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This screenshot is of me transferring a large file (a .VHD) from one logical volume (made of 2 spindles in RAID0) to another one (made of 2 spindles in RAID0) on the same physical machine. (This is not production so don't worry about the sanity of that hardware configuration.) I initiated the transfer via explorer.exe on a third machine on the same LAN, so some TCP protocol may be implicated in the crime here; I don't know.

As the file transfer starts, it peaks out at around 220MB/sec, which is the speed I would expect from a volume to volume transfer on this machine, and then right about 50% of the way through, it drops down to around 50-75MB/s and finishes out at around that rate.

Slowdown halfway through the file

Any ideas as to why the file transfer rate from volume to volume consistently halves half way through the transfer? I tested the same transfer a few times and got the exact same behavior each time.

EDIT: I have tested this again by using robocopy instead of Explorer. I'm using a different file but I'm still initiating the copy from a third workstation:

robocopy transfer speed

I did not observe the drastic slowdown about halfway through the file copy using robocopy, however, if you look at the transfer speed at the end, it is still well above the theoretical limit of gigabit Ethernet.

EDIT #2: Here's the same transfer viewed through Explorer. There's no slowdown with this file. The only difference is that this VHD is about half the size of the first one:

no slowdown

Consistent evidence from two different tools that the file copy speed is faster than what GigE should be able to provide, so I'm still not convinced that the transfer is going over the network. But I still have no idea why the bigger file in the beginning would suffer from the slowdown halfway through the transfer. Perhaps there are two many variables/factors in this experiment.

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maybe fragment issue? –  LiuYan 刘研 Oct 24 '12 at 7:06
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1 Answer

The transfer is going over the network. Assuming these desktops are all connected via Gigabit Ethernet, 60-70MB/s is a fairly typical speed for file transfers. The "third desktop" you mentioned doesn't know or care that the two shares are on the same physical box. All it knows is that it's copying from share A (the source RAID0) to share B (the destination RAID0) and that these are both network destinations.

The fix is simple: Use Remote Desktop to log in to the box with the RAID0 arrays and initiate the file copy that way.

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I definitely understand what you're saying, but why would the transfer speed be more than 2x higher (and also beyond what 1Gb of bandwidth could provide) for the first half of a file transfer, but not the last half? –  Ryan Ries Oct 24 '12 at 3:34
    
The Windows file copy dialog is not a very scientific way to measure file transfer times. It will gladly report 200MB/s while network utilization never goes above the limitations of the Gigabit connection. This is due to filesystem caching, TCP tuning parameters, adjusted estimates and good old-fashioned optimism. –  Joel E Salas Oct 24 '12 at 3:40
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