On Sun, 2005-06-19 at 17:03 +0800, Severin Crisp wrote: > I find that when copying from Disk Image (.dmg) packages the transfer > is initially fast but after about 3MB it slows right down. As it is > essentially a disk to disk copy I can see no reason for this, after > all the disk image has been decompressed, opened and mounted by that > stage - or has it?
I'm not sure it has; it may well be decompressed on the fly, as it's read. Anyway, a possible explanation for the behaviour you're seeing is disk cache. Your OS and/or disk may buffer and cache writes, so that the hard disk may not actually start writing for the first few megabytes of the copy. This permits uninterrupted, and thus much faster reads. Once it starts writing, it has to write a chunk, go and read another chunk, write it, and so on. This is just a guess, but matches my experience on other platforms with modern disk subsystems. -- Craig Ringer
