Comment 35 for bug 595047

This is an attempt at bringing sanity to bug #7372. Please only comment here is you are experiencing high I/O wait times and interactvity on reasonable workloads.

Latest working kernel version: 2.6.18?

Problem Description:
I/O operations on large files tend to produce extremely high iowait times and poor system I/O performance (degraded interactivity). This behavior can be seen to varying degrees in tasks such as,
 - Backing up /home (40GB with numerous large files) with diffbackup to external USB hard drive
 - Moving messages between large maildirs
 - updatedb
 - Upgrading large numbers of packages with rpm

Steps to reproduce:
The best synthetic reproduction case I have found is,
$ dd if=/dev/zero of=/tmp/test bs=1M count=1M
During this copy, IO wait times are very high (70-80%) with extremely degraded interactivity although throughput averages about 29MB/s (about the disk's capacity I think). Even starting a new shell takes minutes, especially after letting the machine copy for a while without being actively used. Could this mean it's a caching issue?