zvol creation as percentage of volume

[Draggeta]

Hi all, this is probably a dumb question, but I haven't been able to find a clear answer yet.

My goal is to create an iSCSI target for VM's for in my home lab. Nothing to fancy, just for practice.
I've created a mirrored volume of 2x2 SSD's and I'm now at the stage when you create a ZVOL.

In the documentation I found this:
"specify size and value such as 10Gib; if the size is more than 80% of the available capacity, the creation will fail with an “out of space” error unless the “Force size” box is checked

I first assumed that this means that you can create a ZVOL filling the whole volume and then use more than 80% of the ZVOL. I felt this was the case as I could create a ZVOL of about 98% of the volume and not get a warning, even though "Force size" wasn't checked.

However, I've also read here on the forums that the size of the ZVOL shouldn't be more than 50% of the total volume space when using iSCSI. Now I'm quite confused. I don't know which one is correct and I'd rather not find out later that I've made a dumb mistake.

Thanks for your time

 

[Mirfster]

However, I've also read here on the forums that the size of the ZVOL shouldn't be more than 50% of the total volume space when using iSCSI.

This is due to performance decreasing. @jgreco has a chart that shows this. Search a bit and you can find it.
Since you didn't post any system specs, I am assuming that you have PLENTY of RAM when considering iSCSI?
In the meantime, have a looksy at jgreco's "Why iSCSI often requires more resources for the same result"

 

[Draggeta]

Hi Mirfster,

Thankfully my specs are indeed ok. I'll not list everything, but basically a Xeon E3 1220-V3, 32 GB RAM and 6x3TB harddisks in Z2. Right now I want to add the 4 SSD's just to run a few VM's on. Thanks for the info. I'll look for the chart. So basically any time you use iSCSI with a ZVOL you shouldn't make the ZVOL itself more than 50% of the volume, but what is done with the ZVOL itself doesn't matter?

 

[Mirfster]

Just quoting jgreco (from the other thread):

What people want -> old repurposed 486 with 32MB RAM and a dozen cheap SATA disks in RAIDZ2

What people need -> E5-1637v3 with 128GB RAM and a dozen decent SATA disks, mirrored

But if this is only for testing, I guess you will be fine. :)

 

[jgreco]

For testing, almost anything is fine, plus, if you test for awhile and start to see really strange stuff, and things seem to progressively suck worse, then you will want to review what @Mirfster points out. . Then you come to this unhappy realization that iSCSI nirvana on ZFS is kinda resource-piggy.

 

[Draggeta]

@jgreco
I've been reading up on everything he said and what I could find on the forum. It is a bit of a shame. I've been loving FreeNAS so far as a NAS/backup storage. I guess it won't be too great for iSCSI and I may need to try and find something like putting the SSD's in the Hyper-V box :) Thank you all for your help!

 

[Mirfster]

I've been loving FreeNAS so far as a NAS/backup storage. I guess it won't be too great for iSCSI

Sure it will, if you provide the correct resources ;)

 

[jgreco]

@jgreco
I've been reading up on everything he said and what I could find on the forum. It is a bit of a shame. I've been loving FreeNAS so far as a NAS/backup storage. I guess it won't be too great for iSCSI and I may need to try and find something like putting the SSD's in the Hyper-V box :) Thank you all for your help!

This has always been the issue with CoW filesystems. Look at WAFL.

The thing to remember is that if you resource it correctly, it will be a LOT faster than the underlying hardware would be if used raw (without ZFS).