You can configure a Flash Read Cache for your virtual machine. When you enable Flash Read Cache, you can specify the block size and cache size reservation.

  • Block size is the minimum number of contiguous bytes that can be stored in the cache. This block size can be larger than the nominal disk block size of 512 bytes, between 4 KB and 1024 KB. If a guest operating system writes a single 512-byte disk block, the surrounding cache block size bytes are cached. Do not confuse the cache block size with the disk block size.
  • Reservation is a reservation size for cache blocks. There is a minimum number of 256 cache blocks. If the cache block size is 1 MB, then the minimum cache size is 256 MB. If the cache block size is 4 K, then the minimum cache size is 1 MB.

For more information about sizing guidelines, search for the Performance of vSphere Flash Read Cache in VMware vSphere white paper on the VMware website.


Set up a virtual flash resource on your client computer.


  1. Right-click a virtual machine in the inventory and select Edit Settings.
  2. On the Virtual Hardware tab, expand Hard disk to view the disk menu items.
  3. Enter a value in the Virtual Flash Read Cache text box.
  4. (Optional) Select a block size and a cache size reservation.
    Client Steps
    vSphere Client You cannot specify block size and cache size reservation in the vSphere Client.
    vSphere Web Client
    1. Next to the Virtual flash read cache text box, click Advanced.

      The Virtual Flash Read Cache Settings dialog box opens.

    2. Click Enable virtual Flash Read Cache.
    3. In the Reservation text box, enter the cache size reservation, and select the units from the drop-down menu.
    4. From the Block Size drop-down menu, select the block size.
    5. Click OK.
  5. Click OK.