VFRC sizing considerations SSD quandry

Sure you know about it.. I am sure you have read about it.. but you even know how to enable it.
But are you sizing it correctly??
Recently a customer I was working with wanted to boot their ESX blades from RAID1 SSD drives!
Wow cool, but not cool. Let me explain…

ESXi is tiny relative to any other OS. Since this hypervisor is so small and loads entirely into memory when it boots, I explained that using the SSD for other options would be more beneficial. Using the SSD for boot would not really be taking advantage of what SSD can do for your Virtualized environment.

I didn’t go as far as to suggest using a stateless auto deploy boot system.. but something a bit easier to manage. Of course they could have used boot from SAN, but based on the reliability of their environment they wanted something a bit more independent.. flash devices fit their use case perfectly.

Now about those SSD drives.. what to do.. what to do..?

This particular customer has FC backed storage and blades so a hyper-converged solution like VSAN wasn’t a good fit. Yes 10G was available but the question was how to maximize the use of the SSD. Disclaimer I didn’t design the components of the compute nodes and local storage, but I wanted to make the most of it..
Queue in…. Enterprise Plus licensing and its additional licensed features.

“vSphere Flash Read Cache”

Some quick bullet items to summarize.

  • Hypervisor-based software defined flash storage tier solution
    • Cache is a high-speed memory that can be either a reserved section of main memory or a storage device
    • Supports Write-Through Cache Mode
    • Improve VMs performance by leveraging local flash devices
    • Ability to virtualize business critical applications
  • Aggregates local flash devices to provide a clustered flash resource for VM and vSphere host consumption
  • Leverages local flash devices as cache
  • Integrated with vCenter, HA, DRS and vMotion
    • VFFS Volume is created
    • Fine-grained control via per VMDK caching
    • Block Size configuration capabilities Min 4KB – Max 1024KB
    • Virtual Flash Host Swap Cache
      • Provides the ability to use virtual flash resource for memory swapping
      • Legacy support for Swap-to-SSD
    • Scale-out Storage Capability: 32 Nodes
    • Requirements
      • vSphere Server 5.5
      • vSphere Web Client
      • VM Hardware version 10
      • License – vSphere Enterprise Plus

In a nutshell 20% of VMDK size is used to determine size of VFRC

Interesting white paper analysis of VFRC
http://www.hds.com/assets/pdf/benefits-of-server-side-flash-with-vmware-vsphere-flash-read-cache-on-hitachi-compute-blade-500.pdf

There are many articles of how to use, how to implement but few articles of benchmarks

http://cormachogan.com/2014/02/14/a-closer-look-at-vsphere-flash-read-cache-vfrc/

http://www.yellow-bricks.com/2013/08/26/introduction-to-vsphere-flash-read-cache-aka-vflash/

http://www.vmware.com/files/pdf/techpaper/vfrc-perf-vsphere55.pdf

http://www.vmware.com/files/pdf/techpaper/VMware-vSphere-Flash-Read-Cache-FAQ.pdf

Advanced professional services would go beyond recommending use of VFRC but actually creating a test bed for the customer to determine the best size of and which block size to use.
default block size is 8kb

Assessment tool would be
vscsiStats
http://nickmarshall.com.au/blog/2013/10/31/rightsizing-vflash-read-cache-vfrc

Notes to share..

-vp

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