Paradoxial performance serving samba files in go

I've written a program in go that acts as a simple HTTP interface to a samba share: the user makes a get request to http://ift.tt/1DmFb8t and \\repository\foo\bar.txt is served up by way of http.ServeFile (more or less).

However, the performance is abysmal. I ran some benchmarks and the results have me stumped. For context, there are three machines in the picture: the samba server (where the file is located), the proxy server (where the go program runs) and the end user's machine (where I ultimately want the file to get). The samba server and the proxy server are co-located, and the end users are fairly far away.

A straight copy from the samba machine to the user machine using windows' copy runs at ~1.5MB/s. That's good enough for me, and what I'm aiming for in the proxy service.

Unfortunately curl 'http://ift.tt/1DmFb8t' > bar.txt from a user's machine clocks in at about 150KB/s.

So, let's see if there's connectivity issues between the samba server and the proxy server. A copy from the samba server to the proxy server looks like it's getting... about 15MB/s.

Hm, maybe it's a go thing? I'll write a go program that benchmarks the transfer speed.

src, _ := os.Open("\\\\repository\\foo\\bar.txt")
start := time.Now()
written, _ := io.Copy(ioutil.Discard, src)
elapsed := time.Since(start)
bytesPerSecond := written/int64(elapsed/time.Second)

Dang, 15MB/s.

Ok, ok, maybe there's something else in the go code causing the issue. Remote onto the proxy server, start up IE, go to http://ift.tt/1DmFb8t, 15MB/s.

Alright, so my code is obviously working great, it must be the connection between the proxy server and the end user. I'll copy bar.txt to the proxy server and use its local path in the url, \mycoolfiles\bar.txt. Huh, 1.5MB/s.

To make things even weirder, I just happen to have C:\mycoolfiles set up as a network share named \\alexscoolfiles, and http://ift.tt/19sA74A clocks in at, dun dun dun, 150KB/s.

Just to confirm this madness, I changed the go program to run in two steps:

1. Copy the file from the share to the local hard drive


2. http.SendFile from there

Lo and behold, after a short pause while the file transfers over at 15MB/s, the download begins at a solid 1.5MB/s.

So, share->proxy is 15MB/s, and proxy->user is 1.5MB/s, but share->proxy->user is... 150KB/s? Ten times slower than it should be? Except not when you're on the same machine as the proxy, because then it's exactly as fast as it should be? And further this problem exists even if it's the exact same file being accessed as long as one is a UNC path and the other is just a local path?

WHAT?

Please help, I just have no idea.