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Scan for outdated or missing drivers - takes under a minuteDriver Scan →Clear out junk files and repair common Windows errorsFree Scan →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →A large ZFS ARC is not, by itself, proof that your Linux host is under memory pressure. The reliable test is a time-correlated comparison: capture operating-system pressure and reclaim evidence while also recording ARC size, targets, and reclaim-related statistics. If the ARC is large while the kernel reports no sustained pressure, its size alone is not a diagnosis. If pressure symptoms and ARC behavior rise together, investigate ARC eviction, competing workloads, and the OpenZFS version before changing a tunable.
What the ARC is—and why its size can mislead you
The Adaptive Replacement Cache (ARC) is OpenZFS’s main in-memory cache for frequently used data and metadata. It is designed to use otherwise available RAM and give memory back when the operating system needs it. That means a machine can show a high ARC allocation at the same time that applications are healthy and the kernel is not struggling to reclaim memory.
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Many dashboards label all non-free RAM as “used.” That number combines page cache, kernel allocations, application working sets, and ZFS memory. It does not tell you whether allocations are stalled, reclaim is excessive, or the ARC is failing to yield memory. Treat the dashboard number as a prompt to investigate, not as evidence that ARC is the cause.
Pressure is established by behavior: stalled or delayed allocations, sustained memory-pressure signals, reclaim activity, swap or compaction trouble, or an out-of-memory event. ARC statistics add the storage-cache context. You need both views, sampled during the same interval, to make a defensible attribution.
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What counts as evidence of memory pressure on Linux?
Start with the host-level symptoms. Check these while users or services are reporting slowdowns, failed allocations, or latency spikes:
- Pressure Stall Information (PSI): If available,
/proc/pressure/memoryreports time that tasks were delayed because memory was unavailable. Compare the changingsomeandfullvalues over time rather than reading one cumulative number. - Reclaim and paging activity:
vmstat 1can show whether pages are being reclaimed or swapped during the incident. A single sample is weak evidence; collect a series. - Kernel memory state:
/proc/meminfoexposes free, available, reclaimable, and swap-related counters. Look for movement that coincides with the reported problem, not merely a low free-memory figure. - Allocation failures and OOM reports: Kernel logs may contain out-of-memory or allocation-failure messages. These confirm a serious event, but absence of an OOM kill does not prove that the system was comfortable.
- Application symptoms: Request latency, failed starts, timeouts, and allocator errors provide the workload-side timestamp needed for correlation.
PSI files and individual counters can be unavailable or interpreted differently in constrained containers. Record the host and container context, and do not infer pressure from a metric that is not actually measuring the affected workload.
Capture a synchronized baseline
Collect a short baseline when the machine is healthy, then repeat the same collection during the reported incident. Use a common clock so ARC and operating-system observations can be lined up later.
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- Record the time and identity of the host. Save the output of
date --iso-8601=seconds, the kernel release, and the OpenZFS release. The available ARC fields and defaults depend on the installed build. - Sample memory pressure. Run
cat /proc/pressure/memoryif the file exists. In another terminal, runvmstat 1 10and save the output. Capturegrep -E '^(Mem|Swap|SReclaimable|Slab|Unevictable)' /proc/meminfobefore and after the sample. - Capture ARC statistics at the same time. On Linux, read
/proc/spl/kstat/zfs/arcstats. The file is a point-in-time counter set; save repeated snapshots rather than relying on one screen view. - Use arcstat when available. The
arcstatutility presents ARC counters as a stream, which is convenient for correlation. Its options and field names differ among OpenZFS releases, so run the installed utility’s help command and select only fields it documents on that host. - Mark the user-visible event. Note when latency, allocation failures, or a workload restart began and ended. Without that interval, a later comparison can mistake normal cache warm-up or an unrelated batch job for pressure.
date --iso-8601=seconds
cat /proc/pressure/memory
vmstat 1 10
grep -E '^(Mem|Swap|SReclaimable|Slab|Unevictable)' /proc/meminfo
cat /proc/spl/kstat/zfs/arcstatsIf /proc/spl/kstat/zfs/arcstats is missing, ZFS may not be loaded, the host may be using a different packaging layout, or you may be inside a container without access to host kernel statistics. Verify the deployment instead of substituting an unrelated memory counter.
