Cant copy big files between VMs unless disable "auto tuning feature” in vnic

We encountered problem while enabling the alwaysOn availability:
We could copy smaller files (generally <900MB) but copying files (these are MS SQL backup files )
>1GB  will drop/freeze between the two VMs running MS SQL 2012.

We overcame this by disabling 'auto-tuning feature' :
“netsh int tcp set global autotuninglevel=disabled”

What's the implication/impact of disabling auto-tuning?

What's the root cause that the copying freeze/drops for files >1GB ?
Is there a VMware or MS KB article for this?

We're on vSphere 5.0 Upd 3  & the VMs are on Win2012 (not sure if it's Standard or DC edition)
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Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
Are you using the VMXNET3 interface in all your VMs, you should be.
sunhuxAuthor Commented:
Yes, we are.  Missed replying on this thread though replied on the other one
AkinsdNetwork AdministratorCommented:
See the comments from Microsoft Below.

If you enable Receive Window Auto-Tuning for WinHTTP traffic, data transfers over the network may be more efficient. However, in some cases you might experience slower data transfers or loss of connectivity if your network uses an older router and firewall that does not support this feature. For example, when you use Windows Internet Explorer to access applications that are hosted in Microsoft Office SharePoint Server, the HTTP traffic may slow down. This occurs because certain routers do not support the Receive Window Auto-Tuning feature.
In your case, this means the vSwitch on your VM Host does not support auto-tuning or it's turned off.

What's the implication/impact of disabling auto-tuning?

Just means you lose the additional benefits of auto-tuning which is mainly TCP Receive Window feature. In this case, you're much better off without it than with it.

What's the root cause that the copying freeze/drops for files >1GB ?
Is there a VMware or MS KB article for this?

Slow speed due to incompatibility. The computer uses more CPU power trying to use the TCP Receive Window feature and then falling back to normal operation

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