vsan expansion calculator?

[ludz1]

Anyone aware of a vsan capacity calculator that if I wanted to add another single disk to each disk group that it would give me an idea of the extra capacity I would gain? thanks

[GreatWhiteTec]

Hi ludz1,

For sizing the main variables that come into play are slack space (25% - 30%), checksums (~5%), and File System overhead (~1% of disk capacity). The one variable you will see right away as soon as you add the drives is the FS overhead (~1%).

You should consider swap space during sizing. I know you are just thinking about adding drives, but wanted to throw this in. Prior to vSAN 6.7, all swap objects were thick provisioned. You can turn that off via CLI, PowerCLI, UI, Host Profiles, etc. I have a blog somewhere about this if you are interested. Starting in 6.7, this is now thin provisioned just like the rest of the objects in vSAN.

[TheBobkin]

Hello ludz1​,

Not to be cheeky but the size capacity added to vsanDatastore would be (number of disks x size of disks) - 1-2% for vSAN FS overhead:

https://docs.vmware.com/en/VMware-vSphere/6.7/com.vmware.vsphere.virtualsan.doc/GUID-581D2D5C-A88F-4...

Do note that Vendors commonly sell for instance "2TB" devices that  have ~5% less usable (e.g. ~1.9TB here) so take this into account.

Other features such as Deduplication & Compression will have a variable overhead depending on a number of factors.

Do of course allow adequate slack space (~25%) for a) gradual growth when using Thin Objects, b) resiliency (if a 4-node or higher cluster) as can repair data after a node/DG/controller loss, c) changing the Storage Policy of Objects and d) temporary extra Objects such as snapshots during backup.

If you mean more "how much USABLE space will I have" then this depends on your Objects being Thick or Thin-provisioned, the FTT and the FTM (e.g FTT=1, RAID1 uses 2x the used space (as the Guest-OS sees it) while FTT=1, RAID5 uses 1.33x space).

Bob

[ludz1]

if you could dig out that article would be interested to have a read. GreatWhiteTec​

[GreatWhiteTec]

Here you go...

https://greatwhitetec.com/2017/03/20/vsan-sparse-swap/

https://greatwhitetec.com/2017/06/08/what-is-vm-overreserved-and-why-is-it-taking-so-much-space/

[ludz1]

Thanks Bob

[ludz1]

So Gents, if I were to give you a scenario, could you give me your results of usable storage?

7 node cluster,

2 disk groups per host

Hybrid disk group:

Each disk group with 800gb cache disk and 3 x 1.2tb capacity disk.

Taking 25% slack, 5% checksum and 1% fs overhead into consideration

FTT=1/RAID1

What would my usable capacity be?

[TheBobkin]

Hello ludz1​,

~17.5TB (USED) at the VM Guest-OS level - do note that you can generally assign a lot more than this with Thin vmdk Objects e.g. a Thin 100GB FTT=1,FTM=RAID1 vmdk with 20GB used on-disk (from the Guest-OS perspective) would consume 40GB on vsanDatastore.

Do of course account for this growing over time as data gets added and consider your strategy for dealing with this (e.g. adding more disks or more nodes).

Bob

[ludz1]

Perfect, great thanks for that TheBobkin​, I had initially not taken the 'doubling' for the mirroring into consideration.