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When an application needs RAM, ZFS can give memory back by evicting entries from its Adaptive Replacement Cache (ARC). The evicted blocks are cache copies, not the only copies of your data, so reclaiming ARC does not delete files or pool contents. The change can be gradual: lowering an ARC limit does not, by itself, force an immediate drop unless the operating system is under memory pressure.
ARC is reclaimable cache, not your pool’s only data copy
The ARC is ZFS’s in-memory cache for recently and frequently used data and metadata. ZFS keeps the authoritative blocks on storage; ARC holds copies to avoid repeated device reads. The FreeBSD Handbook explains that blocks are stored in compressed form in ARC and decompressed when accessed. You can review the architecture in the FreeBSD Handbook’s ZFS chapter.
Because ARC is cache, eviction changes performance rather than data integrity. A block removed from ARC must be read from the pool again when a workload requests it. Frequently reused data may then be cached again, while cold data can remain out of memory without any user-visible problem.
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What happens during application memory pressure
- An application requests pages. A database, virtual machine, jail, or ordinary process allocates memory for its own working set.
- The operating system detects less free memory. Reclaim mechanisms look for memory that can be released without discarding private application data.
- ARC becomes a reclaim target. ZFS can evict cached data and metadata, reducing the amount of RAM devoted to the cache.
- Applications receive more headroom. The reclaimed pages can be used for application allocations, filesystem activity, or other kernel needs.
- Later reads may be slower. Requests for evicted blocks come from storage instead of RAM until those blocks become ARC candidates again.
The FreeBSD documentation describes this as ARC yielding memory under pressure. It does not promise a fixed reclaim speed. A warm cache can release memory over time, and the interval depends on the operating system, OpenZFS version, installed RAM, workload, and severity of pressure.
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Why a large ARC reading is not automatically a fault
ARC grows as demand justifies it, up to its configured maximum. On a storage-focused host with few competing consumers, using a large share of RAM for cache is expected behavior: otherwise that memory would sit idle while repeated reads go to storage.
Interpret an ARC value together with its target maximum and the rest of the system. A current size below the maximum has at least two plausible explanations:
- The workload has not generated enough cacheable activity for ARC to reach the limit.
- Other memory demand caused reclaim, so ARC was reduced.
Operating-system pressure indicators, application latency or out-of-memory events, and ARC hit and miss counters help distinguish those cases. A single snapshot cannot establish that ARC is starving applications or that the host is healthy.
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OpenZFS documents zfs_arc_max as a dynamic maximum in bytes. Lowering it changes the ceiling that ARC should observe, but setting it below the current ARC size does not itself force immediate eviction. The cache shrinks when memory pressure or reclaim activity makes that necessary. The parameter documentation also states that a running system cannot change the value back to zero; zero has the special meaning of deriving a limit from installed memory at initialization.
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FreeBSD uses the corresponding sysctl name vfs.zfs.arc.max. The Handbook gives vfs.zfs.arc.max="8G" as an illustrative cap, says a sysctl change takes effect immediately, and cautions that a warm ARC frees memory gradually. The 8 GB value is an example of syntax and behavior, not a generally correct limit.
Do not confuse “the new maximum is active” with “the cache is already that small.” Verify both the configured target and the current size after a change.
Version and platform differences matter
Parameter names and default calculations vary by operating system and OpenZFS release. Check the current OpenZFS module-parameter documentation for the release installed on your host before applying advice copied from an older guide.
| Item | Documented behavior | Important qualification |
|---|---|---|
zfs_arc_max |
Maximum ARC size, in bytes; can be changed dynamically. | Reducing it below current ARC size does not cause shrinkage without memory pressure. It cannot be returned to zero while running. |
Default when zfs_arc_max=0 |
Since OpenZFS 2.3.0, the documented limit on every platform is the larger of installed memory minus 1 GiB and five-eighths of installed memory. | Older Linux and FreeBSD releases used different calculations, so older “half of RAM” advice is not a universal current rule. |
zfs_arc_sys_free |
Target number of bytes ARC should leave free for the system; it is dynamically changeable. | When set to zero, the documented target is the greater of 512 KiB and one-sixty-fourth of system memory. |
zfs_arc_shrinker_seeks |
Linux control that changes ARC eviction cost relative to other kernel subsystems; documented default is 2. | It is configured before module load and is not a portable FreeBSD setting. |
FreeBSD presents these controls through vfs.zfs.arc.max and related kstat.zfs.misc.arcstats sysctls. Linux OpenZFS exposes module parameters such as zfs_arc_max. Substituting one platform’s name for the other can lead to a failed setting or, worse, a change to a different control than intended.
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How to monitor ARC on FreeBSD
The FreeBSD Handbook says the memory summary in top(1) displays the current ARC size. It also documents ARC statistics under kstat.zfs.misc.arcstats.
| Command | What it shows | How to use it |
|---|---|---|
top |
Current ARC size in the memory summary. | Watch it while starting the application that competes for RAM. |
sysctl kstat.zfs.misc.arcstats.size |
Current ARC size. | Compare the value before, during, and after a memory-intensive operation. |
sysctl kstat.zfs.misc.arcstats.c_max |
Current target maximum. | Confirm that a configured cap reached the running kernel. |
sysctl kstat.zfs.misc.arcstats.hits |
Cumulative ARC hits. | Use with misses and workload timing; a raw count has no meaning without an interval. |
sysctl kstat.zfs.misc.arcstats.misses |
Cumulative ARC misses. | Compare changes over the same observation window as hits. |
filesystems/zfs-stats |
A summarized view of ZFS statistics. | Use it for a broader snapshot when diagnosing cache behavior. |
Record a baseline during normal operation, then repeat the measurements while the competing application is active. Pair the ARC figures with FreeBSD memory and process information and with the application’s own latency or allocation errors. A lower ARC size is evidence of reclaim only when it coincides with other signs of memory demand.
