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Random freezes, missing sound and display glitches usually trace back to one bad driver. Find and replace yours safely.Free scan · under a minuteSet the OpenZFS zfs_arc_max tunable to an explicit byte value that leaves enough memory for the operating system and your applications. Apply the change at runtime, verify the target and actual ARC size, and expect the cache to shrink gradually rather than instantly if it is already larger than the new limit. The right cap depends on your installed OpenZFS version, total RAM, application peaks, and storage workload—not on a universal percentage.
What zfs_arc_max controls
The ZFS Adaptive Replacement Cache (ARC) stores frequently accessed filesystem data and metadata in RAM. It expands when memory is available and gives memory back when the system is under pressure. zfs_arc_max is the upper limit for that cache, expressed in bytes.
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A value of 0 means OpenZFS chooses the maximum automatically. The documented minimum explicit value is 67,108,864 bytes (64 MiB). A cap does not disable ARC; it limits how much RAM ARC can claim, trading some potential cache hit rate for more predictable headroom for databases, virtual machines, compilers, containers, or desktop applications.
The limit is a ceiling, not a reservation. ARC can use less than the ceiling when its working set is small, and setting a large ceiling does not guarantee that all of that memory is occupied. Conversely, lowering the ceiling does not turn the difference into instantly available application memory if ARC is still holding data. Treat the change as a change to future cache growth and reclamation behavior.
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How the automatic limit differs by OpenZFS version
Do not assume that every host reserves the same fraction of RAM. The automatic rule changed between releases and differs in older platform documentation.
| OpenZFS/platform context | Automatic zfs_arc_max rule |
|---|---|
| OpenZFS 2.3.0 and newer documentation | The larger of total system RAM minus 1 GiB, or five-eighths (62.5%) of total RAM. |
| Older Linux behavior documented for OpenZFS 2.0 | One-half of total RAM. |
| Older FreeBSD behavior described in the same documentation | The larger of total RAM minus 1 GiB, or five-eighths of total RAM. |
For example, on an 8 GiB host the two current expressions are 7 GiB and 5 GiB, so the larger result is 7 GiB. On a 2 GiB host they are 1 GiB and 1.25 GiB, so 1.25 GiB wins. These are illustrations of the rule, not recommendations for either machine.
Check the installed OpenZFS release and operating system before comparing a host with advice written for another version. Setting an explicit byte limit removes that particular version-dependent ambiguity. Also remember that a distribution may package a newer or older module than the userland utilities suggest; verify the effective loaded module value.
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There is no universally safe ARC percentage for every application. Start with the memory your non-ZFS workload actually needs, then leave additional room for the kernel, filesystem activity, and short-lived peaks. The cap should be low enough to prevent swapping or allocation failures, but high enough that storage-heavy workloads retain useful cache.
Inventory the memory consumers
- Total physical RAM and current free or available memory.
- Steady-state and peak resident memory for the applications that must remain responsive.
- Kernel, container, virtual-machine, and filesystem overhead that is not visible in one application process.
- Transient demand such as compiles, backups, snapshots, deduplication workloads, or concurrent virtual machines.
- Current ARC target and occupancy, rather than only the configured value.
- Swap use and allocation failures during the busiest period, not just an idle reading.
Use a byte value with an explicit safety margin
Convert the cap to bytes before applying it. For example, a 16 GiB cap is 17179869184 bytes (16 × 1024 × 1024 × 1024). That is an example of the unit conversion, not a recommendation for a particular machine. Select the number only after measuring the application requirement and observing memory pressure.
When in doubt, make a conservative reduction, observe the result under the real workload, and adjust in stages. A lower ARC limit can reduce cache hits and increase storage I/O; a higher limit can leave too little headroom for applications even if the machine appears to have abundant total RAM. Make changes in one direction at a time so you can relate a latency or memory change to the cap you just altered.
