Expert

## Capacity reservation in vSAN

As I know new capacity feature has been added to vSAN : operation reserve and host rebuild reserve also in vmware vsan design guide mentioned :

Here
is an example of hosts with 2 disk groups per host, consisting of 10, 1.92 TB capacity devices, DD&C activates, where both
Operations Reserve (OR) activated, and Host Rebuild Reserve (HBR) activated.

4node cluster: 10% OR + 25% HBR = Approx 30% total reserved capacity
6 node cluster: 10% OR + 17% HBR = Approx 27% total reserved capacity
8 node cluster: 10% OR + 13% HBR = Approx 23% total reserved capacity
12 node cluster: 10% OR + 8% HBR = Approx 18% total reserved capacity
18 node cluster: 10% OR + 6% HBR = Approx 16% total reserved capacity

I know when we have 5 host in cluster the amount of host rebuild reserve = 20%

1- How calculate the amount of operation reserve ?

2- Now,  I have 5 hosts and each host has 5TB capacity thus my vSAN total capacity is 25TB so if I power on host rebuild reserve

Will my total usable capacity 20TB ?

1 Solution

Accepted Solutions

The total capacity of the vSAN Datastore doesn't change. The available free capacity changes. Just take a look at my screenshots..

Nothing enabled, free 25TB

Operational Reserve enabled, free 22.8 TB

Host rebuild enabled and operational reserve, free 16.2:

13 Replies

Usable will be roughly 20TB indeed, minus some overhead and minus the operational overhead as mentioned (10%).

Expert

Thanks. But I have 5 hosts in my lab and each host has 100GB total capacity so my total vSAN capacity is 500GB . According to follow pic I active both operation reserve and host rebuild reserve

But now according to follow pics it shows my total capacity is :500GB

As I reserve both operation and host rebuild and I have 5 hosts so host reserve rebuild should be decrease 20% from total capacity and also 10% for operation . I think Total usable capacity that should be show = 350GB , although it shows 500GB . Why ?

Also in follow doc mentioned the operation reserve can be set between 0-25% but there is not any threshold to set

https://cormachogan.com/2020/10/29/vsan-capacity-management-in-v7-0u1/

Tags (1)

could be because you have extremely small disks, no idea.

Expert

Thanks. I increased the vSAN capacity disks and now my total capacity is 999GB but another while enable operation reserve and host rebuild my total capacity not change and show 999GB also cannot see ant item to set my operation reserve between 0-25

Tags (1)

I have never seen that anywhere myself either, and I don't think it is possible, probably a mistake by cormac. AFAIK in the current releases it is fixed for the operational reserve.

Here's how I understand this:

• The total capacity will be the same, independent of whether the reserve options are enabled, or disabled.
• If enabled, you may not be able to provision the reserved space for e.g. new VMs, but already powered on VMs can consume that space as their virtual disks' space requirement grows.
• I'm no aware of an advanced setting to manually set the Operations Reserve (although I wouldn't rule out that there may be one), and from what I read, most documents mention it's 10%, but I've also seen (can't find it right now) blogs which mention that the Operations Control is variable, and depends on the number of vSAN objects.

André

Expert

Thanks.

1- Is that your means While my total capacity is 1TB if we enable the operation reserve and host rebuild reserve it will also show 1TB ?

2- While my total capacity is 1TB , after enable operation and host rebuild reserve (5 hosts in cluster) finally I can use 700GB on my vsan datastore (20+10% for operational) ?

3- Is that means when I reach to 700GB then I cannot create or provision new vm on vSAN datastore?

Yes, that's how I understand it, and that's what the description in the screenshot that you've posted earlier mentions.

André

The total capacity of the vSAN Datastore doesn't change. The available free capacity changes. Just take a look at my screenshots..

Nothing enabled, free 25TB

Operational Reserve enabled, free 22.8 TB

Host rebuild enabled and operational reserve, free 16.2:

Expert

According to your pic there is 16TB free space .

1- As I understood when we used 16TB we cannot create any new vms on vsan datastore . Is that correct?

Expert

Hi,

let me guess, that's a vSAN Cluster build with nested ESXi.

it might be a good idea to publish the math how such reservation will be calculated.

Currently it's more guessing than knowing whenever someone asks how many storage capacity will be available when feature A, B or C is selected.

Just my 2 cents.

Ralf

Hope this helps a bit.
Greetings from Germany. (CET)
Tags (1)

@baber wrote:

According to your pic there is 16TB free space .

1- As I understood when we used 16TB we cannot create any new vms on vsan datastore . Is that correct?

That is correct, that is what "free space" means.

@kastlr wrote:

Hi,

let me guess, that's a vSAN Cluster build with nested ESXi.

it might be a good idea to publish the math how such reservation will be calculated.

Currently it's more guessing than knowing whenever someone asks how many storage capacity will be available when feature A, B or C is selected.

Just my 2 cents.

Ralf

There's a pretty decent vSAN Sizer available, a Design Guide and countless articles on core.vmware.com which discuss this. I think the problem above is that he is not a native English speaker.

Plus, his environment runs nested, which heavily changes all calculations due to sizing restrictions.