Why is there a difference in USB HDD speed in my two Raspberry Pis?

Question

Summary

USB drives connected to the rev2 running Raspbmc read/write twice as quickly as the rev1 running Raspbian. Why is there a difference in speed?

Testing

I tested the same external USB hard drive connected to both machines. I created a 50 MB test file from random data.

$ dd if=/dev/urandom of=/media/HDD/randomness bs=1M count=50

On the Raspbmc (two replicates):

$ time cp randomness randomness2
real    0m1.922s
real    0m1.984s

On the Raspbian (two replicates):

$ time cp randomness randomness2
real    0m4.649s
real    0m4.709s

I compared the mount options. On the Raspbmc

$ mount | grep media
/dev/sda1 on /media/HDD type ext4 (rw,nosuid,nodev,uhelper=udisks)

On the Raspbian (mounted with -o noatime)

$ mount | grep media
/dev/sdb1 on /media/HDD type ext4 (rw,noatime,data=ordered)

I tested mounting the Raspbian HDD with noatime,nosuid,nodev,data=writeback, but I still get read/write times that are about double that of the Raspbmc

real    0m5.127s
real    0m4.966s

Why is the rev1 Raspbian Pi hard drive not as quick as the rev2 Raspbmc?

Answer 1

This appears to have not much to do with the distribution in use. I tried swapping the rev1 and rev2 boards around. When running the Raspbian system on the rev2 board, I can get similar speeds to the Raspbmc.

I was quite surprised, because I see no references to changes in USB throughput in the rev 2 release notes. However, it seems that there is a difference in RAM in my case.

Adding free -m and GPU allocation from /boot/config.txt, my rev1 has 231+16=247 MB, and the rev2 has 374+128=502 MB. Reading further, it seems that rev1s and early rev2s have 256 MB of RAM, and later-model rev2s have 512 MB (although I'm not sure why the slight discrepancy in my case).