ZFS write throttle only working for quad-core CPU

[Thomymaster]

Hi guys

I run my FreeNAS 8.3.1p2 for supplying my ESXi via ISCSI.

old CPU: Intel XEON 5060
new CPU: Intel XEON E5345

RAM: 8GB
HDDs: 4x Seagate Barracuda 7200.11 (ST31500341AS) configured as a RaidZ2 with "force 4K sector"
ZIL: none
L2ARC: 64GB SSD


When i set it up i noticed a very bad ISCSI 4k write performance and enabled the tunable (vfs.zfs.no_write_throttle) as seen in this thread:

http://support.freenas.org/ticket/1830

However when i had my old 2-core CPU installed in the FreeNAS host this didn't improve performance. Now that i have a quad-core CPU the tunable works, see the 2 below screenshots (CrystalDiskMark is installed in a Win7 x86 VM on my ESXi).

Is there a reson behind it why the results differ so much or is a quad-core CPU needed to make the tuneable work?

Cheers

Thomy

 

[Thomymaster]

Any news on this or is there something i can do?

 

[jgreco]

The tunable probably worked just fine, but wasn't getting you any benefit because the system was the slowest link in the chain. For whatever it is worth, ZFS is big and piggy. Totally awesome if you have resources to throw at it, like lots of CPU and memory. Less awesome if you have older technology. We had some Opteron 240EE fileservers from ~2005 era that were totally capable of saturating gigE, but under FreeNAS they were pretty crummy.

Moral of the story: if you want best performance, use FFS/UFS for storage on slower systems, or resource your system more heavily to offset the larger resource consumption of ZFS.