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Read the ARC data without over-interpreting it
Separate current size from configured limits
ARC statistics normally include a current size and one or more targets or limits. Names and availability vary by OpenZFS version. A current size near a configured maximum is not automatically harmful; it only tells you that the cache is using the space it is allowed to use. The relevant question is whether the kernel is requesting memory and whether ARC reduction follows that demand.
When reviewing arcstats, preserve the field names and units exactly as printed. Some values are bytes, some are counts, and some are ratios or cumulative counters. Do not compare a counter’s absolute value between two hosts without accounting for uptime and sampling interval.
Look for reclaim and throttling behavior
ARC-side indicators can show whether the cache is shrinking, refusing to grow, or throttling activity. The precise names exposed by arcstat and arcstats depend on the release; fields present in one OpenZFS version may be absent in another. Save the complete output so a version-aware reader can identify the relevant counters instead of filtering out unfamiliar fields.
A rising reclaim-related counter during a pressure interval is useful corroboration, but it is not a universal pass/fail threshold. Interpret it alongside PSI, vmstat, application timing, and the direction of ARC size. A counter that only increases because it is cumulative says less than its rate of change during the incident.
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Use repeated samples, not a single snapshot
Take samples at a fixed interval before, during, and after the event. A simple loop can preserve timestamps while recording the kernel file:
while sleep 5; do
printf 'n%sn' "$(date --iso-8601=seconds)"
awk '/^(size|c |c_max |memory_throttle_count |arc_no_grow )/ {print}' /proc/spl/kstat/zfs/arcstats
cat /proc/pressure/memory
vmstat 1 2 | tail -1
doneThe filter is illustrative, not a guaranteed field list. If your version uses different names, remove the filter and archive the full file. The important properties are repeatability, timestamps, and identical sampling during healthy and unhealthy periods.
Correlate the timelines
Build a timeline with one row per sampling interval. Include application symptoms, PSI, reclaim or swap activity, ARC size, ARC target or maximum, and any reclaim-related ARC counters. Then classify the pattern.
| Observed pattern | What it establishes | Next investigation |
|---|---|---|
| ARC is large; PSI and reclaim remain quiet; applications behave normally | Large cache occupancy alone does not demonstrate harmful pressure. | Continue monitoring and document the normal baseline. |
| PSI or reclaim rises, but ARC size is flat or falling | The host is under memory stress, but the ARC is not shown to be the cause. | Inspect application working sets, kernel slabs, containers, swap, and concurrent jobs. |
| PSI, reclaim, and ARC-related eviction or throttling change together | ARC is participating in the pressure interval and warrants deeper review. | Check configured controls, competing demand, and whether eviction keeps pace. |
| A maximum was lowered, but ARC remains above it immediately afterward | The setting change did not force an instant shrink. | Allow memory pressure to induce reclaim and verify behavior with new samples. |
| An incident occurs only during a backup, scrub, build, or batch job | Another workload may be the primary memory demander. | Repeat the test with that workload isolated or instrumented. |
Correlation does not prove that ARC caused every allocation delay. It tells you when ARC activity and system pressure moved together. Confirm causality by checking other consumers and by making one controlled change at a time, with before-and-after measurements.
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Check the OpenZFS version and platform before interpreting fields
OpenZFS’s current master documentation, versioned tuning pages, and arcstat manuals do not always describe the same fields. The available manuals returned for arcstat cover OpenZFS 2.1 and 2.2, while parameter references can reflect newer development documentation. Confirm the version installed on the host and read the matching documentation before assigning meaning to a counter.
Linux and FreeBSD also differ in units and parameter semantics. In particular, Linux uses byte-based zfs_arc_sys_free. A similarly named FreeBSD setting may be expressed in pages. Do not copy a value or unit from a FreeBSD guide into a Linux configuration.
Record the distribution, kernel, OpenZFS package version, architecture, and whether the workload runs on the host or in a container. These details explain why a path, field, or default shown in an example may not exist in your environment.
Understand the relevant ARC controls
| Parameter | What it controls | What it does not prove |
|---|---|---|
zfs_arc_max |
Sets the maximum ARC size. In current OpenZFS documentation, a value of zero selects a default derived from system memory; the exact default and behavior are version-sensitive. | Lowering it below the current ARC size does not itself force an immediate shrink. OpenZFS documents that memory pressure is needed to induce shrinking. |
zfs_arc_sys_free |
Describes a target amount of free system memory that ARC should leave available. The documented default is derived from physical memory and includes a minimum; Linux expresses this setting in bytes. | Its configured value is a policy target, not a report that pressure is occurring now. |
zfs_arc_lotsfree_percent |
Can throttle I/O when free system memory drops below the configured percentage of total system memory. | A nonzero percentage does not establish that the threshold has been crossed or that applications are currently stalled. |
zfs_arc_pc_percent |
Influences ARC interaction with the Linux page cache during memory pressure and reclaim. | The presence of a setting does not show that pressure exists, nor that changing it will fix a particular workload. |
Read the effective values from the running system and keep a copy of the configuration that supplied them. A value shown in a template, distribution default, or documentation page may not match the active module parameters.
