Is Proxmox's ZFS considered software RAID? Why is its read/write performance so high?

🧠 Is ZFS in Proxmox a “software RAID”?

Yes — ZFS is technically a software RAID, but it’s far more advanced than traditional mdadm-style software RAID.

ZFS isn’t just a RAID layer — it’s an integrated storage stack that combines:

File system + Volume manager + RAID + Caching + Integrity protection

This makes it much smarter and faster than both traditional software RAID and many hardware RAID controllers.

🧩 1. ZFS Architecture Overview

ZFS manages an entire storage pool (zpool) by itself.

It knows exactly which disks hold each data block, their checksums, and how to recover them — without relying on any external RAID card.

⚡ 2. Why ZFS Read/Write Performance Is So High

① ARC (Adaptive Replacement Cache)

ZFS uses RAM as an intelligent read cache, called ARC.

It tracks both “most frequently used” and “most recently used” data, and can use 50–75% of system memory by default.

So, if your server has 32 GB RAM, ZFS may use ~16 GB for caching — and cached reads hit RAM speed, not disk speed.

② ZIL / SLOG (Write Cache Layer)

All writes go first into the ZFS Intent Log (ZIL) in memory.

Then, they’re flushed to disk asynchronously.

If you add a dedicated fast SSD (SLOG), synchronous write performance becomes dramatically faster — ideal for databases or Proxmox VMs.

③ Copy-on-Write (COW)

ZFS never overwrites existing blocks.

Instead, it writes new blocks and updates pointers atomically.

Advantages:

1. No filesystem corruption
2. Instant snapshots
3. Mostly sequential writes (less random I/O overhead)

④ Built-in Compression (lz4, zstd)

ZFS automatically compresses every block before writing.

For text or database data, compression ratios can reach 2:1 or better.

→ Fewer bytes to write or read = less physical I/O, higher speed.

⑤ Intelligent Prefetch & Async I/O

ZFS performs smart prefetching — it can detect sequential read patterns and load data ahead of time.

With ARC (RAM cache) and optional L2ARC (SSD read cache), sequential reads can reach SSD-like speeds even from HDDs.

⑥ Fully Multithreaded I/O

ZFS runs with heavy multithreading for compression, checksum, prefetch, etc.

It can utilize all CPU cores — which is why on multi-core Proxmox servers, ZFS throughput often scales linearly with CPU.

⚙️ 3. Why It Often Outperforms Hardware RAID

ZFS’s power comes from using your system’s CPU + RAM as part of the storage engine — so it keeps getting faster as hardware improves.

📊 4. Example Benchmark (Typical Values)

🧮 5. Proxmox ZFS Best Practices

✅ Limit ARC memory usage (optional):

(limits ARC to 8 GB RAM)

✅ For HDD pools:

(disables Linux I/O scheduler — ZFS manages it itself)

✅ For SSDs:

1. Use a small SSD as SLOG (write log)
2. Use another SSD as L2ARC (read cache)

🏁 Summary

ZFS is a “software RAID” — but it’s also a full self-aware storage engine that unifies RAID, filesystem, caching, integrity, and snapshots.

That’s why it often outperforms hardware RAID, especially on modern multi-core, high-RAM Proxmox servers.

Would you like me to draw a “Data Flow Diagram: ZFS vs Hardware RAID vs mdadm” (showing how caching and writes differ)? It visually explains where ZFS gains its speed.