Hello Devs from the QEMU Community,
I’m Trilok Bhattacharya, a postgraduate student at the University of Manchester, working on my dissertation project that involves investigating the effects of snapshot streaming to the I/O throughput, and mitigating
them.
While working on the same, I captured the benchmarks of a standalone disk of size 50G (qcow2) on an Ubuntu-18.04 VM using FIO, and taking snapshots at regular fixed intervals.
The test VM is launched with a bash script as below –
VM_LOCATION=${1:-$HOME/projects/vms/disks/ub-18.04_50G.qcow2}
QEMU=/usr/local/bin/qemu-system-x86_64
NETWORK_ARGS="-nic user,hostfwd=tcp:127.0.0.1:1122-:22"
SERIAL="-daemonize"
sudo $QEMU -smp 4 -m 4G \
-drive id=drive0,if=none,format=qcow2,file=$VM_LOCATION,cache=none,aio=threads,discard=unmap,overlap-check=constant,l2-cache-size=8M \
-device virtio-blk-pci,drive=drive0,id=virtio0 \
--accel kvm \
-display none \
$NETWORK_ARGS \
-chardev socket,path=qemu-ga-socket,server=on,wait=off,id=qga0 \
-device virtio-serial \
-device virtserialport,chardev=qga0,name=org.qemu.guest_agent.0 \
-qmp unix:qemu-qmp-socket,server,nowait \
-monitor unix:qemu-monitor-socket,server,nowait \
-D qemu.log \
$SERIAL
The fio configuration file is –
[global]
size=10g
direct=1
iodepth=64
ioengine=libaio
bs=16k
directory=/mnt/data
log_avg_msec=1000
numjobs=1
runtime=600
time_based=1
[writer1]
rw=randrw
write_iops_log=/tmp/results/randrw-iodepth-1-numjobs-1_iops
Snapshots (10) are taken at an interval of 20 seconds, after an initial wait of 15 seconds.
Command to take snapshots –
"snapshot_blkdev $block_device $snapshot_dir/$name" | sudo socat – UNIX-CONNECT:$socket_location/qemu-monitor-socket
Upon generating a graph for the fio numbers, it produces this –
(Blue lines represent the moment a snapshot is taken)
It is noticed that there is a sharp dip in the I/O throughput as soon as snapshots are taken, and the dip continues for more than 100 seconds,
until the IO goes back to “normal” again.
I have tried finding the root cause for this drop in performance, but could not find anything significant. I have been looking for some explanations
to the above behaviour, or some directions so that I can investigate on the same.
It will be really beneficial to receive some help from the community, and advance my project.
Looking forward to hearing back.
Sincerely,
Trilok