[Qemu-block] 4k Disk I/O issue

[by morphin]

Hello.

I'm getting too much bad return from my customers at 4k random speeds. They claim that SQL databases works much slower than usual, and I can't find a solution.

I've done a lot of disk speed testing and I tried too many different options for increase 4k random speed but still I can only reach half of the physical disk speed.

Because of that I want to ask your advice. Please warn me if there is any wrong in my tests and my xml.

Recently I'm using writeback - threads and "<driver queues='4' iothread='4'/>" and I'm using CEPH with RBD but in this test I used the same SSD RAW disk to be more consistent and fair.

Let's see our test environment:

HOST OS:
Kernel: 4.14.71-1-lts Arch-linux
qemu-headless 3.0.0-1
libvirt 4.7.0-1

HOST HW:
Intel(R) Xeon(R) CPU E5-2630 v4 @ 2.20GHz

68GB++ Free ram

LSI-9300_8e HBA - FW=15.15.06.00

DİSK:

Source path: /dev/disk/by-id/ata-INTEL_SSDSC2KG480G7_BTYM72940CU2480BGN

Options: RAW, Writeback, threads

GUEST:

Win-2016

Driver: virtio-win-0.1.141

Guest xml: https://paste.ubuntu.com/p/w3W9BddzqY/

Also I tried q35-3.0

TEST RESULTS

1. First of all this is Pysical disk result without virtualization.

2. i440FX - cache=none - IOmode=threads 

3. i440FX - cache=directsync - IOmode=native

4. i440FX - cache=writeback - IOmode=threads - <driver queues='4' iothread='4'/>

5. i440FX - cache=writeback - IOmode=threads - <driver queues='8' iothread='4'/>

5. i440FX - cache=writethrough - IOmode=threads - <driver queues='4' iothread='4'/>

6. i440FX - cache=unsafe - IOmode=threads - <driver queues='4' iothread='4'/>

NOW LETS TRY Q35-3.0 CHIPSET

1. Q35-3.0 - cache=none - IOmode=native - <driver queues='4' iothread='1'/>

2. Q35-3.0 - cache=none - IOmode=threads - <driver queues='8' iothread='8'/>

3. Q35-3.0 - cache=writeback - IOmode=threads - <driver queues='4' iothread='1'/>

4. Q35-3.0 - cache=writeback - IOmode=threads - <driver queues='8' iothread='8'/>

AND FINALLY I CHECKED OUR ENEMY TEST RESULTS "vmware 6.5 - local ssd datastore & vmdk " 

They are closer to the physical disk as random and sequential.