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Exploring the vSphere Flash Read Cache (vFRC) APIs Part 1

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I am sure by now you have all probably heard about some of the new storage features introduced in vSphere 5.5 such as Virtual SAN (VSAN) and vSphere Flash Read Cache. In the coming weeks, I will be working on a series of articles that will be looking at these new features from an Automation perspective and demonstrate how you can automate and manage these configurations using the vSphere APIs. This of course will then allow you to automate using your favorite scripting/programming language such as PowerShell, Perl, Java, .NET, Ruby, etc.

In this first article, I will be exploring the new vSphere Flash Read Cache APIs and I will be primarily focusing on consuming vSphere Flash Read Cache for your virtual machines and not on the actual ESXi host configurations. In a subsequent articles, I will look at the necessary APIs to configure vSphere Flash Read Cache for your ESXi 5.5 host. For those of you who are not familiar with this feature I would recommend you check out Duncan Epping's introduction article to vSphere Flash Read Cache also known as vFRC as well as the new What's News vSphere Flash Read Cache whitepaper by Rawlinson Rivera.

To enable vFRC for a particular virtual machine, you will need to use the virtual machine ReconfigVM_Task() method and specify the particular virtual disk you wish to enable vFRC on. There is new property in the vSphere 5.5 API called vFlashCacheConfigInfo under the VirtualDisk object which you will need to configure.

These are the 5 properties:

  • reservationInMB
  • blockSizeInKB
  • cacheConsistencyType
  • cacheMode
  • vFlashModule

Technically speaking, you only need to specify the reservationInMB property as the rest of the properties have system defaults. However, at a minimum you will probably want to configure reservationInMB and blockSizeInKB where the valid values are 4-1024 and the system default is 4KB.

For cacheConsistencyType, even though the vSphere API mentions both "strong" and "weak" type, only "strong" is supported/configurable and this means that the cache data will be consistent upon a crash. If you try to configure it to "weak", you will get a not supported error.

For cacheMode, even though the vSphere API mentions both "write_thru" and "write_back", only "write_thru" is supported/configurable and this means that when writes are written to the cache, they are then de-staged to the underlying storage sub-system. If you try to configure it to "write_back", you will get a not supported error.

The last property vFlashModulespecifies the specific vFRC module to be used and at this current time, only "vfc" is valid and this is also a system default and does not need to be specified.

To demonstrate these new vFRC VM APIs, I created a sample vSphere SDK for Perl script called vflashVMMgmt.pl which can run against a vCenter Server or a standalone ESXi 5.5 host.


The script supports three operations: query, enable and disable.

To enable vFRC for a particular VM, you will need to use the "enable" operation and specify two required options (--disk and --reservation) and --blocksize is optional with default being 4KB. Here is an example configuring vFRC with 8KB blocksize & 1GB reservation:

./vflashVMMgmt.pl --config .vcenter55-1 --vmname TestVM --disk "Hard disk 1" --operation enable --blocksize 8 --reservation 1024

You can query whether a VM has vFRC enabled by using the "query" operation and specify --disk option for a particular VMDK. Here is an example:

./vflashVMMgmt.pl --config .vcenter55-1 --vmname TestVM --disk "Hard disk 1" --operation query

To disable vFRC for a particular VM, you can use the "disable" operation which disables vFRC by setting the reservationInMB property to 0. Here is an example:

./vflashVMMgmt.pl --config .vcenter55-1 --vmname TestVM --disk "Hard disk 1" --operation disable

Hopefully this has given you a good overview of the new vSphere Flash Read Cache APIs from a virtual machine perspective and the necessary information to enable or disable this feature. In the next part of the series, I will take a look at the new vSphere APIs that are required to setup and configure vFRC for your ESXi host, so stay tuned!

How to get rid of "Quicks stats on hostX is not up-to-date" message

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After upgrading my Apple Mac Mini to ESXi 5.5 I noticed the following warning message being displayed after joining my ESXi host to my vCenter Server:

"Quick stats on mini is not up-to-date" 

I have seen this warning message in the past and it usually goes away within a few minutes of connecting to a vCenter Server. However, this time it did not go away and as you can see from the screenshot, I have had my ESXi host up for 4 days now and the message is still there.

UPDATE (9/30) - This looks to be a known issue in vSphere 5.5 and there is a permanent fix which has been documented in the following VMware KB 2061008. Restarting the management service will not prevent the warning message from coming again, please refer to the KB for the solution.

I do not know about you, but I like to have a clean environment and I get annoyed when I see warning/error messages in the UI. From what I can tell, vCenter Server was able to collect the "quick stats" from the ESXi host but perhaps there was a communication problem at some point or just a glitch?

In any case, is is pretty easy to fix the problem, you just need to restart the management service on the ESXi host and this will force a refresh of the stats. You have three ways of doing this:

  1. Restart management service using DCUI connecting to your ESXi console
  2. Restart management service using DCUI via command-line
  3. Restart management service using command-line script

Option 1 is pretty straight forward and both Option 2 and 3 can be performed on the command-line via an SSH session to your ESXi host if you have enabled SSH.

Option 2 - To launch the DCUI, just type "dcui" on the command-line and you will be able to interact with the DCUI as you normally would from the console and restart the management service:

Option 3 - To restart just using the command-line, you can run the following command to restrat the management service:

/sbin/services.sh restart

Quick Tip: New Hyper-V guestOS identifier in vSphere 5.5

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For those of you who are so inclined to run Hyper-V as a nested VM on ESXi 5.5 (not sure why anyone would want to do such a thing), you should be aware that there is a new guestOS identifier that you can configure your VM to for the most optimal configuration. The main reasons you would want to use this configuration is that by default Windows Enlightenment is enabled and this will prevent Hyper-V from running as it will detect it is running inside of a VM. This configuration will disable Windows Enlightenment to allow you to run Hyper-V.

I noticed a new guestOS identifier called "windowsHyperVGuest" while browsing through the vSphere 5.5 API Reference guide, but when I checked the vSphere Web Client, I did not see this guestOS type as an available option. Perhaps this was not a supported guestOS, after all Nested Virtualization is not officially supported by VMware. In any case, you can still configure your VM by leveraging the vSphere API.

Here is a quick vSphere SDK for Perl script called changeGuestOSID.pl which allows you to reconfigure a VM with a valid guestOS identifier from the vSphere API Reference guide. You can of course easily do this using PowerCLI as well as any other language that can speak to the vSphere API.

Once updated, you should now see it reflected in both the vSphere Web/C# Client:

Note: I did not do extensive testing other than basic installation of latest Hyper-V Server and I do not believe you need any additional settings. If you wish to run nested 64-bit VMs, then you will need to enable VHV.