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DrWhy
Enthusiast
Enthusiast

Host shows Double the Disk rate as a VM

I'm performing a restore of a large file server 17TB.  I went to the monitor tab of the VM in the vSphere web client and set it to real-time to monitor the restore performance. I noticed that the disk rate of the VM shows ~250,000 KBps but the disk rate of the ESXi6 HOST shows ~500,000 KBps.  This is the only VM running on this host at this time and I would expect to see the same disk rate on the HOST and VM.  Can anybody explain to me why the disk rate of the HOST is double that of the VM?

HOST Performance Chart:

Host.png


VM Performance Chart:

VM.png

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9 Replies
DrWhy
Enthusiast
Enthusiast

Does anyone have any ideas?

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a_p_
Leadership
Leadership

I can't tell you for sure, so this is more of a guess. With any virtual disk type other than Eager Zeroed, each block will be zeroed out on first access. This is transparent to the VM, thus the doubled data rate on only the host.

André

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pkolom
Enthusiast
Enthusiast

Maybe it is related to host swapping?

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DrWhy
Enthusiast
Enthusiast

I thought this as well, however, I tested this theory and restored the data to a Thick-Eager Zeroed disk and that Disk Rate was actually lower.  100,000KBps lower to be exact.  What do you make of this?

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DrWhy
Enthusiast
Enthusiast

To be clear, are you referring to the page file within the guest VM?  I will try to disable this and see what I find. 

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pkolom
Enthusiast
Enthusiast

No, I'm referring to ESXi swap file. It's located in folder with VM files (if you have default config active).

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DrWhy
Enthusiast
Enthusiast

What aspect of the ESXi swap file would be causing this?  How would I determine if that's the case?  Is there something I can change to improve the performance?

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pkolom
Enthusiast
Enthusiast

Just a note: idea that swapping/balooning is causing this is just my wild guess Smiley Happy It may be the root cause, it may not.

I can't tell you why you have such behaviour, I'd suggest to to read this VMware KB: Troubleshooting ESX/ESXi virtual machine performance issues  (check swapping/balooning first)

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DrWhy
Enthusiast
Enthusiast

I didn't do any of the tests that article says, however, at this time the host has 64GB available and there are only two VM's with 4GB each in them.  So I think this can be ruled out, correct?

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