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Set the OpenZFS zfs_arc_max tunable to an explicit byte value that leaves enough memory for the operating system, your applications, and temporary demand. A live reduction is allowed, but the ARC can remain larger than the new limit until memory pressure causes it to reclaim pages, so verify both the configured target and actual memory behavior after changing it.
There is no universally safe ARC percentage or application-specific number. The right cap depends on installed RAM, the workload’s cache benefit, application demand, and the OpenZFS version and operating system in use.
What the ZFS ARC does and why limiting it helps
The Adaptive Replacement Cache (ARC) is OpenZFS’s in-memory cache for storage data and metadata. It expands when free memory is available and gives memory back when the system needs it. A larger ARC can improve storage performance when the workload benefits from cached reads, but that same memory is unavailable to databases, virtual machines, compilers, containers, and other processes until it is reclaimed.
Setting zfs_arc_max establishes a ceiling for ARC growth. The trade-off is direct: a lower ceiling creates more predictable headroom for applications, while potentially reducing cache hit rates and storage-workload performance. OpenZFS tuning guidance treats ARC as an adaptive cache, so the effect must be judged on the workload rather than assumed from the cap alone.
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Limiting ARC is most useful when the machine has a known competing memory requirement. Examples include a database with a defined working set, a host running several virtual machines, or a build server that periodically allocates much more memory than it uses while idle. It is not automatically necessary just because ARC appears large: a healthy ARC can occupy otherwise unused memory and later release it. First establish that memory pressure, swapping, allocation failures, or application latency is actually associated with cache growth.
Check the system before choosing a cap
Collect these facts before changing anything:
- Total physical RAM and any memory reserved by firmware, a hypervisor, or the operating system.
- The OpenZFS release and operating system. Automatic defaults changed between releases, and Linux and FreeBSD have not always used the same rule.
- The application’s steady-state memory requirement and its peak or burst requirement.
- The current ARC target and actual occupancy.
- Whether the machine is already experiencing swapping, allocation failures, latency spikes, or storage I/O pressure.
Measure during the workload that matters. An idle system can make an ARC limit look harmless, while a backup, build, query, deduplication, or virtual-machine workload may create very different memory and cache demands.
Separate physical RAM from the memory visible to applications. A virtual machine may have less usable memory than the host’s installed total, and firmware or the kernel may reserve part of the address space. The cap must be chosen against memory the running system can actually allocate, not against a label on the hardware.
Also record other consumers before blaming ARC: application caches, kernel slab memory, container limits, virtual-machine reservations, and temporary jobs can all compete for the same pages. A cap can create headroom, but it cannot compensate for an unrelated leak or an application whose peak requirement exceeds physical memory.
Understand zfs_arc_max and its defaults
zfs_arc_max is expressed in bytes. OpenZFS’s current module-parameter documentation defines 0 as the automatic setting and gives a minimum value of 67,108,864 bytes (64 MiB). An explicit value below that minimum is not a valid cap.
| Configuration or release | Automatic maximum described by OpenZFS | What it means |
|---|---|---|
zfs_arc_max = 0 on current OpenZFS (2.3.0 and later documentation) |
The larger of total system RAM minus 1 GiB and five-eighths of total system RAM | OpenZFS calculates the maximum instead of using a hand-selected byte value. |
| Older Linux behavior documented for OpenZFS 2.0 | One-half of total system RAM | This is a historical rule; do not assume it describes a current installation. |
| FreeBSD behavior described in the current documentation | The larger of total system RAM minus 1 GiB and five-eighths of total system RAM | Confirm the rule against the installed release rather than transferring Linux assumptions to FreeBSD. |
| Any explicit nonzero value | The byte value you provide, subject to the documented minimum | The ARC may grow up to that ceiling, but a lower ceiling does not instantly evict existing contents. |
The automatic rule is not a recommendation for every machine. It is simply the default calculation described for the stated versions. A host running memory-intensive applications may need a deliberately lower value; a storage-heavy host may benefit from retaining more cache.
