Virtual Flash feature in vSphere 5.5
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Virtual Flash feature in vSphere 5.5


Article ID: 308470


Updated On:


VMware vCenter Server VMware vSphere ESXi


This article provides an overview of Virtual Flash and its benefit in the vSphere environment.
Virtual Flash Read Cache allows you to locally cache virtual machine read I/O on an ESXi host and even migrate that virtual machines cache to another Virtual Flash enabled ESXi host.


VMware vSphere ESXi 5.5
VMware vCenter Server 5.5.x


Virtual Flash allows you to accelerate virtual machine performance through the use of local SSD disks, which serve flash memory cache to chosen virtual machines running on the ESXi host. Virtual Flash Read Cache can supply low latency for extreme latency sensitive applications, thus enabling the virtualization of some computer systems/applications previously considered too I/O intensive and, therefore, impossible/implausible.

Note: To use vSphere Flash Read Cache, you need Enterprise Plus Licensing.

Flash Memory

Virtual Flash caching is achieved using flash memory in the form of solid state disks (SSDs).

Flash memory
  • Flash Memory is non-volatile electronic storage.
  • It can be electrically erased.
  • There are 2 main types of flash memory: NAND and NOR, with NAND being the most commonly used type.
SSD disks
  • A storage device that stores data on solid-state flash memory.
  • It uses an array of semiconductors, rather than a magnetic or optical media solution for data storage.
  • Common interfaces for SSD disks are SATA, SAS, and PCI-E.
  • Advantages of SSD disks include reduced power usage, faster data access, and higher reliability.

Virtual Flash Cache/Device

There are two main components used to achieve Virtual Flash read caching. Common reads are stored in the Virtual Flash cache and the Virtual Flash device keeps track of the parts of the VMDK disk that are are cached. Together, they can dynamically configure and supply caching of data stored on a chosen virtual machine disk (VMDK).
Virtual Flash Cache
  • Created in the Virtual Flash Filesytems (VFFS).
  • A virtual disk flat file used to store cached data.
  • The size of the Virtual Flash Cache is equal to the amount of configured Virtual Flash Read Cache at the VMDK level.
Virtual Flash Device
  • Created in the Filesystem Device Switch (FDS). This FDS is a vmkernel module which sits between the virtual machine and the storage, reports if the requested I/O is cached or not and provides the optimal path to the data.
  • A node object used to reference the target VMDK.
  • The size of the Virtual Flash Device is equal to that of the VMDK disk, but no actual storage space is consumed as it is just a node.

Benefits of using Virtual Flash

  • Virtual machines configured with Virtual Flash Read Cache provide improved storage performance:

    • Increased read speeds for certain workloads.
    • Increased storage write speeds due to a reduction in common storage read operations.
    • Reduced storage I/O contention and less traffic on your storage network due to local caching of busy virtual machines.

  • Virtual Flash Read Cache is a completely transparent to the virtual machine. No special configuration or agents are required for the virtual machine.
  • Virtual Flash Read Cache can be assigned to a virtual machine or virtual disk (or disks) as required.
  • Virtual Flash can be used as Host Swap Cache, boosting performance of the virtual machines running on that host. For more information, see Configuring Host Swap Cache with Virtual Flash Read Cache resources (2059285).

Using Virtual Flash Read Cache

To start using Virtual Flash Read Cache:
  1. Ensure your environment is fully compatible with vSphere 5.5. You need a certified ESXi 5.5 host, vCenter Server 5.5, and a local SSD disk. To see if your hardware is compatible, see the VMware Hardware Compatibility Guide.
  2. Set up a Virtual Flash resource. For more information, see Setting up a Virtual Flash resource in vSphere 5.5 (2051647).
  3. Allocate Virtual Flash Read Cache to a virtual machine disk. For more information, see Allocating Virtual Flash cache to a virtual machine (2051572).

    You can now leverage the advantages of virtual machine flash read caching. The virtual machine is completely unaware of the change, the only difference should be drastically reduced read times for certain workloads.

Note: Consider further configuration to retain resource allocation if you intend to use features such as vMotion, HA, or DRS on your Virtual Flash Read Cache enabled virtual machines.

Additional Information

For more information, see:

Allocating vSphere Flash Read Cache to a virtual machine
Migrating virtual machines with vSphere Flash Read Cache
Setting up a Virtual Flash resource in VMware vSphere 5.5
Virtual Flash File System in ESXi 5.5
Virtual Flash volume limits in ESXi 5.5
Configuring Host Swap Cache with vSphere Flash Read Cache resources
vSphere 5.5 中的虚拟闪存功能
vSphere 5.5 の仮想フラッシュ機能