I get asked this question a lot so in the spirit of this blog it was about time to write it up.

The only correct answer is “it depends”. Typically, the RAW to usable ratio is 2:1 (i.e. 50%). By default, 1TB RAW capacity equates to approximately 500GB usable capacity. Read on for more details.

In VSAN there are two choices that impact RAW to usable capacity. One is the protection level and the other is the Object Space Reservation (%). Lets start with protection.

Virtual SAN (VSAN) does not use hardware RAID (Disclaimer at the end). Thus, it does not suffer the capacity, performance, or management overhead penalty of hardware RAID. The raw capacity of the local disks on a host are presented to the ESXi hypervisor and when VSAN is enabled in the cluster the local disks are put into a shared pool that is presented to the cluster as a VSAN Datastore. To protect VM’s, VSAN implements software distributed RAID leveraging the disks in the VSAN Datastore. This is defined by setting policy. You can have different protection levels for different policies (Gold, Silver, Bronze) all satisfied by the same VSAN Datastore.

The VSAN protection policy setting is “Number of Failures to Tolerate (#FTT) and can be set to 0, 1, 2, 3. The default is #FTT=1 which means using distributed software RAID there will be 2 (#FTT+1) copies of the data on two different hosts in the cluster. So if the VM is 100GB then it takes 200GB of VSAN capacity to satisfy the protection. This would be analogous to RAID 1 on a storage array. But rather than writing to a disk then to another disk in the same host we write to another disk on another host in the cluster. With #FTT=1, VSAN can tolerate a single SSD failure, a single HDD failure, or a single host failure and maintain access to data. Valid settings for #FTT are 0, 1, 2, 3. If set to 3 then there will be 4 copies of the VM data thus RAW to usable would be 25%. In addition, there is a small formatting overhead (couple of MB) on each disk but is negligible in the grand scheme of things.

#FTT # Copies
RAW-to-usable Capacity %
0 1 100%
1 2 50%
2 3 33%
3 4 25%

Perhaps you create the following policies with the specified #FTT:

  • Bronze with #FTT=0 (thus no failure protection)
  • Silver policy with #FTT=1 (default software RAID 1 protection)
  • Gold policy with #FTT=2 (able to maintain availability in the event of a double disk drive failure, double SSD failure, or double host failure)
  • Platinum policy with #FTT=3 (4 copies of the data).

Your RAW to useable capacity will depend on how many VM’s you place in the different policies and how much capacity each VM is allocated and consumes. Which brings us to the Object Space Reservation (%) discussion.

In VSAN, different policy can have different Object Space Reservation (%) (Full Provisioned percentages) associated with them. By default, all VM’s are thin provisioned thus 0% reservation. You can choose to fully provision any % up to 100%. If you create a VM that is put into a policy with Object Space Reservation equal to 50% and give it 500GB then initially it will consume 250GB out of the VSAN Datastore. If you leave the default of 0% reservation then it will not consume any capacity out of the VSAN Datastore but as data is written it will consume capacity per the protection level policy defined and described above.

That ended up being a longer write up than I anticipated but as you can see, it truly does depend. I suggest sticking to the rule of thumb of 50% RAW to usable. But if you are looking for exact RAW to usable capacity calculations you can refer to the VMware Virtual SAN Design and Sizing Guide found here. https://blogs.vmware.com/vsphere/2014/03/vmware-virtual-san-design-sizing-guide.html
Also, you can check out Duncan Epping’s Virtual SAN Datastore Calculator: http://vmwa.re/vsancalc

Disclaimer at the end: ESXi hosts require IO Controllers to present local disk for use in VSAN. The compatible controllers are found on the VSAN HCL here: http://www.vmware.com/resources/compatibility/search.php?deviceCategory=vsan

These controllers work in one of two modes; passthrough or RAID 0. In passthrough mode the RAW disks are presented to the ESXi hypervisor. In RAID 0 mode each disk needs to be placed in its own RAID 0 disk group and made available as local disks to the hypervisor. The exact RAID 0 configuration steps are dependent on the server and IO Controller vendor. Once each disk is placed in their own RAID 0 disk group you will then need to login via SSH to each of your ESXi hosts and run commands to ensure that the HDD’s are seen as “local” disks by Virtual SAN and that the SSD’s are seen as “local” and “SSD”.

I hope this is helpful. Of course questions and feedback is welcome.