Myujth

I'm measuring random write speeds for a EMMC device with fio.

I'm testing multiple block sizes with multiple io sizes.

After multiple iterations for all test cases on a raw emmc device, I create a logical volume using the entire emmc device and run the same tests on that volume.(such as /dev/new_vol_group/new_logical_volume)

I was expecting a slight performance overhead with LVM but the strangest thing happened.

For small io sizes random write speeds are quite similar between raw and LVM. When IO size is increased(specially when the size is double amount of ram), random write speeds for raw device is quite decreased but that's not the case with LVM. There is no decrease in random write speeds with LVM when file size is increased.

So LVM is much faster than raw device for random write access specially for large filesizes. This is only true for random write. I didn't see this behaviour with sequential read-write or random read tests.

This is my fio file

ioengine=libaio 
direct=1 
buffered=0 
iodepth=1
numjobs=1
ramp_time=5
startdelay=5
runtime=90
time_based
refill_buffers
randrepeat=1

Only reason I could think of was caching, but caching can't affect the tests this much when io size is double the amount ram on the host.

Note: I use fio-3.1 for benchmarking, Ubuntu 18.04 LTS as a host.

I detected this behaviour on multiple devices.