Yellow Bricks

by Duncan Epping

6.5

Which disk controller to use for vSAN

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I have many customers going through the plan and design phase for implementing a vSAN based infrastructure. Many of them have conversations with OEMs and this typically results in a set of recommendations in terms of which hardware to purchase. One thing that seems to be a recurring theme is the question which disk controller a customer should buy. The typical recommendation seems to be the most beefy disk controller on the list. I wrote about this a while ago as well, and want to re-emphasize my thinking. Before I do, I understand why these recommendations are being made. Traditionally with local storage devices selecting the high-end disk controller made sense. It provided a lot of options you needed to have a decent performance and also availability of your data. With vSAN however this is not needed, this is all provided by our software layer.

When it comes to disk controllers my recommendation is simple: go for the simplest device on the list that has a good queue depth. Just to give an example, the Dell H730 disk controller is often recommended, but if you look at the vSAN Compatibility Guide then you will also see the HBA330. The big difference between these two is the RAID functionality offered on the H730 and the cache on the controller. Again, this functionality is not needed for vSAN, by going for the HBA330 you will save money. (For HP I would recommend the H240 disk controller.)

Having said that, I would at the same time recommend customers to consider NVMe for the caching tier instead of SAS or SATA connected flash. Why, well for the caching layer it makes sense to avoid the disk controller. Place the flash as close to the CPU as you can get for low latency high throughput. In other words, invest the money you are saving on the more expensive disk controller in NVMe connected flash for the caching layer.

Watch VMworld session “Top 10 Things to Know about vSAN #STO1264BE” for free …

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I didn’t know, but apparently our session was a featured session at VMworld Europe. For those interested in our VMworld: Top 10 vSAN session there’s a “mini player” below of our European VMworld session. I somehow can’t get it in a proper size but still wanted to share it. Easier probably for you is to go to this link and simply watch the video there as it is displayed in a proper way. Awesome to see our session up there, and congrats Cormac on having 3 sessions in the top list!

List all “thick” swap files on vSAN

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As some may know, on vSAN by default the swap file is a fully reserved file. This means that if you have a VM with 8GB of memory, vSAN will reserve 16GB capacity in total for it. 16GB? Yes, 16GB as the FTT=1 policy is also applied to it. In vSAN 6.2 we introduced the ability to have swap files created “thin” or “unreserved” I should probably say. You can simply do these by setting an advanced setting on each host in your cluster. (SwapThickProvisionDisabled) Now when you have set this and power-off/power-on your VMs the swap file is recreated and the swap file will be thin. Jase McCarty wrote a script that will set the setting for you in each host of your cluster, but the problem of course is how do you know which VM has the “new unreserved” swap file and which VM still has the fully reserved swap file. This is what a customer asked me last week.

I was sitting next to William at a session and I asked him this question. William went at it and knocked out a Python script which lists all VMs in a cluster which have a fully reserved swap file. Very useful for those who are moving to “unreserved / sparse” swap. This way you can figure out which VMs still need a reboot and reclaim that (unused) disk capacity.

Note, the “sparse” / “unreserved” swap files are only intended for environments which do not overcommit on memory. If you do overcommit on memory please ensure you have disk capacity available, as you will need the disk capacity as soon as the hypervisor wants to place memory pages in the swap file. If there’s no disk capacity available it will result in the VM failing.

Thanks William for knocking out this script so fast…

Can I front vSAN with a VAIO Caching Solution?

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I had this question a couple of times already, so I figured I would write a quick post. In short: yes you can put a VAIO Filter in front of vSAN. The question really is, which one would you like to use and why?

First of all, the VAIO Filter needs to be certified to be placed in front of vSAN storage. Just like it needs to be certified for VMFS and NFS. You can go to the VAIO HCL and check this yourself:

In this example Infinio supports VVols, vSAN and VMFS. Great! Now the next question is: why? Well that is the bigger question, personally I don’t see a real compelling reason. For traditional storage it makes a lot of sense, as you want to keep IOs local and add cache in a “cheap way” instead of expanding, a potentially close to end of life, storage system. For vSAN that is different. vSAN has a distributed architecture and every node has a flash device that is being used for write caching, and also read caching in a hybrid scenario. If this is a new deployment invest your money in NVMe instead. If you want to repurpose existing hardware and it is not on the vSAN HCL, ask yourself if you should complicate your deployment or not?

I would personally recommend to keep it simple, but than again I can also understand you do not want to let flash resources go to waste if vSAN does not support the devices. If you want to go VAIO, make sure to check the HCL and take the potential risks and operational complexity in to account and weigh that against the cost.