What is the ideal memory block size to use when copying?

Question

I am currently using 100 megabytes per memory block to copy large files.

Is there a "good" amount that people normally use?

Edit

Thanks for all the great responses.

I'm still quite new to these concepts so I'll try to understand a lot of the ones that have been said (e.g. write back cache). I keep learning new things :)

Answer 1

A block between 4096 and 32KB is the typical choice. Using 100MB is counter-productive. You are occupying RAM with the buffer that can be put to much better use as the file system writeback cache.

Copying files is very fast when the file fits completely in the cache, the WriteFile() call is a simple memory-to-memory copy. The cache manager then lazily writes it out to the disk. But when there's no more room in the cache, the copy speed drops off a cliff when WriteFile() has to wait for space to be made available. It now goes at disk write speeds.

Answer 2

I would recommend you to benchmark this, and remember to include much smaller block sizes. In my own tests on this, I got quite counterintuitive results.

When reading and writing from the hard drive, all (power of two) block sizes between 512 byte and 512 kB gave the same speed. Increasing the block size from 512 kB to 1 MB reduced the copying speed to about 60%. Increasing the block size further increased the speed again, but never all the way back to the speed of using small blocks.

When all the copied data was in the cache memory, the (much faster) copying speed improved with increasing block sizes, flattening out around reaching 32 kB blocks, and then suddenly dropped to about half the previous speed when going from 256 kB to 512 kB blocks, never to return to the previous speeds.

After this test, I dropped read/write block sizes in several of my programs from around 1 MB to 32 kB.