Importance of path change settings in VMware

[aherbert23]

With 1 gig networks it is important to tune per path settings for maximum throughput. The default of 1000 IOs per path can cause micro bursts of saturation and limit throughput. I've done a fair amount of testing and found that the best setting is to actually change paths based on the number of bytes sent per path. The reasoning is that it can be detrimental to change paths too often for small block IO. Setting the path change to bytes optimizes for both.

Here is a real world example from a demo I recently conducted. This was done with 4 1G interfaces on both the Nimble array and the ESX host connected through a Cisco 3750X stack.

SQLIO prior to path optimization :

Server	Tool	Test Description	IO/s	MB/s	Avg. Latency
SQL-06	SQLIO	Random 8k Writes, 8 threads with 8 qdepth for 120 sec	12375	97	4ms
SQL-06	SQLIO	Random 8k Reads, 8 threads with 8 qdepth for 120 sec	14456	113	3ms
SQL-06	SQLIO	Sequential 64k Writes, 8 threads with 8 qdepth for 120 sec	2130	133	29ms
SQL-06	SQLIO	Sequential 64k Reads, 8 threads with 8 qdepth for 120 sec	2147	134	29ms

SQLIO after path optimization :

Server	Tool	Test Description	IO/s	MB/s	Avg. Latency
SQL-06	SQLIO	Random 8k Writes, 8 threads with 8 qdepth for 120 sec	26882	210	1ms
SQL-06	SQLIO	Random 8k Reads, 8 threads with 8 qdepth for 120 sec	28964	226	1ms
SQL-06	SQLIO	Sequential 64k Writes, 8 threads with 8 qdepth for 120 sec	7524	470	8ms
SQL-06	SQLIO	Sequential 64k Reads, 8 threads with 8 qdepth for 120 sec	7474	467	8ms

Notice the large improvement in not only throughput but also the reduction in latency. The latency in the first test was due to the saturation of the 1G links.

The optimization is done with the following command from the ESX 5.x console:

for i in `esxcli storage nmp device list | awk '/Nimble iSCSI Disk/{print $7}' | sed -e 's/(//' -e 's/)//'`; do

  esxcli storage nmp psp roundrobin deviceconfig set -d $i -I 0 -t iops;

done

This can also be done with a PowerShell script that is posted here :

Set VMware RoundRobin PSP through PowerCLI

[pdavies34]

Why did you choose "-B 262144" as the bytes size, what other options/results did you test with?  I assume this was the best overall balance between IOPS and Throughput?

[aherbert23]

I did try several other options. Setting it to 256K per path seemed to be the sweet spot.

[mandersen81]

This is great Adam Herbert greatly appreciated.  I have seen this in a couple installs now, and will put this in my toolbox of changes for 1G installs.

[dhamilton113]

really terrific and important post

Adam Herbert Thanks for this.

[#6]

Can this command be run on an environment that already has VMs on the iSCSI LUNs without an interruption of service?

[pdavies34]

Yes, it has an immediate effect and the way the script is written it only affects Nimble Volumes.

It is also relatively simple to modify the script to only affect an individual volume if you so wished.

Phil

[aherbert23]

Yes, it is safe to run. No downtime needed.

[wen35]

have you guys tried iops=0, this essentially ignore the # of IOPS per path before switching, and relies on queue depth.  Essentially poor man's LQD on ESX!  We are trying to do some testing in tech-marketing lab to get some results.

[jwang131]

Yes setting the policy=iops with both iops=0 and bytes=0 may give better performance, where MPIO doesn't need to wait for 256K before switching path.