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We have a Windows Server 2012 Hyper-V host that has 32 GB of RAM running two Server 2012 VM's. The machines have dynamic memory enabled, but combined are only using 17 - 17.5 GB. We received an alert this morning that the host was using all the memory.

I logged in (extremely slow at this point) and no specific processes seem to be using very much RAM. I went to the Performance tab in Task Manager and noticed that there is 13.5 GB of ram in the "Cached" field. The "Memory composition" shows all that as "Modified".

I assume this means that the OS is not flushing the RAM to disk for some reason, however I can't seem to find any info on how to troubleshoot the problem.

Would really appreciate any suggestions. If more details are needed, I'd be happy to share them.

EDIT: It appears after a few hours it started releasing that memory. It now has 3.2 GB available and is responsive again. I'd still like some ideas on why it would do this to begin with to see if I can prevent it.

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It seems to me that cache is a good thing, and your system should be using the RAM. Not sure why your system is slow though. –  Zoredache Oct 17 '13 at 16:57
    
I agree with you. If it didn't slow down the host to a crawl, I wouldn't really care. It has a battery backed RAID controller so caching is fine. –  Tamerz Oct 17 '13 at 17:10
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No, cached RAM is available for application and the OS if needed. It's not a disk buffer (as your Question seems to imply). It's not bad, not a problem, and not causing any slowness. Check the page fault rate to confirm, I'm sure it's very low. Something else is causing the slowness –  Chris S Oct 17 '13 at 17:16

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