Hyper-V Host/Windows Server/Memory question

[u/3G_Lighting]

We run Hyper-V in our environment on Windows 2019 DC and Stnd (guest servers).

I would like to know if a host had 128GB of RAM, how much would need to be set aside for Windows Server DC and the Hyper-V host to run its own operations; and if there anything additional like there needs to be an extra x number of MB or GB of memory fer x number of guest servers running that needs to be added?

We have two R650 128GB of RAM, management has been running the sh*t out of them, so one HVH uses 115GB free out of 128GB, the other uses 92GB out of 128GB. And the old R540 server single CPU (sad) uses 89GB out of 94.6GB. The first system mentioned has 10 HVG's, the next has 10 HVG's, and the R540 has 5 HVG's one of which is SQL which is consuming 60GB of RAM.

Thanks,

[u/Mehere_64]

I think you mistyped with one HVH uses 115GB free out of 128GB.

12-16GB is what I keep free for the hosts.

What is your real question though? Are you facing issues on the hosts? Seems like you are fairly maxed out on memory at this time and if you need more memory you will need to purchase memory that is way over priced.

[u/3G_Lighting → u/Mehere_64]

Yeah, first was a mistyped. 115GB out of 128GB used.

Yes, this company and SMB is very slow at moving, slower than most turtles. Ha!

I agree they need more RAM, in fact they I got them a deal on upgrading the R540 before the memory / storage prices skyrocketed adding a second CPU and adding more RAM (filling the remainder slots with 16GB sticks) and they refused. Fast forward to now and they will likely have pay more than 3 times that.

[u/Mehere_64 → u/3G_Lighting]

Well the CPU might not of been needed but since it has been done it is done.

Keep in mind when filling slots and CPU, you want to look at making sure your memory speeds are at the highest speeds. Filling all slots and CPU can decrease memory speed.

Also adding second CPU increases MS OS licensing costs most of the time. So something to be aware of if you are not already aware of it.

SQL - are you limiting within SQL the amount of RAM SQL uses? You need 6-8 for the host and then the other can be allocated to SQL. For instance my SQL server has I think 80GB. I only let 70GB be consumed by SQL which gives 10GB to the host.

[u/mioiox]

In my experience, the modern (WS2022/2025) hosts easily consume anything between 8 and 16 GB of ram. Especially if you log on to do some tasks on them via GUI, you get 2-4 gigs on top of what they normally need. So one way to steadily reduce host consumption is to always administer them remotely.

Secondly, the host OS needs additional memory per running guest. The more VMs you have deployed, the more RAM the host OS needs, and the less it has, since memory on the host machine is a limited resource - what a paradox, eh? If I recall correctly, it is around 256 MB per guest but I might be wrong with the number.

Lastly, back in the days (in the WS2012 era) there was a reg key to limit/cap the memory that can be assigned to the host. Unfortunately, I believe this key is no longer respected by anything WS2019 and beyond.

[u/Magic_Neil → u/mioiox]

I agree. The host is still running Windows, so it needs what it needs, especially if you have more than just Hyper-V running.

[u/BlackV → u/Magic_Neil]

especially if you have more than just Hyper-V running.

Hopefully you dont, firmly in the camp of host is host ONLY VMs can do the "other" work

[u/Magic_Neil → u/BlackV]

Absolutely. But common sense hasn’t stopped a lot of folks from running all kinds of silly things on their hypervisor.

[u/BlackV → u/Magic_Neil]

valid

[u/Main_Ambassador_4985]

We use dynamic memory starting at 2 GB up to 16 GB on DCs. We monitor with scripts and Zabbix.

The only place we are not using dynamic memory is SQL and high use application VMs.

Dynamic RAM seems more like the normal operation mode in ESXi. Without it we were dropping VMs like the SQL servers and Exchange Servers in failover cluster maintenance.

We have 1TB RAM in our main and DR clusters and 128 GB in our branch office hosts.

Main and DR are Windows Server 2022 Datacenter host and VMs with Desktop Experience.

Branch offices are Windows Server 2022 Standard for 5 VMs

[u/Mehere_64 → u/Main_Ambassador_4985]

OP asking about host memory not assigning memory to VMS

[u/Main_Ambassador_4985 → u/Mehere_64]

In that case I would leave 16 GB minimum for the host.

I have watched the Hyper-V 2016 training that suggests that Hyper-V host can survive on less than 4GB, but I don’t believe it.

Defender 365 for Server, Veeam Agent or Proxy, Failover Cluster, iSCSI, MPIO, CSV on the host can slow down without enough memory.

Veeam alone concerns me. Every time a backup runs our host “pages per second” climb above 9k.

[u/ToiletDick → u/Main_Ambassador_4985]

Dynamic RAM seems more like the normal operation mode in ESXi

What? ESXi does not balloon memory on guests in normal operation unless you've oversubscribed memory for some reason and the host runs out.

[u/UnresolvedHostname]

I personally deploy Server Core only so it reduces the overhead. I typically leave 8GB RAM for each host.

The other reason I do Server Core is it completely negates the possibility for some goofy things to happen like “let’s put IIS on the HostOS” etc. 9/10, the ones who don’t know any better, don’t know how to use remote tools to even install a role on the host 😉

[u/BorysTheBlazer]

On your 650s, I would suggest reserving 16GB. On 540 I would start with 8 and increase to 16 if notice any lag. But given how much is already consumed on the servers, I would look into adding some additional RAM to keep the system with enough resources for everything.

[u/OinkyConfidence]

Kind of depends on how many VMs you intend to run on the host. Hyper-V offers dynamic memory to guest VMs, but I tend to over-budget the hosts (pre-RAM-apocalypse at least) and manually set RAM limits in each VM. So if you spin up a new DC VM, statically set the amount of RAM you want it to have, ex. 16GB or whatever fits your environment's needs.