Re: fstream "write" faster than Windows "WriteFile"
stefan.kniep@gmx.de wrote:
Hello,
On my system, sequentially writing 40 MB to disk with fstream "write"
is up to 3 times faster than doing the same using the "WriteFile"
function. I need to use the Windows API for file operations because I
have to handle files that are larger than 4GB. I tested the
"FILE_FLAG..." flags, but they make little difference when writing
files. The write performance is quite crucial in my application, so I
would like to know if there's a solution for this problem. Below is a
small test program that demonstrates the performance difference.
I'm not going to study your code to speculate why you got the results that
you did (but it has to do with buffering and your test not actually
measuring disk write speed).
Bottom line is this: there's no way that ofstream is faster than WriteFile.
Under the covers - under about 4 layers of abstraction - writing with
ofstream in fact calls WriteFile to do the writing.
To get the fastest write performance you should:
- Use CreateFile with FILE_FLAG_NO_BUFFERING and FILE_FLAG_OVERLAPPED.
- Do asynchronous writes using WriteFile.
- Write only in aligned multiples of the disk sector size.
- Write as much data from memory in a single call to WriteFile as you can.
Be sure to read all the notes in the documentation for CreateFile and
WriteFile. Both async IO and unbuffered IO are fussy, with quirks that you
need to handle correctly.
-cd
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