Can I use fio on a mounted device?

I'm using fio to get some broad read IOPS performance data for various storage configurations like this: fio --name=readiops --filename=/dev/md1 --direct=1 --rw=randread --bs=4k --numjobs=4 --iodepth=32 --direct=1 --iodepth_batch=16 --iodepth_batch_complete=16 \ --runtime=100 --ramp_time=5 --norandommap --time_based --ioengine=libaio --group_reporting readiops: (g=0): rw=randread, bs=4K-4K/4K-4K/4K-4K, ioengine=libaio, iodepth=32 ... fio-2.1.11 Starting 4 processes Jobs: 4 (f=4): [r(4)] [100.0% done] [3504MB/0KB/0KB /s] [897K/0/0 iops] [eta 00m:00s] readiops: (groupid=0, jobs=4): err= 0: pid=10458: Thu Jan 15 05:49:28 2015 ... I'd like to compare the figures I'm getting with an array that is in production use at a quiet time, is this possible to do without affecting the data on the array? There is a --readonly option (duplicated for some reason) [in the man page](http://linux.die.net/man/1/fio) but it isn't 100% clear to me that this is what I'm after: > --readonly > Enable read-only safety checks. > ... > --readonly > Turn on safety read-only checks, preventing any attempted write.