9pfs (Plan 9 File System via virtio-9p) and virtio-blk are two different methods for providing storage to virtual machines, differing primarily in their level of abstraction (file-level vs. block-level).
9pfs (virtio-9p)
9pfs exposes a specific directory on the host machine directly to the guest. It is often used for "shared folders" where the host and guest need simultaneous access to the same files.
Pros:
- Elasticity: Storage is only consumed by actual files on the host; no need to pre-allocate a large image file.
- Ease of Use: Useful for development, where you want to edit code on the host and run it immediately in the guest.
Cons:
- Performance: Generally slower than block devices because every file operation must go through the 9P protocol and host system calls.
- Compatibility: While Linux support is excellent, Windows support is limited and often requires third-party drivers.
- Modern Alternative: virtio-fs is the successor to 9pfs in many modern QEMU setups, offering significantly better performance and POSIX compliance.
virtio-blk
This presents a virtual raw block device (a "hard drive") to the guest. The guest OS treats it like a physical disk and manages its own filesystem (e.g., ext4, XFS) on top of it.
Pros:
- High Performance: Optimized for low latency and high throughput. Features like IOThread Virtqueue Mapping (introduced in QEMU 9.0) allow it to scale across multiple vCPUs efficiently.
- Full Feature Support: Supports TRIM/Discard (to reclaim space), bootable partitions, and standard disk management tools.
Cons:
- Isolation: The host cannot easily "see" or edit files inside the block device while the guest is running without risking corruption.
- Fixed Size: Typically requires pre-allocation or using "sparse" image formats (like QCOW2) which can have their own performance trade-offs.
When to use which
Use 9pfs when:
- You need to share a host directory into a guest for development, simple file exchange, or testing.
- You value ease of sharing over raw performance and strict isolation.
Use virtio-blk when:
- You are provisioning a root disk or data disk for a VM.
- You care about performance, isolation, and standard disk semantics (snapshots, independent filesystems per VM).
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