[OmniOS-discuss] OmniOS / Nappit slow iscsi / ZFS performance with Proxmox

[Steffen Wagner]

Hi everyone!

 

I just setup a small network with 2 nodes:

* 1 proxmox host on Debian Wheezy hosting KVM VMs

* 1 napp-it host on OmniOS stable

 

The systems are currently connected through a 1 GBit link for general WAN
and LAN communitcation and a 20 GBit link (two 10 GBit links aggregated) for
the iSCSI communication.

Both connection's bandwidth was confirmed using iperf.

 

The napp-it system currently has one pool (tank) consisting of 2 mirror
vdevs. The 4 disks are SAS3 disks connected to a SAS2 backplane and directly
attached (no expander) to the LSI SAS3008 (9300-8i) HBA.

Comstar is running on that Machine with 1 target (vm-storage) in 1 target
group (vm-storage-group).

 

Proxmox has this iSCSI target configured as a "ZFS over iSCSI" storage using
a block size of 8k and the "Write cache" option enabled.

This is where the problem starts:

 

dd if=/dev/zero of=/tank/test bs=1G count=20 conv=fdatasync

 

This dd test yields around 300 MB/s directly on the napp-it system.

 

dd if=/dev/zero of=/home/test bs=1G count=20 conv=fdatasync

 

This dd test yields around 100 MB/s on a VM with it's disk on the napp-it
system connected via iSCSI.

 

The problem here is not the absolute numbers as these tests do not provide
accurate numbers, the problem is the difference between the two values. I
expected at least something around 80% of the local bandwidth, but this is
usually around 30% or less.

 

What I noticed during the tests: When running the test locally on the
napp-it system, all disks will be fully utilized (read using iostat -x 1).
When running the test inside a VM, the disk utilization barely reaches 30%
(which seems to reflect the results of the bandwidth displayed by dd).

 

These 30% are only reached, if the locical unit of the VM disk has the
writeback cache enabled. Disabling it results in 20-30 MB/s with the dd test
mentioned above. Enabling it also increases the disk utilization.

 

These values are also seen during the disk migration. Migrating one disk
results in slow speed and low disk utilization. Migrating several disks in
parallel will evetually cause 100% disk utilization.

 

I also tested a NFS share as VM storage in proxmox. Running the same test
inside a VM on the NFS share yields results around 200-220 MB/s. This is
better (and shows that the traffic is going over the fast link between the
servers), but not really yet as I still lose a third.

 

I am fairly new to the Solaris and ZFS world, so any help is greatly
appreciated.

 

Thanks in advance!

 

Steffen

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