When I tried that on the "sister" VM, I saw the transfer rate peak at 96mb/s! Very desirable, but rather puzzling. Jan 2017, 09:16 Primary OS: MS Windows 7 VBox Version: PUEL Guest OSses: Ubuntu 16.04 圆4, W7 I expect the performance of Bridged mode would also increase significantly in this scenario, since it makes no sense for it to ever be slower than NAT, but that'll have to wait til the next time it's convenient to restart the guest. So, the VB networking layer is obviously much more performant if fed data in large chunks - even if the natsettings are at their default small 64K buffers.
Regardless, the meaningful amount of time spent at the high transfer rate result in an average throughput speed of ~58mb/s. It's hard to guess at "why" - it could be determined by the buffer sizes of either the host or the host NIC, but I don't really have any solid ideas. Towards the end of the transfer (but much much less than just a few MB away) the rate falls off a cliff again to less than 40 mb/s. whenever you have a pipeline, it's important to keep the pipe as full as possible, but it looks like different sets of buffers (guest, vNIC, NAT) are all draining at different times, resulting in the sine-wave for transfer rates mentioned earlier.įorcing the guest to use SMB3 instead changes the CIFS buffers to "rsize=1048576,wsize=1048576", and things get pretty interesting then: once it's "warmed up", the transfer spends a lot of time at 60mb/s - on par with using Bridged - and even creeps into the 70mb/s range at times, surpassing it. looking just at the guest though, the default settings for CIFS are "rsize=61440,wsize=65536", and from what I can see these are a non-trivial factor in the poor performance with NAT. To drift into somewhat-guest-specific aspects for a moment: a linux host (Ubuntu 16.04) with the same configuration as the problematic guest still easily sustains line rate on transfers to the NAS.
After poking around some more, it looks like part of the problem is interaction between the NAT layer and SMB: that is, the settings/behavior of the two of them happen to be working out to produce especially-poor results. I didn't go into the details before, but the NAS is being mounted as a Windows share (ie SMB).
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