@stormi XCP-ng QCOW2 vs. VHD Performance Feedback on NVMe
First of all, I would like to thank the team for all the hard work in bringing QCOW2 support to a production-ready state. It is a very welcome feature.
I have performed some quick I/O benchmarks comparing the new QCOW2 format against the traditional VHD. In my tests, QCOW2 appears significantly slower than VHD on my hardware.
Test Environment
Hypervisor: Dell PowerEdge R420
CPU: Intel Xeon E5-2470 v2
Storage: Intel SSDPELKX010T8 NVMe
VM OS: Debian 13
VM Specs: 2 vCPUs, 1GB RAM
Setup: One 10GB VHD and one 10GB QCOW2 disk, both pre-filled from /dev/random.
Methodology
I used a custom test suite available here: https://vm01.unsoft.hu/~ventura/fio/fio_test_20250408.tar.gz
[image: 1778009249525-vhd_bandwidth_summary.png] [image: 1778009256705-vhd_latency_summary.png]
[image: 1778009281033-qcow2_bandwidth_summary.png] [image: 1778009286521-qcow2_latency_summary.png]
I also ran a simplefio loop with the following results:
VHD:
root@Debian-13-CloudInit-20250810:/mnt/vhd# for mode in read write; do for jobs in 1 16; do for bs in 4 64; do for t in "" rand; do printf "%2i qd %2ik % 4s " $jobs $bs $t; fio --name=random-write --rw=$t$mode --bs=${bs}k --numjobs=1 --size=1g --iodepth=$jobs --runtime=10 --time_based --direct=1 --ioengine=libaio|grep -e BW -e runt ; done; done; done; done
1 qd 4k read: IOPS=9625, BW=37.6MiB/s (39.4MB/s)(376MiB/10001msec)
1 qd 4k rand read: IOPS=5414, BW=21.2MiB/s (22.2MB/s)(212MiB/10001msec)
1 qd 64k read: IOPS=2657, BW=166MiB/s (174MB/s)(1661MiB/10001msec)
1 qd 64k rand read: IOPS=2575, BW=161MiB/s (169MB/s)(1610MiB/10001msec)
16 qd 4k read: IOPS=45.7k, BW=178MiB/s (187MB/s)(1785MiB/10001msec)
16 qd 4k rand read: IOPS=45.9k, BW=179MiB/s (188MB/s)(1794MiB/10001msec)
16 qd 64k read: IOPS=16.7k, BW=1041MiB/s (1092MB/s)(10.2GiB/10001msec)
16 qd 64k rand read: IOPS=16.7k, BW=1042MiB/s (1093MB/s)(10.2GiB/10001msec)
1 qd 4k write: IOPS=8842, BW=34.5MiB/s (36.2MB/s)(345MiB/10001msec); 0 zone resets
1 qd 4k rand write: IOPS=8880, BW=34.7MiB/s (36.4MB/s)(347MiB/10001msec); 0 zone resets
1 qd 64k write: IOPS=6095, BW=381MiB/s (399MB/s)(3810MiB/10001msec); 0 zone resets
1 qd 64k rand write: IOPS=6006, BW=375MiB/s (394MB/s)(3755MiB/10001msec); 0 zone resets
16 qd 4k write: IOPS=49.3k, BW=193MiB/s (202MB/s)(1928MiB/10001msec); 0 zone resets
16 qd 4k rand write: IOPS=47.3k, BW=185MiB/s (194MB/s)(1848MiB/10001msec); 0 zone resets
16 qd 64k write: IOPS=14.3k, BW=891MiB/s (934MB/s)(8910MiB/10001msec); 0 zone resets
16 qd 64k rand write: IOPS=15.5k, BW=966MiB/s (1013MB/s)(9663MiB/10001msec); 0 zone resets
QCOW2
root@Debian-13-CloudInit-20250810:/mnt/qcow2# for mode in read write; do for jobs in 1 16; do for bs in 4 64; do for t in "" rand; do printf "%2i qd %2ik % 4s " $jobs $bs $t; fio --name=random-write --rw=$t$mode --bs=${bs}k --numjobs=1 --size=1g --iodepth=$jobs --runtime=10 --time_based --direct=1 --ioengine=libaio|grep -e BW -e runt ; done; done; done; done
1 qd 4k read: IOPS=5866, BW=22.9MiB/s (24.0MB/s)(229MiB/10001msec)
1 qd 4k rand read: IOPS=4000, BW=15.6MiB/s (16.4MB/s)(156MiB/10001msec)
1 qd 64k read: IOPS=2229, BW=139MiB/s (146MB/s)(1394MiB/10001msec)
1 qd 64k rand read: IOPS=2161, BW=135MiB/s (142MB/s)(1351MiB/10001msec)
16 qd 4k read: IOPS=16.9k, BW=66.2MiB/s (69.4MB/s)(662MiB/10001msec)
16 qd 4k rand read: IOPS=17.6k, BW=68.8MiB/s (72.1MB/s)(688MiB/10001msec)
16 qd 64k read: IOPS=7244, BW=453MiB/s (475MB/s)(4529MiB/10002msec)
16 qd 64k rand read: IOPS=6994, BW=437MiB/s (458MB/s)(4372MiB/10002msec)
1 qd 4k write: IOPS=5551, BW=21.7MiB/s (22.7MB/s)(217MiB/10001msec); 0 zone resets
1 qd 4k rand write: IOPS=5159, BW=20.2MiB/s (21.1MB/s)(202MiB/10001msec); 0 zone resets
1 qd 64k write: IOPS=4024, BW=252MiB/s (264MB/s)(2515MiB/10001msec); 0 zone resets
1 qd 64k rand write: IOPS=4027, BW=252MiB/s (264MB/s)(2517MiB/10001msec); 0 zone resets
16 qd 4k write: IOPS=14.5k, BW=56.8MiB/s (59.6MB/s)(568MiB/10002msec); 0 zone resets
16 qd 4k rand write: IOPS=14.0k, BW=54.7MiB/s (57.4MB/s)(547MiB/10001msec); 0 zone resets
16 qd 64k write: IOPS=6360, BW=398MiB/s (417MB/s)(3976MiB/10002msec); 0 zone resets
16 qd 64k rand write: IOPS=6090, BW=381MiB/s (399MB/s)(3807MiB/10002msec); 0 zone resets
I would be interested to know if I'm overlooking something, or if the qcow2 format simply provides lower performance compared to VHD for the time being?
