Benchmarking file output strategies

[Rafael Ávila de Espíndola]

It seems that the common wisdom on the fastest way to create a file is

* create the file
* resize it to the final size
* mmap it rw
* write the data to the mapping

I benchmarked that against doing 1 MB writes to create a 1GB file with pseudo random data.

The test program is attached. The results I got were (in seconds, mmap is the first):

btrfs
1.752698e+00
1.112864e+00

tmpsfs
1.484731e+00
1.113772e+00

hfs+ (laptop)
4.015817e+00
2.240137e+00

windows 7 (vm)
1.609375e+00
3.875000e+00

ext2 on arm (old google chrome book):
5.910171e+01
6.566929e+01

So on Windows it is true, mmap seems to be faster than writes. On Linux and OS X x86_64 the situation is inverted. On arm mmap is a bit faster.

It would be interesting to see if someone else can reproduce these numbers. It would be particularly nice to try a newer arm system and windows outside a vm.

Also, does anyone have a theory of where the difference comes from?

Cheers,
Rafael
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