Quick question: Dataset and ZVOL

[BlazeStar]

Hi guys,

Using FreeNAS-9.2.1.3-RELEASE-x64

New to ZFS.

I've been reading this:
http://doc.freenas.org/index.php/Volumes#Creating_ZFS_Datasets
and this:
http://doc.freenas.org/index.php/Volumes#Creating_a_zvol
and a few other stuff

I'm still unsure why you would use ZVOLs...

Also I don't understand why you CAN created ZVOL nested inside a DATASET.
But why you can not create a DATASET nested inside a ZVOL.

So first... I understand how to manage ZVOLS and DATASETS but what are their usages?

I think DATASETS are quite simple: folders.

And ZVOLS have the feature of allowing you to create a block and reserve space on the volume.

I don't need to use it as a iSCSI device extent, or use a different file system.
I just want to take advantage of the reserved space (quota).
But then you can't manage this ZVOL using DATASETS or else...
So I'm wondering if I should be using ZVOLS at all.

What I'm trying to accomplish :

4 X 4TB drives in RAID 10 (stripped mirrors) = 8TB

I would like to keep 4TB for virtualization and 4TB for shares.

Dataset won't let me set quotas.

That's why I turned to ZVOLS.

What do you think?

 

[eraser]

You can create quotas on Datasets. It is under the Web GUI -> Storage -> select your Dataset -> Edit ZFS Options (wrench icon) -> Advanced Mode button

 

[BlazeStar]

Nice! Thanks!

So in my case, I really don't need to use a ZVOL right?

 

[eraser]

I see that you plan use some space for virtualization. Two common ways that I can think of to present storage to the virtualization host are NFS or iSCSI.

If plan to use iSCSI, then you will share out a ZVOL. If you plan to use NFS, then you will share out a Dataset (or a subfolder of a dataset).

 

[BlazeStar]

I plan on using NFS so I'll go with a DATASET.

Thanks a lot!

 

[eraser]

Excellent idea. I run NFS for my virtualization storage myself.

Some recommendations:

• Stay away from ZFS deduplication. It is almost never worth the massive increase in RAM requirements. ZFS Compression works great though and is enabled by default in recent versions of FreeNAS.
• Are you planning to use VMware ESXi as your virtualization host? If so you will notice horrible write performance over NFS. Go read here for why and the proper solution (add a SLOG): http://forums.freenas.org/index.php...xi-nfs-so-slow-and-why-is-iscsi-faster.12506/
  • A cheap dirty hack workaround is to set sync=disabled on the dataset but that is risky. I would never make that change if I was storing data belonging to someone else.