Calculating Usable disk space of vSAN 6.6

In the hybrid vSAN 6.6 of 4 nodes, every node has 800 GB SSDs, and 4 HDDs of 1.8 TB each. To calculate usable disk space, I multiply 4 with 1.8 to get 7.2 TB for each node and then multiply that by 4 to get 28.8 raw disk space for entire vSAN. Out of this, 10% will be lost in overhead, giving me available disk space of about  25.9 TB. To have FTT=1, I need to take out 25% space which will be lost if one node goes down, so I am left with about 19.4 TB usable space.

However I am told by a sales person that I will really have only 7.7 TB - due to mirroring and threshold, whatever they may be.

Please help me ascertain usable disk space for this vSAN. Is mirroring essential? Thanks.
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Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
We would recommend you use this resource, and then discuss output with vendor, if your figures agree
AkulshAuthor Commented:

This link is good, but I still want to know how Usable disk space is calculated from raw disk space. Can you offer some guidance? (That link does not show how such calculations are made.) Thanks.
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AkulshAuthor Commented:

Not really. That link is about SAN/NAS, not about vSAN. Also I am familiar with vSAN 6.0 usable space -- have been using it for last 2 years. I wish to know how usable space is calculated for latest vSAN. So the link cannot be from 2015. Thanks none the less.
Here is another link tailored to vSAN 6.6

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Something else to consider, which might be why the sales rep is saying the size
7.7 TB - due to mirroring and threshold..
Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
Do you have you answer, or do you want the exact equation to use ?

and know why ?
AkulshAuthor Commented:

Your MrvSAN link hit the mark. It shows how disk space is calculated in vSAN 6.6.
Before closing this question, I wish someone would tell me how vSAN is attractive with some much waste (65%) of disk space. I myself now want to stay far away from it. Thanks.
Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
Availability and Resilience Clustered SAN

Also Performance
Read the second Article I posted : 

If I am reading this correctly, 65% wasted space would be relative to if the vservers are thin or thick provisioned.

If you have a 100GB thick provisioned server on the san in a MIRROR with no compression then you are going to have to use 100GB if additional storage.

If you have a 100GB thin provisioned box on the san in a MIRROR then it will only be a fraction of that x2

Storage, Speed, redundancy, HA would be the reasons to go forward with this product.

I am actually interested to hear if you agree.
AkulshAuthor Commented:

I can have all that with external SAN also -- and with much less headache.

(For last 4 months, we have been constantly struggling with vReplicaton issues where replicated data grows to 100X or more due to malfunctioning storage policy of vSAN 6.0. Two teams of VMware Support engineers have not come up with any answer/fix in more than 2 months. Cases are still open.)
Failures To Tolerate=1 (FTT=1) is the same as the configuration of 'RAID1' in that it has two mirrored data components, so technically yes 100GB of disk at the VM-level will use 200GB on vsandatastore.
However this is only going to be the case if all/some VMs are using a proportion of their disk-space or are Thick-provisioned (don't go Thick unless the application/OS benefits from this).
So for example, if you had a Thin-provisioned 100GB vmdk, but the guest OS was only using 50GB, then this will be 50GB+50GB=100GB on vsandatastore.
Thus whether you have Thick or Thin vmdk Objects (VM disks) they will use as much space as they are provided or using (in the case of thin) immediately on creation/migration to vSAN.
Another thing to consider is the extra space that is used by vswp Objects that are the a swap-space the size of the VMs RAM, these can also be configured so that they are Thin-provisioned (they are Thick by default)
Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
vSAN not for you then!

If you already have performance availability and resilience in order if your current SAN fails.
AkulshAuthor Commented:

I had seen that VMware Communities link also. It is "Not answered" (which can be misleading sometimes) with only one response. I don't see how Thin and Thick provisioning has anything to do with calculation of Usable space. Of course, for any given usable space, going with Thin provisioning is much wiser, and is default in most cases --- except when some rare application requires Thick provisioning due to performance needs.

(By the way, VxRail calculations show less than 30% usable disk space. Does anyone like those appliances?)

In a primary location, we have Compellent SAN, and it has "Storage, Speed, redundancy, HA" -- all of them. And rebooting an ESXi server is so much simpler since it is not involved in storage.

Thanks for all your help. I will close this question shortly.
Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
Plenty of VxRail appliances in use!

vSAN not for everyone!
Personally, I prefer Simplivity or Nutanix for Hyper Convergence.
AkulshAuthor Commented:
Thanks you both ITguy565 and Andrew. It was a nice discussion.
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