Question about Extending a ZFS Pool

[augeaz]

Hi,

My FreeNAS is currently based on 4 disks each 3gb in a RAIDZ2 configuration I believe.
Recently I came in a possession of 4 additional disks each 4tb.

As my FreeNAS is nearing 80%, I started researching an idea of extending my current volume.

Going through FreeNAS® 11.1-U6 User Guide I found section 8.1.1.4. about Extending a ZFS Volume and it says
"to extend a RAIDZ2 requires a minimum of four additional drives."

However, it does not say or I do not understand clearly if adding 4 disks each 4tb I will gain extra 8tb of volume or only 6tb as it may want to match the size of my original setup.

What is your opinion here, what is the best way to use the additional 4 disks each 4tb or maybe you would rebuild the system from scratch using 4 x 3tb + 4 x 4tb but what would be the most efficient configuration?

Looking forward to hearing your opinions and advises.

Below a couple of screenshots showing how does it look.

 

[Johnny Fartpants]

First thing is based on your screen shots then your pool is not a RAIDz2 but 2 x mirror vdevs. Therefore I would extend the zpool by adding another 2 x mirror vdevs with your 4TB drives. Spacewise you will gain approx 8TB of space.

 

[Chris Moore]

My FreeNAS is currently based on 4 disks each 3gb in a RAIDZ2 configuration I believe.

The illustration is striped mirrors, not RAIDz2. You can add another mirrored pair, called a vdev, to extend the array. You should probably read this introductory material to better understand.

Slideshow explaining VDev, zpool, ZIL and L2ARC
https://forums.freenas.org/index.ph...ning-vdev-zpool-zil-and-l2arc-for-noobs.7775/

Terminology and Abbreviations Primer
https://forums.freenas.org/index.php?threads/terminology-and-abbreviations-primer.28174/

What is your opinion here, what is the best way to use the additional 4 disks each 4tb or maybe you would rebuild the system from scratch using 4 x 3tb + 4 x 4tb but what would be the most efficient configuration?

We need to know more about what you already have and it sounds like you are not sure what you have because it is not a RAIDz2. If you have 8 drives that are all 4TB, a RAIDz2 configuration of all those drives would be more resistant to failure than a pool of mirrored vdevs.

 

[augeaz]

First thing is based on your screen shots then your pool is not a RAIDz2 but 2 x mirror vdevs. Therefore I would extend the zpool by adding another 2 x mirror vdevs with your 4TB drives. Spacewise you will gain approx 8TB of space.

Thanks for your reply Johnny

I built my FreeNAS rig back in 2015 and it seems my memory and knowledge faded a lot since then if this is not RAIDz2 configuration.

I will do some more research and most likely go ahead with your advice to extend the zpool by adding another 2 x mirrors with my 4TB drives to gain up to 8TB of space!

Sounds like a winner for me.

 

[augeaz]

The illustration is striped mirrors, not RAIDz2. You can add another mirrored pair, called a vdev, to extend the array. You should probably read this introductory material to better understand.

Slideshow explaining VDev, zpool, ZIL and L2ARC
https://forums.freenas.org/index.ph...ning-vdev-zpool-zil-and-l2arc-for-noobs.7775/

Terminology and Abbreviations Primer
https://forums.freenas.org/index.php?threads/terminology-and-abbreviations-primer.28174/


We need to know more about what you already have and it sounds like you are not sure what you have because it is not a RAIDz2. If you have 8 drives that are all 4TB, a RAIDz2 configuration of all those drives would be more resistant to failure than a pool of mirrored vdevs.

Thanks for your reply Chris,

My memory and knowledge in this department faded a lot since 2015 it seems.

I will go through the material and see if I can grasp a little more of the topic before moving forward with the upgrade.

To clarify my current situation.
I have an up and running FreeNAS® 11.1-U6 with 4 disks each 3TB.
I came across an additional 4 disks but 4TB each.

Is it still viable option to go for RAIDz2 with four 3TB drives and four 4TB drives?

 

[Chris Moore]

Yes. I will give you more information once I get to work.

Sent from my SAMSUNG-SGH-I537 using Tapatalk

 

[Chris Moore]

Only if you are able to take all the data off the system while the pool is reconfigured.

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[augeaz]

Only if you are able to take all the data off the system while the pool is reconfigured.

