ZFS Pool questions - help me understand.

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styx0r

Limp Gawd
Joined
Jul 28, 2008
Messages
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I've just spent the last few hours reading about ZFS configurations, and I have a working install of napp-it up and running to play with.

I understand that pools are the basic storage organization method. They are made up of Virtual Devices, or Vdevs.

Let's assume I have 5 identical 2TB drives.

Let's say I created a pool tank with two Vdevs, and fill it with 1TB of data.

Tank:
(2) 2TB drives mirrored = vdev 1
(3) 2TB drives RAIDZ = vdev 2

Now, I know this isn't a recommended configuration - all vdevs should have identical redundency characteristics - but I want to understand this:

Now, let's say I buy a 6th 2TB drive, and I want to convert the 2TB mirrored vdev into a (3) 2TB RAIDZ, so I end up with:

Tank:
(3) 2TB drives RAIDZ = vdev 1
(3) 2TB drives RAIDZ = vdev 2

Can I force the data onto the RAIDZ(vdev 2) and off the mirror(vdev 1), before I remove vdev 1? And then, (re)create vdev 1 as a RAIDZ with my 6th drive?
 
As far as I know you can't change the type of vdev in a pool. If you create a mirror your stuck with a mirror even if you add a third drive. You would have to create an entirely new pool to convert it. Here is a link to an article where someone has done just that (link). It is also not recommended that you don't combine different types of VDEV's within a pool as this will just complicate things when you go to expand. What I recommend would be to create a pool with a single 3 disk raidz vdev. Then expand your pool by adding a second raidz vdev of 3 disks when you get the 6th disk.

Or just buy the 6th disk and make a single raidz2. (2x 3 disk raidz1 < 1x 6 disk raidz2)
 
A ZFS pool does not care about the vdevs it consists of.
You can mix any type of vdevs to increase capacity but you can currently
not convert a vdev from one type to another beside basic to mirror.

You should only care about keeping overall redundancy level.
The rest is a question of economy and a balanced structure for better performance.
 
I don't believe you can ever remove a vdev at all from a pool. And as others have mentioned you can't ever change a vdev's type (with the exception of adding or removing mirrors from a mirror, but it's still only a mirror).
 
Ok, I appreciate your replies - I feel like I did a bad job of explaining.

I know you can't convert the mirror to a RAIDZ.

I was imagining something like:

1. Delete mirror vdev
2. Create RAIDZ vdev

-Without losing data. In my example, there's only 1TB of data being stored, which can easily fit on vdev 2. My question was specifically if I can flush the data stored on the mirror vdev before removing it from the pool.
 
styx0r said:
Ok, I appreciate your replies - I feel like I did a bad job of explaining.

I know you can't convert the mirror to a RAIDZ.

I was imagining something like:

1. Delete mirror vdev
2. Create RAIDZ vdev

-Without losing data. In my example, there's only 1TB of data being stored, which can easily fit on vdev 2. My question was specifically if I can flush the data stored on the mirror vdev before removing it from the pool.
Click to expand...


ZFS spreads data over all vdevs for speed, capacity and redundancy.
If you delete a vdev you kill the pool

You can only backup to a second pool and destroy/ recreate the pool
 
Last edited:
styx0r said:
1. Delete mirror vdev
Click to expand...

Simply, ZFS does not support this. You can not remove a vdev from a pool. You can destroy the pool and start over, but then it's obviously up to you to move your data around.
 
Thanks again for all the input - I really appreciate it! I now understand - vdevs are there for the life of the pool.
 
Create 2 different pools then

Pool 1
2 x 2tb drives Mirror < vdev1

Pool 2
3 x 3tb Drives Raidz < vdev1

When you want to move the 2 x 2tb mirror into a RaidZ

Copy the data off of the Pool 1 - Onto Pool 2
Destroy Pool 1

Recreate Pool 1 as a 3 x 2tb RaidZ

Copy your data back from Pool 2 onto Pool 1

.;)
 
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