Do not size the cap by subtracting only the largest application’s nominal requirement from total RAM. The operating system, ZFS metadata, services, guests, and bursts all need room. Conversely, do not assume every byte of ARC is permanently unavailable: ARC is adaptive and is designed to release memory under pressure. The purpose of an explicit cap is to put an upper bound on that adaptive behavior when the automatic policy is too generous for your workload.
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Inspect the current target and actual ARC size on Linux
OpenZFS exposes ARC statistics through /proc/spl/kstat/zfs/arcstats. The c_max entry is the current maximum target. Compare it with the ARC’s current occupancy, commonly shown by the size entry.
grep -E '^(c_max|c_min|size) ' /proc/spl/kstat/zfs/arcstatsTo see the module parameter itself, read the value exported by the loaded ZFS module:
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cat /sys/module/zfs/parameters/zfs_arc_maxThe current command-line monitor is zarcstat. On OpenZFS releases before 2.4.0, the utility was named arcstat.
zarcstat 1Use a sampling interval that covers both idle and busy periods. A single reading can catch ARC while it is growing, shrinking, or waiting for demand and therefore says little about the application’s peak behavior. If the command is not installed, continue using arcstats and ordinary system memory monitoring. A target value alone does not prove that ARC has already reclaimed memory; always compare the target with actual occupancy and the application’s behavior.
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Apply a live limit on Linux
A running system can change zfs_arc_max without unmounting pools. The following example sets a 16 GiB maximum; replace the byte value with the limit calculated for your host.
sudo sh -c 'echo 17179869184 > /sys/module/zfs/parameters/zfs_arc_max'
cat /sys/module/zfs/parameters/zfs_arc_maxWriting the value requires root privileges. If the parameter path does not exist, the ZFS module may not be loaded, the host may use a different packaging layout, or ZFS may not be the active filesystem module. Resolve that platform issue before treating the write as successful. A shell redirection must run with elevated privileges; putting sudo only before echo does not grant permission to open the parameter file, which is why the example elevates the shell.
Immediately recheck the statistic:
grep -E '^(c_max|size) ' /proc/spl/kstat/zfs/arcstatsThe new maximum can be below the ARC’s current size. OpenZFS documents that reducing the maximum does not force an instant contraction; the cache may remain above the new target until memory pressure causes it to shrink. Do not interpret an unchanged size value immediately after the write as proof that the setting failed. Observe it while the application allocates memory or another legitimate workload creates pressure.
A live change is useful for a controlled test because it avoids committing an unproven value to boot configuration. Record the old value first, test the real workload, and persist the setting only after you know the balance between application headroom and storage performance is acceptable.
Apply a live limit on FreeBSD
FreeBSD exposes the tunable through sysctl. A typical live change is:
sudo sysctl vfs.zfs.arc_max=17179869184Use the installed release’s ZFS parameter names and inspect the returned value after the command. The same shrink behavior applies: a lower target can take time to result in lower occupancy, and the cache may only release enough memory when the system needs it. If the returned value differs from the requested one, stop and verify the parameter’s documented units and bounds for that release.
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Make the limit persistent across reboots
A runtime write normally lasts only until the next unload or reboot. Persistence is operating-system and distribution specific, so verify the mechanism used by your release rather than copying a path blindly.
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Many Linux distributions load module options from a file under /etc/modprobe.d/. A commonly used pattern is:
options zfs zfs_arc_max=17179869184Save that line in the distribution’s ZFS module-options file, regenerate the initramfs if the distribution requires module options there, and reboot during a maintenance window. After reboot, confirm both the module parameter and c_max in /proc/spl/kstat/zfs/arcstats. Some systems load ZFS through a different early-boot mechanism, so the exact file and initramfs command are not universal.
Keep the value in a change record along with the OpenZFS version and the reason for the cap. Package upgrades can replace modules, alter early-boot ordering, or change the automatic default. A post-upgrade check should therefore read the live parameter, not merely confirm that a configuration file still contains the line.
