Solved: Guest Memory Usage

jt30605 · ‎01-23-2008

I have a VM that has 1GB of ram assigned to it. VCenter reports Guest Memory Usage of 300MB. When I look at the performance using task manager in the OS, it says it has 124MB available so it's using more than 300MB. I just can't seem to wrap my mind around why this is different. If the the guest is currently using 300MB of memory on my host, how is the guest OS be using more? I ran ESXTOP for an entire work day last week and pulled memory snapshots every 5 minutes. Based on those numbers it seems that a lot of our VM's have too much memory assigned to them. But, if the number I get from there is not the actual usage, how can I know if I can pull the memory down a little? Or should I only rely on the number reported in the OS? I did decrease the memory on several VM's based on my esxtop results and havent seen a performance hit from doing this. We're getting to the point where I need to scale back the memory usage on our hosts in order to maintain our HA failover capacity. If anyone could clarify this for me I would greatly appreciate it.

Rumple · ‎01-23-2008

If you have over subscribed your memory then the ballon driver is probably artifically using up available RAM within the VM so that memory can be used elseware which will make windows think its using all that RAM.

You also have to remember that windows sucks at memory management so its probably paging out memory as well.

The basic math I use is to create single CPU VM's with 512MB of RAM. Unless I find VC reporting that the VM was using 80+% of the RAM all the time then its fine. I've even dropped servers down as low as 384MB of RAM. I think out of 36VM's I have 3 servers with 1GB (BES w/msde), spam server and a test Citrix server.

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Rumple · ‎01-23-2008

If you have over subscribed your memory then the ballon driver is probably artifically using up available RAM within the VM so that memory can be used elseware which will make windows think its using all that RAM.

You also have to remember that windows sucks at memory management so its probably paging out memory as well.

The basic math I use is to create single CPU VM's with 512MB of RAM. Unless I find VC reporting that the VM was using 80+% of the RAM all the time then its fine. I've even dropped servers down as low as 384MB of RAM. I think out of 36VM's I have 3 servers with 1GB (BES w/msde), spam server and a test Citrix server.

jt30605 · ‎01-23-2008

We haven't over subscribed our memory, so there's no ballooning. All hosts are using between 40-60% of memory. Most of our 50 VM's are 1GB+ and according to my esxtop results I can pull them back quite a bit. So you think it is safe to go by the number I get from VCenter and esxtop?