I/O performance/tweaking question

[Oleg Goldshmidt]

Hi,

I noticed something strange after upgrading two relatively aged
computers from Fedora 14 to Fedora 17. Both 'puters are a few years
old, one is a desktop, another a laptop. Both are connected to a
vanilla home 802.11g wireless router, the desktop by cable, the laptop
wirelessly. Each has an external USB disk attached (from time to
time).

Occasionally I copy files from one USB disk to another using scp or
rsync - some files small, some large. I noticed long ago that copying
over the network was faster than connecting both disks to the same
computer, for whatever reason (single bus in both directions?). With
the described setup I don't know what the bottleneck is - for all I
know it may still be USB/disk if some buffer is kept full.

What bothers me that after the upgrade the transfer speed went down
markedly. When both 'puters were running Fedora 14 I typically got
2.3MB/s or even more. I was never surprised because the laptop has an
Intel 5100 AGN wireless adapter that probably has 802.11n disabled (by
default? not by me...), and if it is in 802.11g mode then I should
expect less than 3MB/s, I think. However, with Fedora 17 on both ends
it is always 1.0MB/s (e.g, as reported by scp). A transfer of a large
file may start at 2.5MB/s but very rapidly converges to 1.0MB/s,
occasionally fluctuating between 900KB/s and 1.1MB/s. Unattended
backup through rsync is no problem, but when I copy something largish
interactively it is much more annoying than before.

I am surprised. Naively, I would expect a newer system to be at least
as fast as the old one. Nothing but the Fedora version changed - same
HW, the disks have not been reformatted, etc. I suspect that something
may be optimized differently, but I have no idea what to tweak - or
even what to check. Or mabe some obscure piece of firmware is missing? 
Does anyone have any suggestions?

Obviously, I have no way to experiment with Fedora 14 anymore - it's
gone.

Thanks in advance for any suggestions.

-- 
Oleg Goldshmidt | pub at goldshmidt.org