5.0

With vSphere 5.0 VMware changed the way Mem.MinFreePct worked. I had briefly explained Mem.MinFreePct in a blog post a long time ago. Basically Mem.MinFreePct, pre vSphere 5.0, was the percentage of memory set aside by the VMkernel to ensure there are always sufficient system resources available. I received a question on twitter yesterday based on the explanation in the vSphere 5.1 Clustering Deepdive and after exchanging > 10 tweets I figured it made sense to just write an article.

https://twitter.com/vmcutlip/status/345289952684290048

Mem.MinFreePct used to be 6% with vSphere 4.1 and lower. Now you can imagine that when you had a host with 10GB you wouldn’t worry about 600MB being kept free, but that is slightly different for a host with 100GB as it would result in 6GB being kept free but still not an extreme amount right. What would happen when you have a host with 512GB of memory… Yes, that would result in 30GB of memory being kept free. I am guessing you can see the point now. So what changed with vSphere 5.0?

In vSphere 5.0 a “sliding scale” principle was introduced instead of Mem.MinFreePct. Let me call it “Mem.MinFree”, as I wouldn’t view this as a percentage but rather do the math and view it as a number instead. Lets borrow Frank’s table for this sliding scale concept:

Percentage kept free of –>	Memory Range
6%	0-4GB
4%	4-12GB
2%	12-28GB
1%	Remaining memory

What does this mean if you have 100GB of memory in your host? It means that from the first 4GB of memory we will set aside 6% which equates to ~ 245MB. For the next 8GB (4-12GB range) we set aside another 4% which equates to ~327MB. For the next 16GB (12-28GB range) we set aside 2% which equates to ~ 327MB. Now from the remaining 72GB (100GB host – 28GB) we set aside 1% which equates to ~ 720MB. In total the value of Mem.MinFree is ~ 1619MB. This number, 1619MB, is being kept free for the system.

Now, what happens when the host has less than 1619MB of free memory? That is when the various memory reclamation techniques come in to play. We all know the famous “high, soft, hard, and low” memory states, these used to be explained as: 6% (High), 4% (Soft), 2% (Hard), 1% (Low). FORGET THAT! Yes, I mean that… forget these as that is what we used in the “old world” (pre 5.0). With vSphere 5.0 and up these water marks should be viewed as a Percentage of Mem.MinFree. I used the example from above to clarify it a bit what it results in.

Free memory state	Threshold in Percentage	Threshold in MB
High water mark	Higher than or equal to Mem.MinFree	1619MB
Soft water mark	64% of Mem.MinFree	1036MB
Hard water mark	32% of Mem.MinFree	518MB
Low water mark	16% of Mem.MinFree	259MB

I hope this clarifies a bit how vSphere 5.0 (and up) ensures there is sufficient memory available for the VMkernel to handle system tasks…