I have an odd situation on my machine. I have multiple external USB 3.0 drives doing high amounts of IO (about 70-100MBps each, copying disk images or using tools like dd) but very low IO on my system SSD drive.
I can run a high IO operation on each of these drives fine, but once I'm using 3+ drives my system gets unusably laggy even though my CPU, Memory, and System IO usage is low. I can ionice the operations which helps a little but I still run into the same bottleneck.
Where might this bottleneck be?
Specs:
i7 processor 12GB memory,
internal system drive is SSD
all externals are USB 3.0 5400-7200RPM,
Ubuntu 14.04 x64.
When the bottleneck is hit, RAM usage is <50% swap has <1GB used, system IO is just whatever is required to run Ubuntu not running any heavy operations there, CPU is <30% usage across all cores.
The reason this is happening is that the page cache is being filled up by these large read/write operations, and the page cache is system-wide, not per disk. I posted a similar question at the Unix StackExchange and got an answer, which enabled me to skip the cache using the
ddcommand...