KVM I/O explorations


This post is primarily going to be a resource about what I learned about KVM/QEMU I/O performance.


In a nutshell; the defaults are terrible options to use. writethrough is the default QEMU caching option and it makes everything painfully slow. Additionally, using a non-metadata-preallocated qcow2 will also slow writes down immensely.

Caching modes and read performance

The default, mentioned nowhere in libvirt’s UI, configs, or QEMU, is writethrough. This has a huge impact on read I/O, as mentioned in the fine print of Proxmox’s PVE Wiki and some other obscure places on the internet.

This also seems to be impacted mostly if you’re using qcow2, though I have no hard data on this at this time (only experience).

I’m going to recommend switching your caching to none or writeback, especially if you use qcow2-backed disk images.

Write performance and image formats

As summarized in my first paragraph, you’re going to want to be using either raw sparse files, LVs, or metadata-preallocated qcow2 images.

You may be asking yourself what a metadata-preallocated qcow2 image is. In a nutshell, it’s the -o preallocation=metadata flag when passed to qemu-img’s create or convert subcommands.

I still would not recommend the use of qcow2 files, unless you absolutely need snapshotting features and don’t have a LVM volume group spare. My reasoning goes that, when I converted a 66 GiB Windows Server to a sparse raw disk, the raw ended up taking up only 50 GiB. 16 GiB in metadata seems a bit much for snapshotting in my book.

I would recommend using either sparse raw disk images, or LVM logical volumes. With LVM you even get snapshots back.

Thin provisioning and dynamic sizing

This chapter isn’t much about performance and mainly about thin raw disk’s sizes and how to keep them down.

The virtio block driver does not let you set (or pass through to the VM) discard=unmap. Thus, I would recommend using the virtio-scsi SCSI controller instead, which does let you pass disk images through as discard=unmap. At that point, any VM discard commands will free up space on the host.

Final words

I hope this post can be of some use to someone, and save them of the despair of using qcow2-backed storage in writethrough mode, and wondering if their hard drives / fancy new RAID / hypervisor is dying.

Additionally I would like to ask the developers of QEMU and Libvirt to recognize these immense performance hurdles and indicate them as such, before someone makes the experiences I’ve made.