Why does it take so long to copy a 900MB vs. 200MB file on windows 7?

Question

If i copy a 213MB file, it copies INSTANTLY - well, within milliseconds If i copy a 936MB file, it takes approx 1min.

Both files are located on the Root of my E:\ drive.

Why would there be such a difference? Even if the 213MB took 999 milliseconds, the latter should copy in only a few seconds, but it doesnt.

The machine has 4GB Ram, duo core, etc. - both are on a single drive.

Answer 1

I have noticed that there is a high degree of caching that goes on with Windows 7. If you copy a file that will fit completely into available RAM, it will return control back to you very fast, so you can do work while it finishes your copy in the background.

That said, caching doesn't really explain the difference in file copy times, unless the file is already in memory when you attempt to copy it, where the other is not. One minute for a disk-to-disk copy of a 936MB file is about 15 megabytes per second, which is a respectable transfer rate.

In other words, I would expect a 936MB file to take a minute or so to copy.

Answer 2

There are a series of commands that take place every time you perform a file copy routine. If the file is 213MB then you would perform these commands (lets say 3 steps), and a single read/write command to copy the file.

If your file is 936MB then you would still do the 3 steps previously mentioned, and an additional 7 read/write commands to actually copy the file.

Thus, 100 small files of totaling 900MB comprises 100 mandatory steps, and 100 "copy" steps and it will take longer than copying a single 900MB file

So the difference is between the number of steps needed to copy.

Answer 3

Are your source and destination drives physically the same in both of your tests? Copying files on the same drive takes more time because of all the going back-and-forth the drive head has to do. The respective heads can read/write sequentially (pending fragmentation amount) when copying from drive to drive.

Answer 4

It has been my experience that Windows will report the file as copied once it has been read into memory. If it has buffer space for the 231MB file, but not the 900MB, you will have to wait until it has written enought of the file to read the end of it into memory. This may be a large number of read write cycles on different areas of the disk. With the larger file the space may be further across the disk resulting in slower read write cycles.

Windows also tends estimate copies high initially. I often get estimates of minutes, and the copy completes in a few seconds or tens of seconds.

Copying a file to a disk area near the source will be faster than copying to a location further across the disk. Head movement latency used to be higher than rotational latency. Both increase with distance. I haven't looked at latency figures lately, but expect head movement is still slower than rotational latency.