When measuring the impact of using DRBD on a system’s I/O throughput, the absolute throughput the system is capable of is of little relevance. What is much more interesting is the relative impact DRBD has on I/O performance. Thus it is always necessary to measure I/O throughput both with and without DRBD.

	Caution
	The tests described in this section are intrusive; they overwrite data and bring DRBD devices out of sync. It is thus vital that you perform them only on scratch volumes which can be discarded after testing has completed.

I/O throughput estimation works by writing significantly large chunks of data to a block device, and measuring the amount of time the system took to complete the write operation. This can be easily done using a fairly ubiquitous utility, dd, whose reasonably recent versions include a built-in throughput estimation.

A simple dd-based throughput benchmark, assuming you have a scratch resource named test which is currently connected and in the secondary role on both nodes, is one like the following:

# TEST_RESOURCE=test
# TEST_DEVICE=$(drbdadm sh-dev $TEST_RESOURCE)
# TEST_LL_DEVICE=$(drbdadm sh-ll-dev $TEST_RESOURCE)
# drbdadm primary $TEST_RESOURCE
# for i in $(seq 5); do
    dd if=/dev/zero of=$TEST_DEVICE bs=512M count=1 oflag=direct
  done
# drbdadm down $TEST_RESOURCE
# for i in $(seq 5); do
    dd if=/dev/zero of=$TEST_LL_DEVICE bs=512M count=1 oflag=direct
  done

This test simply writes a 512M chunk of data to your DRBD device, and then to its backing device for comparison. Both tests are repeated 5 times each to allow for some statistical averaging. The relevant result is the throughput measurements generated by dd.

	Note
	For freshly enabled DRBD devices, it is normal to see significantly reduced performance on the first dd run. This is due to the Activity Log being "cold", and is no cause for concern.