Hello!
I have two hard drives and I want to determine time of copying files from one to other.
So, I do:

1) Create file system on the second HDD and mount it
2) Copy set of files from / to folder mounted on step 1
3) Call sync to write all data to disks
3) Repeat step 2-3 10 times to make results more smooth

So, results are too strange:
The first time is a much greater that other 9, for example:
1000 51 50 49 52 ....

I don't have reasons why the first time is mutch greater than other 9: May be on first copying allocated internal structures of file system on the second HDD, or may be sync works on asynchronous manner, it started to write memory buffers to disk, and exits, this process continues then first copying is already started.

at the first run some data will be cached.