The ZFS setting behind this performance question is zfs_arc_max, the cap on memory ZFS can use for its Adaptive Replacement Cache (ARC). Proxmox VE sets a default cap on newer installations, but that does not mean every host has the same value—or that raising the cap will improve a particular workload. Check the running value and available memory before changing it.
What the ZFS ARC limit controls
The ARC caches data in memory to help ZFS serve storage reads efficiently. zfs_arc_max sets the maximum size of that cache; it is a ceiling, not a command to reserve that amount of RAM. Proxmox cautions that adequate ARC memory matters for I/O performance, while also warning administrators to reduce it carefully.
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The setting is distinct from ZFS compression. Compression is a dataset property affecting new blocks written after the property is set; it does not retroactively rewrite existing blocks. Proxmox recommends lz4 for low CPU overhead and notes that compression can sometimes improve I/O, depending on how compressible the data is. Proxmox VE Administration Guide: ZFS.
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Proxmox says new installations starting with Proxmox VE 8.1 set the ARC limit to 10% of installed physical memory, with a maximum of 16 GiB. That description is specific to new installations; do not assume an older installation or an upgraded host has that value. Inspect the actual configuration and runtime value.
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The VE 9 beta installation guide also lists “ARC max size” among the advanced ZFS installation options. The installer option does not establish that every installation uses the same setting. Proxmox VE Administration Guide.
Decide whether a change is warranted
First identify the Proxmox VE release and whether the host was installed fresh or upgraded. Then compare the configured cap and actual ARC use with available host memory, guest memory pressure, and the storage workload. A low-looking cap alone does not establish that ARC is limiting performance, and increasing it can leave less memory for the host and guests.
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Proxmox gives a rule of thumb of 2 GiB base plus 1 GiB per TiB of available storage; its example for an 8 TiB pool is 10 GiB. These are vendor guidance figures, not a benchmark or guaranteed optimum. The available documentation does not establish the right value for an individual workload.
Inspect the current value
Check the runtime parameter and the module configuration before editing anything. The sysfs parameter reports the current boot’s value in bytes:
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cat /sys/module/zfs/parameters/zfs_arc_maxTo see whether a persistent module option is configured, inspect the ZFS module options file used by your system, commonly /etc/modprobe.d/zfs.conf. A configured value and the runtime value may differ until the system has been rebooted. Consult the administration guide for your installed release before editing configuration.
Apply a temporary cap
A runtime write changes the cap for the current boot only. Proxmox documents this example for a 10 GiB cap; the arithmetic produces a byte count:
echo "$[10 * 1024*1024*1024]" >/sys/module/zfs/parameters/zfs_arc_maxDo not treat 10 GiB as a recommendation for every machine. Choose a target based on the host’s memory headroom and observed workload, and keep a way to reverse the change if the host or guests become memory-constrained.
Make a cap persistent
For a persistent setting, Proxmox documents adding or changing a ZFS module option. This example sets an 8 GiB maximum, expressed in bytes:
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options zfs zfs_arc_max=8589934592- Back up the existing module-options configuration and record its current value.
- Add or update the ZFS module option with the intended byte value, following the guide for your exact Proxmox VE release.
- If the root filesystem is ZFS, run
update-initramfs -u -k allafter changing the value. - Reboot to activate the persistent module option, then check the runtime parameter again.
Proxmox warns that a maximum at or below zfs_arc_min can be ignored unless the minimum is adjusted. The guide also gives 64 MiB as the minimum ARC value. Avoid setting the maximum at or below the configured minimum; verify the interaction in the documentation for your release.
What to do if performance does not improve
Changing a memory ceiling does not prove that the cache was the bottleneck. If the host still performs poorly, reassess memory pressure and the workload rather than repeatedly raising the cap. Keep ARC tuning separate from compression changes: compression has its own CPU and data-compressibility trade-offs, and changes affect new writes rather than old blocks.
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