Sent from my SAMSUNG-SGH-I537 using Tapatalk

I can take all the data off the system safely if required.

There is one more small detail that came to my attention.
The four additional drives are not exactly the same.
Three of the drives are ST4000DM000 and one of the additional drives is what would appear a newer version ST4000DM004.

Do you think this could be a potential problem?

 

[danb35]

Do you think this could be a potential problem?

Not in the least.

Is it still viable option to go for RAIDz2 with four 3TB drives and four 4TB drives?

In two separate RAIDZ2 vdevs? Of course, though you won't have any more usable capacity that way than you would in striped mirrors. As a single RAIDZ2 vdev? Yes, but in that case ZFS will limit the pool's capacity based on the smallest disk.

 

[augeaz]

Not in the least.

In two separate RAIDZ2 vdevs? Of course, though you won't have any more usable capacity that way than you would in striped mirrors. As a single RAIDZ2 vdev? Yes, but in that case ZFS will limit the pool's capacity based on the smallest disk.

Thanks for replying dab35!

I am finishing backing up all the data.
Once done I will start with adding two mirror vdevs based on the additional four 4TB drives.
If that fails for any reason I will try RAIDz2 option.
However what I read RAIDz2 is much slower and it may affect my users currently enjoying 4 port link aggregation connection.

I will keep you all updated on the work progress! :)

 

[Chris Moore]

Once done I will start with adding two mirror vdevs based on the additional four 4TB drives.

No need to backup if you are extending the pool by adding mirror vdevs. The reason to backup was if you wanted to gain additional space by using RAIDz2.

If you put the four 3 TB drives and the four 4 TB drives into a single RAIDz2 vdev, they would all be used as if they are 3 TB drives, so you would loose around 1 TB of space from each 4 TB drive until such time as you are able to replace the 3 TB drives with 4 TB drives. Once all the drives are 4 TB drives, you would gain access to all the space. If you use mirrors, you are sacrificing half of all your drive space to the mirror. If you use RAIDz2, you are only using the capacity of two drives for the redundancy.
Your existing pool of mirrors should have about 5.2 TB of capacity. By adding two more mirrors, you would gain about 7 TB of capacity, for a total of about 12 TB, which you are only supposed to use 80% of, or about 9.5 TB of usable capacity.

If you go with a pool of eight drives, mixed capacity, you would have about 14.9 TB of total capacity with 11.8 TB being usable after allowing for only filling the pool to 80%. So, even though the 4 TB drives would work like 3 TB drives, you would get significantly more capacity by using RAIDz2 and the entire pool would be protected against two drive failures.

 

[Chris Moore]

If that fails for any reason I will try RAIDz2 option.

Why would it fail?

However what I read RAIDz2 is much slower and it may affect my users currently enjoying 4 port link aggregation connection.

Is this system being used to host virtualization or just simple file sharing? How many users?
If they are not using 10Gb network, they will likely not see the difference in speed between mirrors and RAIDz2. The difference is largely in high IOPS applications like hosting VDI.

 

[Johnny Fartpants]

@Chris Moore is right. If you want a quick easy fix that doesn't require backing up your data and rebuilding your pool then add the mirrors however if you want or are happy to start from scratch then an 8 disk Z2 sounds like a better idea. Pays your money takes your choice.

 

[augeaz]

Thank you for all the input.

I finished backing up the data just in case.
I will try to upgrade this weekend to avoid downtime during the week.
The server is used as a simple file sharing service accessed by at least four workstations.
We are still on 1Gb network with the only difference having FreeNAS setup with 4 port link aggregation.
In practice, each of the workstations has always around 1Gb speed access.

If the drop in performance is negligible for my current use and we will gain extra space I am really drawn towards the RAIDz2 solution as from your comments it appears to be more of a long-term solution.
I will do some more research, make up my mind and take some action.

I will keep you updated on the upgrade progress if any problems arise!

 

[Chris Moore]

Please feel free to update us regardless of whether problems arise or not

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[pschatz100]

Perhaps another consideration... A larger RAIDz2 solution will be more resilient to disk failures than a bunch of mirrors, given that any two drives can fail in RAIDz2 and you won't lose data - whereas with mirrors, if two disks in the same mirror fail then you lose data.