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If your Proxmox host uses ZFS, the setting worth checking is zfs_arc_max, the maximum size of ZFS’s Adaptive Replacement Cache (ARC). New Proxmox VE installations starting with version 8.1 use a default limit of 10% of installed physical memory, capped at 16 GiB. That is an installation default—not proof that your host is using that value, short of memory, or in need of a higher cap.
What the ARC limit controls
ZFS uses ARC to cache data in memory. zfs_arc_max sets the maximum memory the cache may use; it is a ceiling, not a command to reserve that amount immediately. Proxmox’s VE 9 beta installation guide also lists “ARC max size” among the installer’s advanced ZFS options.
Proxmox’s VE Administration Guide cautions: “Allocating enough memory for the ARC is crucial for IO performance, so reduce it with caution.” That makes the cap relevant to investigate, but it does not establish that ARC is the bottleneck on any particular host or that raising the cap will improve a workload.
Check your host before tuning
Establish which default may apply
Find the installed Proxmox VE version and whether the machine was newly installed or upgraded. The documented 10%-of-memory limit, capped at 16 GiB, applies to new installations starting with VE 8.1; it should not be assumed for older installations or upgraded systems. Check the actual configuration instead of inferring it from the version alone.
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Read the active limit
On the Proxmox host, inspect the runtime parameter:
cat /sys/module/zfs/parameters/zfs_arc_maxThe value is expressed in bytes. A value of 0 means there is no explicit maximum set through this parameter; it does not mean ARC uses no memory. To look for a persistent module option, inspect the ZFS-related configuration under /etc/modprobe.d/ and check for a line setting zfs_arc_max. Runtime and persistent settings can differ, so check both.
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Look for evidence of pressure
Compare ARC behavior with available host memory, guest memory pressure, and what the workload is doing. A host that is short of memory for guests is a different problem from a workload that benefits from a larger cache. The available Proxmox guidance gives configuration rules of thumb, not a diagnostic test or a performance benchmark; do not treat the default cap by itself as evidence of a performance problem.
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Proxmox’s VE 8 Administration Guide gives a rule of thumb of 2 GiB base plus 1 GiB per TiB of available storage. It illustrates the calculation with 8 TiB of available pool storage, yielding 10 GiB for ARC. The same guide identifies 64 MiB as a minimum ARC value.
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These figures are vendor guidance, not a guaranteed optimum, a benchmark result, or a recommendation that every host should use the calculated amount. Available memory, guest needs, the workload, and the installed release all matter. Use the rule as context for reviewing a configuration, not as a substitute for observing the machine.
Change the cap only with a reason and a recovery plan
Temporary change for the current boot
Proxmox documents this runtime example for setting a temporary 10 GiB maximum; the expression writes the value in bytes:
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echo "$[10 * 1024*1024*1024]" >/sys/module/zfs/parameters/zfs_arc_maxA runtime write applies only for the current boot. The 10 GiB value is an example, not a universal recommendation. Choose a target based on the host’s memory headroom and observed workload, and consult the guide for the exact Proxmox VE release before applying a change.
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Persistent change
For a persistent limit, the Proxmox guide documents a ZFS module option like this:
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options zfs zfs_arc_max=8589934592This example sets an 8 GiB cap. Add or change the option in the appropriate ZFS module configuration for your host, then reboot to activate the persistent setting. If the root filesystem is ZFS, Proxmox instructs administrators to update the initramfs whenever the value changes:
update-initramfs -u -k allRetain a recovery path before changing boot-related configuration, and verify the runtime value after reboot. If the requested maximum is at or below zfs_arc_min, Proxmox warns that the maximum may be ignored unless the minimum is adjusted. Avoid setting the two limits without checking their interaction.
Do not confuse ARC sizing with compression
ZFS compression is a separate setting, not another way to set the ARC cap. Proxmox recommends lz4 for low CPU overhead and notes that compression can sometimes improve I/O, depending on how compressible the data is. Changing a compression property affects newly written blocks; it does not recompress existing data retroactively. Evaluate compression as a separate workload-dependent choice rather than changing it as a proxy for ARC tuning.
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