Let's benchmark multiple disk write performance:

1. Get the total number of disks in use with the Ceph OSD; in my case, it's three disks:

        # mount | grep -i osd | wc -l

2. Drop caches:

        # echo 3 > /proc/sys/vm/drop_caches

3. The following command will execute the dd command on all the Ceph OSD disks:

        # for i in `mount | grep osd | awk '{print $3}'`;
          do (dd if=/dev/zero
         of=$i/deleteme bs=10G count=1 oflag=direct &) ; done 

To get the aggregated disk write performance, take the average of all the write speeds. In my case, the average comes out to be 127 MB/s: