Reading a whole file on a network drive the fast way (Windows, C/C++, C#, ...)

Question

Lately I've been having problems reading big files on a network drive and I just can't pinpoint what I may be doing wrong. I tried both in C++ (Unmanaged) and in C# and had about the same performances on both...which were somewhat abysmal.

Sometimes it will read at 4 KB/s a file on the network, but if this file is located on the local HD it will achieve easily the maximum data rate the HD can output. That is with reading 64 KB chunks at a time... I tried with bigger buffers up to insane numbers, or smaller and it doesn't make much differences.

I tried async IO in C# with BeginRead on the FileStream and OVERLAPPED IO in C++ as well as synchronous reads and they all had the same problems, which is being slow on the network.

The only solution we came up with is to copy the file using the OS CopyFile function on the local HD before actually reading the file but I'm not too satisfied with this approach. It just seems like CopyFile is doing something we are not that makes it incredibly faster than our approach.

Anyone has a clue as to why this is?

Answer 1

We would have to guess, since you aren't showing us your code. So my guess is that Windows file copy is opening the file with the FILE_FLAG_SEQUENTIAL_SCAN flag which in turn causes the file system/cache to choose optimal block sizes and submit read requests in anticipation of read calls that havn't been submitted yet.

Answer 2

We only can assume that you have been trying really all possible methods of reading/writing. Have you been reading synchronously or asynchronously? Did you try I/O completion ports? Or ReadFileEx() function? I would guess that the Windows CopyFile() function detects that you want to read a file from network and will use different method for reading then it would use for disk access.

If you have really exhausted all possible reading methods, and if you really need thing to be solved, then I would suggest to check out a bit on what is the CopyFile() function doing. There are numerous tools for doing that. E.g.: this one (or some other -- links on the same page).