I was trying to do some performance testing between using iSCSI on the host as a diskfile to a guest vs the VM guest using the iSCSI device directly.
However, in the process of trying to establish a baseline performance figure, I started increasing the MTU settings on the PCI-express NICs with RTL8168B chips.
First bottleneck was discovering the max MTU allowed on these is 7K instead of 9K but googling seems to indicate that the RTL8168B is only capable of 4K frames.
I assumed 4K would still be better than nothing but unfortunately bumping up the MTU to anything else but 1.5K caused the file transfers (using NFS for easy testing), to hang at random points or more accurate slow to a crawl.
Checking the syslog, I discovered warnings that increasing MTU with this adapter may cause problems.
Searching around, it seems to be a common problem but there doesn't appear to be any clear cut fix, including some suggestions to use a third party driver.
Does anybody know of a proven solution or is the Realtek chip itself irrevocably broken/bugged that anything above the default 1500 will simply not work?