How can I run a job when the server load is low?

I have a command that runs a disk snapshot (on EC2, freezing an XFS disk and running an EBS snapshot command), which is set to run on a regular schedule as a cron job. Ideally I would like to be able to have the command delayed for a period of time if the disk is being used heavily at the moment the task is scheduled to run.

I'm afraid that using nice/ionice might not have the proper effect, as I would like the script to run with high priority while it is running (i.e. wait for a good time, then finish fast).

Thanks.

UPDATE:

This is what I ended up going with. It checks /proc/diskstats and runs my job when the current IO activity hits 0, or we timeout. I'll probably have to tweak this when I look at what kind of IO activity our servers actually get in production:

#!/bin/bash

DEVICE=sdf

# we want to make a snapshot when IO in progresses reaches this:
LOW_THRESHOLD=0

TIMER=0
MAX_SEC_DELAY=120

# Get the number of IO operations in progress:
ioInProgress(){
    grep $DEVICE /proc/diskstats | awk '{print $12}'
}

# Wait for a good time to run snapshot, else timeout:

while [[ $TIMER -lt $MAX_SEC_DELAY && $(ioInProgress) -gt $LOW_THRESHOLD  ]]; do
    TIMER=`expr $TIMER + 1`
    sleep 0.5
done

# Recording delay required:
echo $TIMER
echo "Executing snapshot"
run-the-snapshot