FreeBSD system configuration
FreeBSD commonly persists tunables in /etc/sysctl.conf with a line such as:
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vfs.zfs.arc_max="17179869184"Reload or reboot according to the release’s administration procedure, then verify the effective value. Keep the configured value in your change-management notes so a future upgrade does not silently restore the automatic setting.
Restoring automatic sizing
The documented runtime behavior does not allow changing an active value back to 0. To return to automatic sizing, remove or change the persistent override and reboot (or use the platform’s supported module reload procedure when safe). Verify after startup that the automatic rule for your OpenZFS version is active. If you need a temporary experiment, plan the rollback before applying the cap so you do not mistake an unsupported live write of zero for a successful restoration.
Verify that applications actually gained headroom
- Record the configured target from the module parameter or platform tunable.
- Record
c_maxand the current ARCsize. - Observe available memory and swap activity while the application reaches its normal peak workload.
- Run the storage workload that matters to you and watch for increased latency, I/O, or reduced cache hit behavior.
- Leave the cap in place only if application headroom improves without unacceptable storage-performance loss.
Compare measurements taken before and after the change under similar workload conditions. An idle host can make any cap appear harmless because neither applications nor ARC are competing for memory. Include backups, snapshots, compilation, guest boot, or other events that previously caused pressure. If memory headroom improves but storage latency rises, the result is a trade-off to tune, not a binary success or failure.
A lower ARC target is successful when it provides the intended memory margin under the real workload. It is not necessary for ARC to equal the cap; the cache is adaptive and can remain below the maximum when demand is low.
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Related ARC controls and when to leave them alone
zfs_arc_max is the primary capacity control. Other tunables change how ARC responds to pressure, but they do not replace selecting an appropriate maximum.
| Tunable | Documented behavior | Practical interpretation |
|---|---|---|
zfs_arc_sys_free |
Target amount of system free memory ARC should leave available. With 0, the effective target is the larger of 512 KiB and total RAM divided by 64. |
Can influence when ARC begins giving memory back; it is not a substitute for an application-sized maximum. |
zfs_arc_grow_retry |
Controls how long ARC waits after a memory-pressure event before trying to grow again. A value of 0 uses the built-in five-second default. |
Changes growth timing, not the maximum amount of ARC. |
zfs_arc_lotsfree_percent |
Throttles I/O when free memory falls below a percentage of total RAM. | Use only when observed low-memory behavior justifies pressure-based I/O throttling. |
zfs_arc_no_grow_shift |
Prevents ARC growth when free memory is below a fraction of the ARC target. | A specialized pressure response; changing it without measurements can make behavior harder to predict. |
zfs_arc_sys_free is expressed as a memory target, while zfs_arc_lotsfree_percent uses a percentage and zfs_arc_no_grow_shift uses a fraction-related threshold. Do not mix their units when reviewing a configuration. Change one control at a time and document the old value. Otherwise, a later performance change becomes difficult to attribute.
Troubleshoot common results
The value changes, but ARC remains large
This is expected when the new target is below current occupancy. Give the system a realistic workload that creates memory demand and monitor size again. A live cap is not an immediate purge command. Avoid forcing a purge simply to make a graph move; the documented behavior is pressure-driven reclamation.
The write is rejected or the value snaps back
Check that the command ran with root privileges, the ZFS module is loaded, and the value is at least 67,108,864 bytes. Also check whether an automated boot configuration or management tool is overwriting the parameter. Confirm that you are writing the parameter for the active module and that the requested number is in bytes.
Applications still run out of memory
Confirm that the effective target, rather than only a configuration file, is lower after reboot. Check for other consumers such as virtual machines, containers, kernel slabs, or a workload spike. If ARC has not reclaimed yet, continue observing under pressure; do not assume the configured cap is immediately available as application RAM. A cap cannot compensate for an application whose own peak requirement already exceeds the machine’s practical capacity.
Storage performance gets worse
Reducing ARC removes cache capacity. If the workload is read-heavy or metadata-heavy, a cap that is too low can increase device reads and latency. Raise the limit in measured increments, or keep the lower value only if application reliability is worth the performance trade-off. There is no documented universal hit-rate loss or fixed percentage penalty to apply in advance; measure the workload that matters.
