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Important! Default HBA Device Queue Depth Changes Between vSphere Versions

Michael Webster — Wed, 24 Apr 2013 23:50:38 +0000

A little while ago I wrote an article titled 5 Tips to Prevent 80% of Virtualization Problems. This article was all about storage and how to configure your storage and the dangers to watch out for. This is because problems in virtualized environments are predominantly caused by or related to storage in one way or another. In that article I explained the impact of queue depths on performance and also some of the dangers of making the HBA device queue depths too high. What I didn’t know at the time I wrote the previous article was that the default queue depth for QLogic HBA’s was changed between vSphere 4.1 and 5.x. This article will being you up to date on the changes and the impacts of the change in default values between vSphere 4.x and 5.x.

The HBA device queue depths are important as I outlined in 5 Tips to Prevent 80% of Virtualization Problems because it has a big impact on the number of parallel IO’s that a VM can issue and that can be serviced. It also has an impact on the number of LUNs that you need to support to achieve the same performance. If your queue depth is too low you can have very high latency when your VM’s are trying to issue a high number of parallel IO’s as they’ll all queue up inside the hypervisor. If your HBA device queue depth is too large you could have lots of IO’s either queueing up in the HBA itself, or you could overload your storage array. So you need to strike the right balance.

This article is a follow up to Cormac Hogans article titled Heads Up! Device Queue Depth on QLogic HBA’s that was published in response to some queries VMware had received from one of their Technical Account Managers. I would recommend you read Cormac’s article for the reasons why the change to the defaults was made, as I won’t cover that here.

The following are the default HBA device queue depths when using QLogic HBA’s for FibreChannel or FCoE SAN connectivity:

ESXi 4.1 U2 – 32
ESXi 5.0 GA – 64
ESXi 5.0 U1 – 64
ESXi 5.1 GA – 64

Note: This change does not effect Emulex HBA’s, only QLogic.

The VMware KB 1267 – Changing the queue depth for QLogic and Emulex HBAs, which documents the process for changing device queue depths for QLogic and Emulex HBA’s has been updated to include the default queue depths for the adapters per vSphere version.

So is this change to the default really significant? I think it’s significant in that it wasn’t documented anywhere and in fact the QLogic HBA documentation still lists the default as 32. It’s also significant due to the impact of an overload condition can be quite a dramatic negative storage performance hit, which could take a while to troubleshoot. But for a very long time it had been a common best practice for VMware to recommend changing the HBA device queue depth on QLogic HBA’s to 64 from the default of 32. In most cases this had a positive impact on performance with reduced IO latencies. If you are using Storage I/O Control it will dynamically adjust queue slots between different VM’s on a shared datastore and you don’t need to worry about the device queue depths.

Storage I/O Control takes away the worry and will adjust performance to ensure the latency thresholds are met (by default 30ms). If you have vSphere Enterprise Plus (4.1 and above) and you have Multiple VM’s per Datastore you should be making use of Storage I/O Control. The device queue depth is used when there is only one VM per datastore and Disk.SchedNumReqOutstanding is used when there are multiple VM’s per datastore, in which case the per device queue depth is ignored. As Paudie O’Riordan, one of VMware’s Senior Staff Technical Support Engineers says “let the computer (SIOC) make the decision, not the finger and the wind”.

However there are a few cases where the queue depth of 64 had a detrimental impact and that was largely when non-virtual systems were sharing the same storage array as the vSphere hosts. In this case the vSphere hosts got a far larger proportion of the array’s IO resources and this could impact the performance of the non-virtual systems. I would recommend that where possible you don’t share storage arrays between your virtual and non-virtual environments, which would avoid these types of impacts. In cases where that is not possible you will need to carefully consider the quality of service and storage IO isolation requirements and impacts that high performance vSphere hosts could have on the overall storage array.

The Queue Depth for all devices on the QLogic HBA is a total of 4096. So if you have a per device (per LUN) queue depth of 32 you can support 128 LUN’s at full queue depth, without queueing in the HBA. If you increase the queue depth to 64 (as is the new default in 5.x) then you can support only 64 LUN’s at full queue depth. You can still have more LUN’s configured based on the assumption that not all LUNs will be using all the queue depth all at the same time, so you can effectively overcommit queues in essence. But it would pay to consider the impact of a large queue depth if all VM’s do start issuing IO’s. As Cormac says in his article “If you hit the adapter queue limit, then you won’t be able to reach the device queue depth, and may possibly have I/Os retried due to queue full conditions.”

Now this is only for the HBA queue depths. What about the target ports or storage processor ports on the array? A lot of array storage processor ports will have a queue depth of 2048. You should check with your storage vendor what the Target Port Queue Depth is for your array, if any. As you can see if the HBA is only configured to issue IO’s to one target port a single HBA could easily overwhelm the storage processor and this could cause a QFULL. Fortunately your design should have LUN’s configured across multiple target storage processor ports and multiple storage processors to reduce the risk of overloading. So what happens in a QFULL scenario? Well you can read the QLogic Document titled Execution Trottle and Queue Depth with VMware and QLogic HBA’s. In essence the vSphere Host will set the queue depth to the minimum, which is 1. You can just imagine what this would do to your performance.

