🧠 PVE Disk Cache Cheat Sheet

VM Type / Workload	Recommended Cache Mode	Why	Notes
Windows VM (general desktop, small random I/O)	writeback	Better write performance; Windows often has less efficient disk caching	Slightly higher risk of data loss if host crashes; good with UPS; add iothread=1 and discard=on for SSDs
Linux VM (server workloads, ext4/XFS/ZFS)	none	Bypasses host cache, Linux handles caching itself efficiently	Slightly lower write performance than writeback, safer for integrity; add discard=on for SSDs
Linux VM (high-performance DB, heavy writes, sync important)	writeback or directsync	Improves throughput while maintaining some safety	directsync safer for transactional DBs; writeback for analytical workloads; always use UPS
VM with mostly read workloads	none or directsync	Avoids double caching; minimal performance impact for reads	Almost same performance as writeback for reads; much safer; add discard=on for SSDs
Mixed workloads on SSD/NVMe	writeback with iothread	Maximizes performance on fast storage	iothread helps NVMe most, but also useful for fast SSDs; always use discard=on
Critical Data VM (business essential, cannot lose data)	none or writethrough	Maximum data integrity and safety	writethrough provides read caching with write safety; performance secondary to reliability
Development/Test VM (disposable or frequently backed up)	writeback or unsafe	Maximum performance for testing	unsafe only for truly disposable data; regular backups recommended for writeback

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⚙️ Essential Parameters for All Configurations

• SSD/NVMe: Always add discard=on (enables TRIM)
• Performance: Always add iothread=1 for virtio-scsi disks
• Safety: Use UPS, especially with writeback or unsafe modes

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🧩 Quick Cache Mode Reference

Cache Mode	Description	Safety	Performance
none	Direct disk access, bypasses host cache	🔒 Highest	⚙️ Moderate
writethrough	Safe writes + read caching	🔒 High	⚙️ Moderate
directsync	Sync writes direct to disk	🔒 High	⚙️ Moderate
writeback	Host page cache, delayed flush	⚠️ Medium	🚀 High
unsafe	No sync flush, full cache	❌ Very low	⚡ Maximum

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💡 Notes for ZFS/Btrfs Users

If your Proxmox storage backend uses ZFS or Btrfs, the cache parameter has minimal effect since these filesystems manage caching internally.
However, cache=none is still the safest and most compatible choice.

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✅ Summary

• Use writeback for performance-sensitive Windows or test workloads.
• Use none or writethrough for critical data and Linux servers.
• Always enable discard=on for SSD/NVMe.
• Always use iothread=1 on high-speed storage.
• Protect with UPS if using writeback or unsafe.

examples

• Windows VM

qm set 101 -scsi1 /dev/nvme0n1p2,cache=writeback,discard=on,iothread=1

• Linux VM

qm set 102 -scsi1 /dev/nvme0n1p3,cache=none,discard=on,iothread=1