1 vs 2 zpools for mirrored vdevs

[Deleted47050]

Hi there, this is the latest entry in the community's noob list speaking. I am looking for your opinions on a possible HDD design in a small FreeNAS box, because I haven't really found a straightforward answer to my questions.

I have installed FreeNAS on an HP Microserver that was lying around unused to see how it looked like. At the time I only had 1 spare 1 TB drive so that's what it's currently running with. I have just installed a jail with Plex and transferred a few movies. So far it's working fine, but before making a final decision to keep it or not I would like to increase the drive capacity and put it under some more serious testing.

Now, I know I could just get rid of what's on the drive, get a new one and create a mirrored vdev right from the start, but that wouldn't be fun enough. I am planning on transforming the current striped vdev in a mirrored one (yes, I have read the info for noobs -> I know this is the only change to a vdev you are allowed to make :P ) to start with, just to add some redundancy, but this is what I haven't decided/understood yet:

1. Suppose that in the future I buy two more drives, would it be better to create a new mirrored vdev inside the existing zpool, or to create it in a new zpool? Two different zpools would mean that even if both drives failed inside the same vdev, the other zpool would not be impacted (not true with the single zpool option), but I am looking for confirmation.
2. Should I decide to want both performance and redundancy, should I
   • a) stripe two mirrors
   • b) mirror two stripes?
3. Since the current HDD I am using was just a spare one, if I just begin with buying one new drive, it won't probably be the same brand/type, but only the same size. Would there be any catastrophic issues coming out from this configuration?

Let's start with these, I will likely have more later on ;)

 

[Ericloewe]

You can't mirror vdevs. That means you can only stripe mirrors.

1. Suppose that in the future I buy two more drives, would it be better to create a new mirrored vdev inside the existing zpool, or to create it in a new zpool? Two different zpools would mean that even if both drives failed inside the same vdev, the other zpool would not be impacted (not true with the single zpool option), but I am looking for confirmation.

Right.

1. Since the current HDD I am using was just a spare one, if I just begin with buying one new drive, it won't probably be the same brand/type, but only the same size. Would there be any catastrophic issues coming out from this configuration?

Not a real problem, unless they're completely different (WD Green with a 15k SAS drive).

 

[gpsguy]

Rather than buying another 1TB drive, I'd buy a 3 or 4TB drive and mirror it with the original one. No, you won't see any additional storage now. Down the road, you could replace the 1TB drive with a larger one and your storage would increase.

BTW, you didn't say which Microserver you have. If you didn't already know, you can put 16GB RAM in the NxxL series boxes.


Sent from my phone

 

[Deleted47050]

You can't mirror vdevs. That means you can only stripe mirrors.

Great, one problem solved then :) Would you recommend another implementation with 4 drives or does this one look good to you?

Rather than buying another 1TB drive, I'd buy a 3 or 4TB drive and mirror it with the original one. No, you won't see any additional storage now. Down the road, you could replace the 1TB drive with a larger one and your storage would increase.

BTW, you didn't say which Microserver you have. If you didn't already know, you can put 16GB RAM in the NxxL series boxes.


Sent from my phone

That's an interesting suggestion actually, certainly something I could consider. Thanks!

This is an N40L and yes, 16 GBs is the goal. I am looking at the Crucial value series sticks, they don't seem to bad for the price for a test/media FreeNAS box to be honest.

 

[gpsguy]

I'd either look at either striped mirrors or RAIDz2 with 4 or 6 disks. Striped mirrors or separate volumes on mirrored disks allow you to grow your storage over time. But the overhead for parity is steep. If instead of using 2 sets of mirrors, you created a 4 disk RAIDz2 volume, your total amount of storage would be the same, but any 2 of the disks could fail - whereas if 2 disks in a mirror failed you'd have data loss.

Another option would be to stuff 2 more disks in the ODD bay. You'd need to use one of the hacked BIOS' so you could enable AHCI on eSATA and ODD ports. At that point, you might want to consider 6 disks in RAIDz2.

As far as RAM is concerned do make sure that you buy ECC RAM for it.



Sent from my phone

 

[Deleted47050]

I'd either look at either striped mirrors or RAIDz2 with 4 or 6 disks. Striped mirrors or separate volumes on mirrored disks allow you to grow your storage over time. But the overhead for parity is steep. If instead of using 2 sets of mirrors, you created a 4 disk RAIDz2 volume, your total amount of storage would be the same, but any 2 of the disks could fail - whereas if 2 disks in a mirror failed you'd have data loss.

Another option would be to stuff 2 more disks in the ODD bay. You'd need to use one of the hacked BIOS' so you could enable AHCI on eSATA and ODD ports. At that point, you might want to consider 6 disks in RAIDz2.

As far as RAM is concerned do make sure that you buy ECC RAM for it.



Sent from my phone

RAIDZ2 looks good for a 4- or 6-drive setup, yep. But just to make sure I am not missing anything here, should I stripe two mirrors instead, those would have to end up in the same zpool right? So one single volume? I suppose that this is where you start seeing the advantage of RAIDZ2 though.

Also, in technical terms, what really happens when you stripe mirrors? I mean, with drives A and B in one mirror, and C and D in the other. Would this mean that A and C will be the "sources" and B & D the mirrors, and each pair will be seen by the system as a single drive of twice the capacity?

 

[gpsguy]

Yes, if you stripe the mirrors, they will end up in the same pool and single volume. As you said in your first message, you could create mirrors and not stripe them. In the first scenario, if both drives in a mirror were to fail at the same time, all the data in the volume would be lost. In the second example, if both drives in a mirror failed, you'd only loose the data in that volume. The other mirror/volume would be intact. But, most users don't want to manage two volumes. With RAIDz2 you could loose any 2 drives, without data loss.

Here are some configuration examples, using 2TB disks.

Code:

6x2TB RAIDz2 ~7.2TB
4x2TB RAIDz2 ~3.6TB
2x(2x2TB) (2 striped mirrors) ~ 3.6TB
3x(2x2TB) (3 striped mirrors) ~ 5.4TB

So, you can see that a 6 disk RAIDz2 volume will give you more storage than 3 sets of mirrors. But, to create a 6 disk RAIDz2 volume, you need to start with all 6 disks. With mirrors, I can start with a pair and add another pair down the road and so on.

For more information, read cyberjock's guide: https://forums.freenas.org/index.ph...ning-vdev-zpool-zil-and-l2arc-for-noobs.7775/

RAID isn't the same as backup. No matter what you do, you want to ensure that you have backups of any critical data (family pictures, etc.)

 

[Deleted47050]

Yeah I know it's not backup, that's already been taken care of. I want either mirrors or RAIDz2 because of the extra reliability, I don't want to lose everything just because one drive fails.

I guess this leaves me with one last question: if I start with a mirror, I know I can add a second one easily later on, but I suppose there is no easy way to go from a mirror to a RAIDz2 right? I suppose I would need to reinitialize the drives.

 

[Ericloewe]

Yeah I know it's not backup, that's already been taken care of. I want either mirrors or RAIDz2 because of the extra reliability, I don't want to lose everything just because one drive fails.

I guess this leaves me with one last question: if I start with a mirror, I know I can add a second one easily later on, but I suppose there is no easy way to go from a mirror to a RAIDz2 right? I suppose I would need to reinitialize the drives.

Right.