2 x 2TB drives in a pair, (preferably 4TB or larger in your case), your pool will grow to the 3TB disk size. Kinda hard to explain. Here, both RAID-Z2;

2 x 4TB + 2 x 3TB + 2 x 2TB = 6 x 2TB, about 7TB usable

But, after first upgrade;

2 x 4TB + 2 x 3TB + 2 new 4TB = 6 x 3TB, about 11TB usable

You get the 2TB of additional space from the existing 2 x 3TB drives. Plus, 2TB from the 2 upgraded drives, (2TB -> 3TB gives you 1TB, times 2 drives).

Basically RAID-Z2 gives you different upgrade options than 3 x mirrored VDEVs. BUT, the ability to loose 2 disks without data loss.

 

[robsonj]

Yep I got it. Thanks for all the help. In my playpen a raidz2...

4x2 + 3x2 + 2x2 = ~6.9Tb usable
4x4 + 3x2 = ~10Tb usable
4x6 = ~14Tb usable

Think I'll stay with mirrored pool though as when it comes time to upgrade beyond 4Tb - I won't realize any of the new drive space (say I move to 6Tb drives) until I've bought 6 new 6Tb drives, versus mirrored where I get incremental increases every 2 new drive purchases - though less overall at the end of the day.

With the mirrors I can loose 3 drives - as long as they are the right 3. The freeness is already my secondary backup anyway, plus I plan to have a tertiary backup behind the freeness and then crash plan.

 

[robsonj]

It is possible. Either use an eSATA card with a supported chipset (e.g. Sil3132), or use a bracket to expose internal SATA ports as eSATA ports, if you have enough.

Yea I'm gonna expose an internal with a bracket. Do you just backup to it using command line tools, or is there ea way to schedule that through the freeness ui - I couldn't see any options, but early in my research on that. Thanks!

 

[Robert Trevellyan]

Seems like you're making an informed decision about your pool layout.

is there ea way to schedule that through the freeness ui

You can set up a snapshot task and a companion replication task using the GUI. The trick with the replication task is that you need to use localhost or 127.0.0.1 as the destination.