Illustrative automatic-limit calculations
The following examples apply the current formula only to show how it works. They are not suggested caps.
| Total RAM | RAM minus 1 GiB | Five-eighths of RAM | Larger automatic value |
|---|---|---|---|
| 16 GiB | 15 GiB | 10 GiB | 15 GiB |
| 64 GiB | 63 GiB | 40 GiB | 63 GiB |
| 128 GiB | 127 GiB | 80 GiB | 127 GiB |
These are binary GiB calculations. The tunable itself still requires the equivalent integer number of bytes. For example, a nominal 8 GiB cap must be converted to an integer byte count before it is written; do not pass an unverified string such as “8G” unless the particular parameter interface explicitly documents that syntax.
The version distinction matters during upgrades. A machine that once followed the Linux half-of-RAM behavior may calculate a different automatic maximum after an OpenZFS upgrade. Record the installed release along with any tuning decision, and recheck c_max after upgrades instead of assuming the old default remains in force.
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Choose an explicit byte limit
Work backward from the memory your applications must retain. Reserve space for the operating system and filesystem services, the application’s normal working set, peak or burst allocations, and a margin for simultaneous activity. The remainder is the most you can consider giving to ARC; it is not automatically the value you should configure.
- Write down the application’s measured high-water memory requirement, not just its average.
- Add operating-system and service overhead that must remain available during that workload.
- Allow for transient demand such as compiles, checkpoints, compactions, imports, or additional virtual machines.
- Compare that total with physical RAM and the current ARC occupancy.
- Select a byte ceiling that leaves observable headroom, then test the workload and adjust only from measurements.
Do not choose a cap solely because it is a round percentage of RAM. The OpenZFS documentation does not establish a universally safe percentage, a guaranteed performance penalty, or a correct value for a particular application.
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A practical first value is one that protects the application’s measured peak plus the operating system’s required reserve, while leaving some room for unplanned bursts. Treat that value as a test hypothesis, not a permanent rule. If the workload remains stable and storage latency is acceptable, it may be suitable. If ARC pressure causes excessive reads or latency, increase it in measured steps; if applications still encounter pressure, lower it or investigate other consumers.
Do not confuse an ARC ceiling with a reservation. Setting a maximum does not allocate that amount to ARC, and it does not guarantee that every byte below the ceiling will remain free for applications. ARC can use less than the maximum, while other kernel and user-space activity still consumes memory. Verify the whole system rather than inferring free memory from the tunable alone.
Inspect the current target and ARC occupancy on Linux
OpenZFS identifies the c_max statistic in /proc/spl/kstat/zfs/arcstats. As root or with permission to read the file, inspect that entry to see the current maximum target:
grep '^c_max' /proc/spl/kstat/zfs/arcstatsUse the current zarcstat utility to observe ARC statistics while the workload runs. Before OpenZFS 2.4.0, the utility was named arcstat; use the name shipped with your installation. Compare the configured target with the cache’s current occupancy and with system-level free memory, swap activity, and application behavior.
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The target and the current occupancy answer different questions. A target tells you how large ARC is allowed to become. Occupancy tells you how much memory it is using at that moment. Both are needed to determine whether a cap has actually created headroom.
Take observations over time rather than relying on one sample. Note values before the change, while the competing application reaches its high-water mark, and after the workload subsides. A target may be correct even when occupancy has not yet fallen, because reclamation waits for pressure. Conversely, a low occupancy reading during an idle interval does not prove that the cap is sufficient for the next burst.
Apply a live cap
OpenZFS documents zfs_arc_max as dynamically changeable, with caveats. Use the module-parameter interface supported by your operating system and installed OpenZFS release, and provide the limit as an integer byte value. Perform the change with the privileges required by that interface.
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- Record the existing
c_maxvalue and current ARC occupancy. - Convert your selected GiB or MiB limit to bytes and check that it is at least 67,108,864 bytes.
- Apply the nonzero value through the release’s supported runtime ZFS module-parameter mechanism.
- Re-read
c_maxand monitor ARC occupancy, free memory, swap, application latency, and storage performance. - Test the actual peak workload rather than relying on an idle observation.