What to do when the evidence points to real pressure
Find the competing memory demand
Inspect service-level working sets, container limits, JVM or database caches, build tools, backup processes, and kernel slabs. Compare their timestamps with the pressure interval. If pressure continues while ARC is shrinking, the non-ARC demand may be large enough to explain the incident.
Verify that ARC can react
Check whether ARC size and reclaim-related statistics change when pressure rises. A cache that falls as demand increases is behaving differently from one that remains pinned while the system reports reclaim stress. Also check whether a recent setting change has only altered a target; a target below the current size is not an instant eviction command.
Change one control, then measure again
If you decide that the ARC’s allowed footprint is too large for the workload, change the documented parameter using the configuration method supported by your distribution and OpenZFS release. Record the old and new values, the time of change, and the workload state. Repeat the same PSI, vmstat, meminfo, and ARC collection. Do not claim success because the configured number changed; success means the pressure symptoms and application impact improve under comparable demand.
Plan for persistence and rollback
Runtime module settings may not survive reboot, while a persistent configuration may be applied before or after services start. Document where the value is set, how it is validated after boot, and how to restore the previous value. Keep a rollback plan if the change reduces cache hit performance or moves pressure elsewhere.
Common diagnostic mistakes
- Calling every used byte “pressure.” Used memory includes useful cache. Require pressure, reclaim, or application evidence.
- Reading one snapshot. A single ARC size or free-memory number cannot show direction or causality.
- Mixing clocks. An ARC sample from after the incident cannot be correlated reliably with an application graph from before it.
- Assuming field names are universal. Verify the installed OpenZFS version and preserve unknown fields rather than silently dropping them.
- Copying FreeBSD units to Linux. Confirm whether a parameter is bytes, pages, a percentage, a count, or a derived default.
- Expecting an immediate shrink after lowering
zfs_arc_max. The documented behavior requires memory pressure to induce shrinking. - Changing several tunables at once. Multiple simultaneous changes make it impossible to attribute an improvement or regression.
- Ignoring non-ARC consumers. Applications, containers, page cache, kernel slabs, and temporary jobs can create the demand that triggers reclaim.
A repeatable checklist
- Write down the exact symptom, affected service, and start and end times.
- Identify the Linux kernel and OpenZFS versions and the host or container boundary.
- Collect PSI,
vmstat, selected/proc/meminfocounters, and kernel logs during a healthy interval. - Collect the complete
/proc/spl/kstat/zfs/arcstatsfile or version-appropriatearcstatoutput at the same cadence. - Repeat the collection during the incident and after recovery.
- Compare directions and rates: pressure, reclaim, ARC size, ARC targets, and ARC reclaim indicators.
- Check effective values and units for
zfs_arc_max,zfs_arc_sys_free,zfs_arc_lotsfree_percent, andzfs_arc_pc_percentwhere present. - Investigate other memory consumers before attributing the event to ARC.
- If a control is changed, make one documented change and repeat the measurement under comparable load.
When a large ARC is healthy
A large ARC with low or stable memory-pressure signals, no abnormal reclaim, and healthy application behavior is generally consistent with normal caching. In that state, forcing the cache smaller may discard useful data without solving a problem. Keep the synchronized measurements as a baseline so a later workload change can be distinguished from normal ARC occupancy.
When ARC is only one part of the problem
Pressure can coincide with ARC activity without being caused primarily by ARC. For example, a sudden application allocation can make the kernel reclaim page cache and ask ARC to yield memory; the application is the demand source, while ARC is a responder. Conversely, an ARC policy may delay or throttle I/O during low-free-memory conditions while another service consumes most RAM. The timeline and the behavior of all major consumers determine which explanation fits.
The practical standard is therefore simple: do not diagnose from ARC size. Diagnose from synchronized operating-system pressure, reclaim behavior, application impact, and ARC statistics interpreted for the exact OpenZFS version and platform in use.
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