VM disks: VirtIO SCSI, cache, discard, IO threads

Why this matters in production

A VM disk’s performance is dominated by the combination of disk controller, cache mode, and underlying storage. Misconfiguring any of these produces either data loss or slow I/O.

Disk controllers

qm set 100 --scsihw virtio-scsi-single

Cache modes

qm set 100 --scsi0 local-zfs:vm-100-disk-0,cache=writethrough

Discard / TRIM

Enabling discard=on allows the guest to issue TRIM commands, which the host forwards to the underlying storage. Critical for SSD performance and thin-provisioned storage.

qm set 100 --scsi0 local-zfs:vm-100-disk-0,discard=on

In the guest, ensure TRIM is enabled:

systemctl enable --now fstrim.timer

IO threads

For multi-queue performance, use IO threads:

qm set 100 --iothread 1

Each disk can be assigned to a specific thread for parallel I/O.

SSD emulation

For older OSes that don’t have VirtIO drivers, you can emulate an SSD:

qm set 100 --scsi0 local-zfs:vm-100-disk-0,ssd=1

This signals the guest that the disk is an SSD, enabling the guest’s SSD-optimised I/O scheduler.

Production considerations

Common mistakes

• Using cache=writeback without guest agent or without frequent backups.
• Disabling discard “to save CPU.” The SSD performance cost is worse.
• Using IDE for new VMs.

Key takeaways

• VirtIO SCSI single is the modern default.
• Cache mode balances performance vs durability; document the choice.
• TRIM is mandatory for SSD performance.

Knowledge check