The setting disappears after a reboot or upgrade
Persistence is distribution and release specific. Inspect the boot-time module or sysctl configuration, regenerate any required initramfs, reboot, and verify the live value rather than relying on the presence of a file. Recheck after OpenZFS upgrades because automatic sizing rules and utility names can change between releases.
The monitoring tool name is unfamiliar
Use zarcstat on current releases; documentation for versions before OpenZFS 2.4.0 calls the utility arcstat. If neither is available, the kernel statistics file remains the authoritative place to inspect entries such as c_max and size on Linux.
A repeatable operating procedure
- Identify the operating system, OpenZFS version, total RAM, and the application’s peak memory requirement.
- Capture the current
zfs_arc_max,c_max, ARCsize, and application memory behavior. - Choose an explicit byte cap that leaves room for the operating system, applications, and transient demand.
- Apply the cap at runtime using the platform’s supported interface.
- Verify the target immediately and understand that ARC may shrink later under pressure.
- Exercise the real workload while watching application memory, swap, ARC size, and storage performance.
- Persist the value using the operating system’s module or sysctl configuration, then verify again after reboot.
- Adjust only after measurements show that the current balance between application headroom and cache performance is unsuitable.
Frequently asked questions
Can I set zfs_arc_max to zero to free memory?
Zero means automatic sizing, not a zero-sized ARC. It delegates the maximum to OpenZFS’s version-specific automatic rule. The documented active-value behavior also does not support changing a live setting back to zero; remove the override and reboot to restore automatic sizing.
Does lowering the cap immediately free the difference?
No. The target can change while current ARC occupancy remains above it. Reclamation occurs as memory pressure causes ARC to shrink, so verify both c_max and size while the workload is active.
What is the minimum value I can configure?
Current OpenZFS module documentation gives a minimum explicit value of 67,108,864 bytes, or 64 MiB. A value below that may be rejected or otherwise fail to provide the intended setting.
Should every server use the same ARC percentage?
No. Application peaks, kernel overhead, guest workloads, storage access patterns, and OpenZFS version all matter. Choose a byte ceiling from measured requirements and validate it under realistic pressure.
Which value should I monitor: the parameter, c_max, or size?
Use all three when troubleshooting. The module parameter or platform tunable shows what was requested, c_max shows the current ARC maximum target, and size shows current occupancy. Comparing them distinguishes a configuration problem from normal delayed shrinkage.
Do the related ARC tunables replace an ARC maximum?
No. zfs_arc_sys_free, zfs_arc_grow_retry, zfs_arc_lotsfree_percent, and zfs_arc_no_grow_shift alter pressure or growth behavior. They are specialized controls and should be changed only for an observed problem; they do not provide a workload-sized capacity limit by themselves.
Frequently Asked Questions
Can I set zfs_arc_max to zero to free memory?
No. Zero selects automatic sizing. Remove a persistent override and reboot to restore that automatic policy.
Why is ARC still above the new limit?
A live reduction changes the target but does not force an immediate purge. ARC can remain above it until genuine memory pressure causes shrinkage.
What is the minimum explicit ARC maximum?
OpenZFS documentation specifies 67,108,864 bytes (64 MiB).
How do I know whether the cap helped?
Compare the configured target, c_max, current size, application memory and swap behavior, and storage performance during the same peak workload.
Is there a standard percentage of RAM to reserve for ARC?
No. Select a byte value from measured application, operating-system, guest, and transient memory requirements, then validate cache performance.
The Bottom Line
Choose zfs_arc_max from measured workload needs, not a universal percentage. Set an explicit byte value at runtime, verify the module target and actual ARC occupancy, and remember that reclamation is pressure-driven. Persist the setting with the mechanism used by your Linux distribution or FreeBSD release, then recheck after reboot and after upgrades. If applications gain the required headroom without unacceptable storage impact, the cap is doing its job.
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