If you created an EC2 instance or a volume itself from a snapshot/image, then you may wish to "initialize" it for a boost in performance (but up to 50%). This is simply a matter of reading every block on the device, which can be performed with dd by using the following command:
sudo dd if=/dev/xvd[x] of=/dev/null bs=1M
[x] with the letter of your drive. e.g.
A Note About Snapshots
Since you are not writing any data to the drive, I should not see this causing an increase in the cost of your next snapshot, since snapshots record the diff between now and the previous snapshot.
If you make periodic snapshots of a volume, the snapshots are incremental so that only the blocks on the device that have changed after your last snapshot are saved in the new snapshot. - source