How to monitor ARC on Linux
On Linux, inspect /proc/spl/kstat/zfs/arcstats. OpenZFS documents the c_max field there as the way to verify the ARC maximum. For ongoing observation, the project references zarcstat; releases before OpenZFS 2.4.0 commonly called the tool arcstat. Confirm which name your package provides instead of assuming both commands exist.
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A practical, safe way to tune a shared host
- Identify the competing consumer. Measure the database, virtual-machine, jail, container, or process that reports allocation failures or latency. Do not assume ARC is responsible solely because it is large.
- Capture an ARC baseline. Save current size, target maximum, hits, misses, and system pressure indicators during representative workload periods.
- Check release-specific documentation. Confirm whether the host is FreeBSD or Linux and which OpenZFS release supplies its parameter names and defaults.
- Choose a conservative cap only when evidence shows competition. A cap can make memory available to other workloads, but it also removes room for frequently used data. There is no universal safe ARC size or reclaim-time guarantee.
- Apply the platform’s native control. Use FreeBSD’s
vfs.zfs.arc.maxsysctl or Linux’szfs_arc_maxmodule parameter as documented for that release. Treat an 8 GB FreeBSD example as an example, not a target. - Verify the running target. Check
c_maxor the equivalent FreeBSD statistic, then watch current ARC size rather than expecting an instant drop. - Measure the trade-off. Compare application latency, memory pressure, storage I/O, ARC hit behavior, and system stability before and after the change. Keep the setting only if the competing workload gains more than ZFS loses.
Workload trade-offs: storage server versus shared host
| Host pattern | Likely ARC strategy | What to watch |
|---|---|---|
| Mostly ZFS storage services with ample RAM | Allow ARC to use available memory up to its documented limit. | Whether useful data is repeatedly reread from storage and whether other consumers remain comfortable. |
| ZFS plus a large database | Consider a cap only after measuring both database memory needs and ARC behavior. | Database cache efficiency, query latency, ARC misses, and storage I/O. |
| ZFS plus virtual machines or jails | Reserve predictable headroom for guests before changing ARC. | Guest reclaim or ballooning, host pressure, and uneven latency on both sides. |
| Small-memory system with bursty applications | Use short observation windows around application bursts; a static cap may not solve transient pressure. | Allocation failures, swap or reclaim activity, and whether ARC recovers after the burst. |
“The ARC yields memory under pressure, but on systems that share RAM between ZFS and other large consumers such as databases, virtual machines, or jails, the competition can cause uneven performance on both sides.”
That warning from the FreeBSD Handbook captures the central trade-off: protecting one workload by shrinking ARC can make ZFS reads less predictable, while leaving ARC unrestricted can make application memory less predictable.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Common symptoms and what they mean
The ARC is near its maximum and an application cannot allocate memory
Check actual system pressure and the application’s resident memory before changing ZFS. If pressure is genuine, a tested lower cap or additional physical memory may provide headroom, but neither is a universal prescription. After a cap change, monitor the current ARC size and application behavior rather than expecting an immediate release.
The ARC is below its maximum but the application is still slow
A below-maximum ARC does not prove that it is being aggressively reclaimed. It may simply not have filled. Look for storage latency, application-level contention, and miss-rate changes over time instead of treating the size alone as the diagnosis.
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The configured cap changed but memory usage barely moved
This is documented behavior when the host is not under pressure or the cache is warm. Confirm the new target through the platform’s statistics, then observe during representative workload activity. Do not repeatedly lower the cap in response to a single snapshot.
ARC misses rise after a cap reduction
More misses can mean that useful working data no longer fits in RAM, forcing additional storage reads. Compare the miss trend with application latency and device I/O. If the competing application gained little, restore a less restrictive value and continue measuring.
A setting copied from another operating system has no effect
Check the platform-specific name and release documentation. FreeBSD sysctls such as vfs.zfs.arc.max are not interchangeable with Linux module parameters such as zfs_arc_max, and Linux-only controls such as zfs_arc_shrinker_seeks should not be presented as FreeBSD knobs.
What ARC reclaim does not do
- It does not delete files, snapshots, or pool blocks.
- It does not guarantee that every byte above a newly lowered maximum disappears immediately.
- It does not prove that an application is memory-starved merely because ARC occupies much RAM.
- It does not provide a universal cap that suits every amount of RAM, OpenZFS release, or workload.
Use ARC statistics, operating-system pressure signals, and application outcomes together. That evidence-based loop is more reliable than treating a single ARC number or a borrowed tuning recipe as a diagnosis.
Key takeaway
When applications need memory, ZFS can reclaim ARC pages and let the operating system reuse that RAM. The data remains on the pool; the cost is that future reads may miss the cache. Reclaim may be gradual, and lowering the maximum alone is not an immediate eviction command. Tune only after identifying real contention, use the parameter names and defaults documented for your OS and OpenZFS version, and verify the result with ARC and application measurements.
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