dd: How to calculate optimal blocksize?

Question

How do you calculate the optimal blocksize when running a dd? I've researched it a bit and I've not found anything suggesting how this would be accomplished.

I am under the impression that a larger blocksize would result in a quicker dd... is this true?

I'm about to dd two identical 500gb Hitachi HDDs that run at 7200rpm on a box running an Intel Core i3 with 4GB DDR3 1333mhz RAM, so I'm trying to figure out what blocksize to use. (I'm going to be booting Ubuntu 10.10 x86 from a flash drive, and running it from that.)

Answer 1

The optimal block size depends on various factors, including the operating system (and its version), and the various hardware buses and disks involved. Several Unix-like systems (including Linux and at least some flavors of BSD) define the st_blksize member in the struct stat that gives what the kernel thinks is the optimal block size:

#include <sys/stat.h>
#include <stdio.h>

int main(void)
{
    struct stat stats;

    if (!stat("/", &stats))
    {
        printf("%u\n", stats.st_blksize);
    }
}

The best way may be to experiment: copy a gigabyte with various block sizes and time that. (Remember to clear kernel buffer caches before each run: echo 3 > /proc/sys/vm/drop_caches).

However, as a rule of thumb, I've found that a large enough block size lets dd do a good job, and the differences between, say, 64 KiB and 1 MiB are minor, compared to 4 KiB versus 64 KiB. (Though, admittedly, it's been a while since I did that. I use a mebibyte by default now, or just let dd pick the size.)