If the new maximum is below the ARC’s current occupancy, the ARC can remain above the target temporarily. The setting does not itself force an immediate purge; memory pressure is what induces shrinking. A delayed reduction therefore does not prove that the setting failed.
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The documentation also states that the setting cannot be returned to 0 dynamically. If you need to restore the automatic calculation, plan the change using the reboot or release-specific procedure documented for your platform rather than assuming that writing zero at runtime will work.
Make the limit persistent across reboots
A live change normally affects the running kernel only. To keep an explicit cap after reboot, configure the ZFS module or equivalent startup mechanism used by your operating system and distribution. The exact persistence file and syntax vary by Linux distribution, OpenZFS release, and FreeBSD version; there is no single path or command that is correct for all of them.
After editing the platform’s configuration, reboot during a maintenance window or use the supported reload procedure, then verify the result by checking c_max again. If the value reverted to the automatic setting, the startup configuration was not loaded, was overridden by another module option, or was written for a different ZFS installation.
Keep the configured byte value, software version, date, and reason for the cap in your system documentation. This prevents a future administrator from treating a workload-specific limit as a universal ZFS requirement, and it makes version changes easier to audit.
Verify that the change solved the memory problem
Verification should cover both memory reclamation and application outcomes:
- Confirm that the configured maximum equals the byte value you intended.
- Watch whether ARC occupancy falls when the application reaches its peak demand.
- Check that swap use, allocation failures, or out-of-memory events improve rather than merely moving to another process.
- Measure application latency and throughput during representative storage activity.
- Look for a sustained increase in storage reads or other I/O caused by a smaller cache.
Do not declare success immediately after writing the parameter. The ARC may need real pressure before it shrinks, and a cap that prevents swapping on one workload can still make another workload slower.
Use matched comparisons where possible: run the same query, build, backup, or virtual-machine workload before and after the change, with similar data and concurrency. Compare both the application’s memory behavior and storage metrics. A successful cap is one that removes the original pressure without introducing an unacceptable cache-related bottleneck.
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Related ARC pressure controls
These tunables influence how ARC responds to pressure, but they do not replace selecting an appropriate maximum.
| Tunable | Documented behavior | When to consider it |
|---|---|---|
zfs_arc_sys_free |
Sets a target amount of system free memory that ARC should leave available. With zero, the effective target is the larger of 512 KiB and total system memory divided by 64. | When you need ARC to preserve a defined free-memory reserve and have evidence that the default behavior is insufficient. |
zfs_arc_grow_retry |
Controls how long ARC growth waits after a memory-pressure event. Zero uses the built-in five-second default. | When growth timing after pressure is the observed issue, not simply excessive maximum size. |
zfs_arc_lotsfree_percent |
Throttles I/O when free memory falls below a percentage of total system memory. | For specialized pressure behavior tied to observed free-memory thresholds. |
zfs_arc_no_grow_shift |
Prevents ARC growth when free memory is below a fraction of the ARC target. | Only when the documented growth-stop behavior matches a diagnosed pressure problem. |
Changing several pressure controls at once makes cause and effect difficult to identify. Start with the maximum, measure, and change a secondary control only when the symptom clearly matches its documented purpose.
These controls are not substitutes for application sizing. For example, changing the growth retry interval cannot make an application with an insufficient working set fit in RAM, and a free-memory threshold cannot guarantee a particular ARC hit rate. Use each tunable for the behavior it documents.
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The reported maximum did not change
Check that you changed the ZFS instance actually serving the pools, used a byte value rather than a human-readable suffix, and applied the setting through the interface supported by the installed release. Re-read c_max instead of relying on the command’s lack of an error message.
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This is expected when the new ceiling is below current occupancy. Allow representative memory pressure and continue observing occupancy. The documented behavior is gradual reclamation rather than an immediate purge.
Applications have more headroom, but storage performance fell
A smaller ARC can reduce the amount of data and metadata retained in memory. Compare application memory stability with storage latency and I/O during the same workload. If the application has excess headroom, raise the cap incrementally and measure again; if memory pressure remains, retain the lower ceiling or reduce another source of demand.
