As promised more content is finally available! Unfortunately I cannot share any screenshots from this actual issue I worked on – you will have to take my word for it.
At my new job I was tasked with solving some issues that had been observed on a remote site on two Cisco C220 M4’s with local disks. The hosts run a set of redundant services but nothing is shared between them except network equipment. The issues were 1) some times around 04.00 AM the software running in the virtual machines would throw an alarm briefly and 2) powering on/off or snapshotting a VM would cause the software to throw alarms as well but on other VMs than the one being powered on/off or snapshotted. The event log on both hosts showed intermittent I/O latency warnings not connected to any of the above issues but nothing alarming.
Now one note though. The software running on these VMs is very latency sensitive so something like snapshots could potentially be a problem in any case but powering on VM A should not affect VM B unless the host is hurting for resources which is not the case.
Before diving in I asked out in the vExpert Slack if anyone had seen issues like this before or had any ideas of what to look for. James Kilby and Bilal Ahmed were quick to throw some ideas on the table. James suggested that vswp file being created on power on might cause the problem and Bilal suggested looking at the network. With those things in mind I started debugging.
First off I had already decided to update the ESXi version to latest 5.5 U3 + patches – it was a little out dated. Also decided to firmware upgrade the servers with the latest 3.0.3c release for Standalone C220 M4s. I had also found the latest supported drivers for the Cisco RAID controller (lsi-mr3) and the Intel NIC (igb) to rule out any compatibility issues. Also it was my hope that an update would remove some of the I/O latency warnings.
Now before upgrading anything I tested to see if the problem was still there. It was. Powering on a test VM with no OS and just 1 core and 4 GB RAM – instant alarms kicking off in the application. However powering off did not cause any noticeable problems. I proceeded to firmware update as my hope was this would solve the issues. Firmware upgrading on a remote site through a small connection is painful! It took a while. But once the first host was updated I proceeded to test if the issues were still there. There were. Damnit. Time to dig deeper. I tried out James’ idea of vswp being the problem and setting a 100% reservation seemed to solve the problem. However this was not a viable solution as this would only solve the problem if the powering on VM has a reservation. If anyone powered on a VM without it, it would still affect all other VMs, regardless of reservation on those.
I booted up our favorite debugging tool ESXTOP and vent into HBA mode and set delay down to 2 seconds. I then observed the Cisco RAID controllers behavior during power on operations and that freaked me out. It would happily do anything between 100 and 1000 IOPS at 5 to 150 ms while not powering on. The latency would spike high but nothing I was that scared of on a small set of local 10k disks. However when powering on a VM without reservation the HBA would stop doing any operations for upwards of 4 refreshes in ESXTOP (at 2 second intervals!). All indicators showing 0. No IO was passed so no latency was observed. This scared me a bit. Latest firmware and supported drivers. Damn. We weren’t seeing the same issue on another site with Dell servers but they also had SSDs instead of 10k disks. Was this the 10k disks not performing enough?
We had a short talk internally about what to do. My boss suggested that we looked at this Cisco bug:
This bug references an issue for some C220 M4’s that were installed with a specific version of the 5.5 ESXi Cisco Custom ISO. It was not the ISO these hosts were installed with but the solution was to use a different driver than the one ESXi default selects for the Cisco RAID controller HBA – the lsi-mr3 driver. Instead it instructs to make sure that the megaraid-sas driver is installed and to remove the lsi-mr3 and lsi-msgpt3 drivers and reboot which will make the megaraid-sas the active driver. We decided to try this. Downloaded the latest support megaraid-sas driver for the server and remove the lsi-mr3 and lsimsgpt3 drivers. Reboot and wait.
After getting online again with one host we tested. Powered on the test VM – and nothing. No alarms. What a difference. Tried it again looking at ESXTOP. No drops in IO. It was now doing 8.000 IOPS @ 15ms no problem. Major difference. Mass powering on VMs 15 at a time had taken minutes before the actual power on task was done and the machines would start booting. It took seconds now.
So what is the moral here? Apparently it could benefit you as a Cisco UCS server user to use the megaraid-sas driver instead of the lsi-mr3 driver. Both are included on the Cisco Custom ISO but it defaults to the lsi-mr3 so you actively have to do something to change that.