ARC stands for Adaptive Replacement Cache, and it’s one of the defining features of the ZFS file system.

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What Is ARC?

ARC is a RAM-based cache used by ZFS to store the most frequently or recently accessed data blocks. It intelligently balances:

•   Recency: data you’ve accessed recently
•   Frequency: data you access repeatedly

This makes it more effective than a traditional “least-recently-used” cache.

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Why ARC Matters

Benefit	Explanation
Massively speeds up reads	Data is read from ultra-fast RAM instead of the SSD/HDD
Reduces disk wear	Fewer reads from storage devices = longer SSD lifespan
Improves VM responsiveness	Especially useful for Windows VMs or active datasets
Self-adjusting	Grows and shrinks to fill available memory (unless capped manually)

How ARC Works with RAM

•   ZFS will use as much free RAM as possible for ARC
•   On a system with 32 GB RAM, it might use 16–24 GB for ARC if the rest isn’t needed
•   On a system with 64 GB RAM, ARC can grow much larger, storing more working data in memory

You can also add a secondary ARC (L2ARC) using a dedicated fast SSD, but the primary ARC is always in RAM.

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Should You Care About ARC?

If you use ZFS as your Proxmox storage backend or even as your Windows Server’s VM disk store:

•   ARC gives you extremely fast read performance
•   More RAM = better caching = faster Sage access, faster boots, less I/O bottleneck

If you’re not using ZFS, ARC isn’t relevant — it’s specific to ZFS-backed systems.

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Would you like a guide to enabling ZFS with ARC in your Proxmox install or a diagram showing how ARC interacts with disks and RAM?

May 25, 2025

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