The setting disappears after reboot
Recheck the distribution’s module configuration and boot-time ordering, then verify the value after restart. Runtime success does not prove that a persistent configuration exists.
Documentation appears to disagree with the machine
Identify the OpenZFS version and operating system first. The historical Linux half-of-RAM rule differs from the current v2.3.0-and-later calculation, and utilities changed names in OpenZFS 2.4.0. Apply the rule and tool name for the software actually installed, not for a different manual page.
The cap is valid but pressure continues
Confirm that the application is using the expected amount of memory and inspect other consumers. A lower ARC limit cannot solve a leak, an undersized virtual-machine host, or a workload whose peak demand already exceeds physical RAM. Check swap and allocation failures while the application is busy, not only after the job completes.
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The machine has plenty of free memory but the cap seems ineffective
ARC may not shrink simply because a lower target was written. Reclamation is induced by pressure, so an idle test can leave occupancy unchanged. Exercise the workload that previously caused pressure, then compare occupancy and application behavior.
A practical decision framework
Use this sequence when deciding whether to keep, lower, or raise a cap:
- If applications are meeting their memory targets and ARC is not causing pressure, leave the current setting alone.
- If applications need more RAM, establish their peak requirement and set an explicit cap that leaves that amount plus system and transient headroom.
- If the cap prevents pressure but causes unacceptable storage slowdown, test a modestly higher value while watching application and I/O metrics together.
- If the machine still enters pressure with the cap, inspect other memory consumers and consider the specialized ARC pressure controls only when their documented behavior matches the symptom.
- Document the chosen byte value, software version, workload conditions, and verification results so a later upgrade does not silently change the assumptions.
The reliable outcome is not the smallest ARC. It is a measured balance in which applications retain the memory they require and ARC remains large enough for the storage workload to perform acceptably.
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Frequently Asked Questions
What is the minimum value for zfs_arc_max?
OpenZFS documents a minimum of 67,108,864 bytes, or 64 MiB. The tunable is expressed in bytes, so convert any MiB or GiB choice before applying it.
Will lowering zfs_arc_max immediately free RAM?
Not necessarily. A live reduction is allowed, but ARC can remain above the new target until memory pressure causes it to shrink. Check occupancy over time under the workload that needs the memory.
What does zfs_arc_max=0 mean?
Zero selects the automatic maximum. Current OpenZFS documentation for version 2.3.0 and later describes that maximum as the larger of total RAM minus 1 GiB and five-eighths of total RAM. Older Linux documentation described half of RAM, so verify the installed version and platform.
How do I verify the ARC limit on Linux?
Read the c_max entry in /proc/spl/kstat/zfs/arcstats. Use zarcstat to watch ARC statistics; releases before OpenZFS 2.4.0 used the name arcstat.
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The documented behavior says the setting cannot be returned to zero dynamically. Use the reboot or platform-specific procedure documented for your OpenZFS installation if you need to restore the automatic calculation.
Is there a recommended percentage of RAM for ARC?
No universal percentage is established. Choose a byte ceiling from measured application peaks, operating-system overhead, transient demand, and acceptable storage performance, then validate it under load.
Why did storage performance decline after capping ARC?
A smaller ARC retains less data and metadata, which can lower cache effectiveness and increase storage I/O. Compare application memory stability with storage latency and throughput, then test a modestly higher cap if the application has sufficient headroom.
Do zfs_arc_sys_free and zfs_arc_grow_retry replace zfs_arc_max?
No. They influence free-memory reserves and growth timing. zfs_arc_sys_free has a documented zero-value target of the larger of 512 KiB and total RAM divided by 64, while zero for zfs_arc_grow_retry uses a five-second default. Neither chooses an appropriate maximum for your workload.
The Bottom Line
Set zfs_arc_max to a measured, explicit byte value that protects application and system headroom, then verify c_max, ARC occupancy, memory pressure, and storage performance under real load. Expect gradual reclaim, account for version-specific defaults, and make persistence platform-specific rather than assuming one universal command.
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