Final Word

I recommend you read Cormac’s article Heads Up! Device Queue Depth on QLogic HBA’s and read the QLogic document Execution Trottle and Queue Depth with VMware and QLogic HBA’s. Overall the change in default queue depth for QLogic HBA’s should be positive for performance in most environments. In some environments however you may need to adjust the settings to reduce the risks that I have outlined here. It’s far better to be armed with this knowledge than suddenly have your storage performance fall off a cliff and not know what might have caused it. If you have vSphere Enterprise Plus (4.1 or above) and you have Multiple VM’s per Datastore you should be making use of Storage I/O Control.

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This post first appeared on the Long White Virtual Clouds blog at longwhiteclouds.com, by Michael Webster +. Copyright © 2013 – IT Solutions 2000 Ltd and Michael Webster +. All rights reserved. Not to be reproduced for commercial purposes without written permission.

Latest ESXi 5.0 Patch Improves VMFS Heap Size Limits

Michael Webster — Thu, 04 Apr 2013 07:25:49 +0000

This is a brief article to let you all know that VMware has greatly improved the default and maximum limit of open VMDK’s per host in the latest patches to ESXi 5.0. Once you apply patch ESXi500-201303401-BG the default and maximum amount of open VMDK storage per host will be 60TB on VMFS5, up from 25TB previously. The VMFS Heap Size is increased from a default of 128MB to a default of 640MB.

As some background I originally wrote about this problem in my article The Case for Larger Than 2TB Virtual Disks and The Gotcha with VMFS and Marcel van den Berg has recently followed up with an article titled A Small Adjustment and a New VMware Fix will Prevent Heaps of Issues on vSphere VMFS Heap. Once the latest patch is applied customers can safely run up to 60TB of VMDK’s per host. This will allow for a lot larger VM’s in terms of storage footprint to be run on each vSphere host. Note this patch is only for ESXi 5.0, ESXi 5.1 Update 1 contains the same change. The release notes for the patch also highlight another problem that was fixed which impacted VM’s with 18 or more VMDK’s which were each configured with more than 256GB per disk (also addressed in 5.1 Update 1). Note that the original KB article with regard to the VMFS heap size KB 1004424 – An ESX/ESXi host reports VMFS heap warnings when hosting virtual machines that collectively use 4 TB or 20 TB of virtual disk storage has been updated with the new limits.

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HP Advisory – vSphere Hosts Disconnecting from Network

Michael Webster — Wed, 13 Mar 2013 21:12:06 +0000

I have had a number of customers who are running HP server hardware report that their hosts are constantly getting disconnected from the network, including their management NIC’s (sometimes causing isolation events), and also they sometimes are getting Purple Screens of Death (PSOD). As you can probably guess this is causing them some major pain. HP has issued an advisory regarding these problems that you need to review if you have any of the affected NIC’s – NC522SFP, NC523SFP, NC375T, NC375i, NC522m, CN1000Q.

I have previously written about the problems I experienced with a few customers running the NC522 and NC523 NIC’s in my article HP Critical Advisory – NC522 and NC523 10Gb/s Server Adapters. The customer that I was working with when I came across this problem originally (well before the advisory went out) had a particularly serious problem as the NIC’s were also used for storage access and management. This lead partially to me writing When Management NIC’s Go Down. Fortunately for my customer they now have a stable environment, but they went through dozens of firmware and driver updates, and eventually had to get the cards replaced.

Now there is a new advisory as of December 2012 regarding a broader set of NIC’s and systems that are having some serious problems and causing VMware vSphere hosts to become disconnected from the network and causing PSOD’s. You can find the HP Advisory Here – HP ProLiant and HP StorageWorks Systems: HP NC375i, NC375T, NC522m, NC522SFP, NC523SFP, CN1000Q Network Adapters – FIRMWARE UPGRADE REQUIRED to Avoid the Loss and Automatic Recovery of Ethernet Connectivity or Adapter Unresponsiveness. The title of the advisory really says it all. VMware has issued KB 2012455 regarding this problem. Note that this is not a VMware issue, it’s a hardware issue, and you should upgrade to the firmware / driver combination that resolves the problem as soon as possible.

I hope that once you upgrade the firmware / drivers your environment will become stable as you would normally expect. When working with HP on these types of issues I have found them to be generally responsive when you get to the right people. I would encourage you to work with your account manager and the HP technical support teams to get these issues resolved. If the problems persist after upgrading the firmware as advised then I would strongly recommend you consider replacing the NIC’s with an alternative model after discussions with HP.

Final Word

NIC disconnections and PSOD’s of this type should be extremely rare in the overall scheme of things. I have not come across many of these types of situations in the 10 years I’ve been working with VMware solutions. But when you come across these types of problems they need to be resolved as soon as possible. The best way to approach it is to log support requests with both VMware and your hardware vendors. Hopefully you strike these types of hardware problems during QA testing before your infrastructure goes live into production, but that is not always the case. If you don’t have a QA process for your hardware that includes burn in then I would recommend you consider it.

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