Use two layers of evidence: read the raw counters in /proc/spl/kstat/zfs/arcstats, then watch interval output from arcstat (OpenZFS releases before 2.4.0) or zarcstat (2.4.0 and newer). A large ARC is not automatically a memory problem. To judge whether it can give RAM back, compare its size and target with avail and need, and observe how those values change during real memory pressure.
This guide shows the commands, explains each important field, accounts for OpenZFS version differences, and gives a practical test for distinguishing healthy caching from reclaim trouble.
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What the ZFS ARC is—and why its size is not the same as unreclaimable RAM
The Adaptive Replacement Cache (ARC) is ZFS’s in-memory cache for recently and frequently used data and metadata. Linux administrators often see a large ARC in system reports and conclude that applications have lost that amount of memory. That conclusion is unsafe: ARC is designed to be reclaimed when the system needs memory.
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The useful question is not “How many gigabytes does ARC hold right now?” but “Is ARC responding to changing memory availability?” OpenZFS exposes estimates and state indicators for that purpose. Read the current ARC size and target, then check avail and need while the workload is running. A single snapshot, especially one taken while the machine is idle, cannot establish that reclaim is failing.
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Linux-wide tools such as free, vmstat, or cgroup metrics remain necessary for total application headroom. ARC’s own free and avail values are estimates used by ARC; they are not replacements for the kernel’s complete memory accounting.
First identify your OpenZFS release
Check the installed release before selecting a monitoring command or quoting an automatic limit. OpenZFS changed the utility name at version 2.4.0:
- OpenZFS versions before 2.4.0 use
arcstat. - OpenZFS 2.4.0 and later use
zarcstat.
The naming change and module-parameter behavior are documented in the OpenZFS Module Parameters documentation. Distribution package names and backported utilities can differ, so verify what is actually installed rather than inferring the command from a distribution label.
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command -v zarcstat || command -v arcstatIf neither command is found, you can still inspect the kernel counters directly through procfs. Installing a reporting utility is optional for one-time checks; the procfs interface is the authoritative place to start for the values exposed by the loaded module.
Read the raw ARC counters
Run this command as the account that can read the file (usually an unprivileged account can read it):
cat /proc/spl/kstat/zfs/arcstatsThe file contains named counters and values, including the ARC statistics and tunable-related entries. To locate the most relevant lines quickly:
grep -E '^(size|c |c_max|avail|need|free|data_size|metadata_size)' /proc/spl/kstat/zfs/arcstatsField formatting can vary by OpenZFS release, so treat the names as the stable interface and inspect the complete output when a value is absent or presented differently. The documented verification interface for the configured maximum is the c_max entry in this same file.
Values are generally reported as bytes or counters whose units are identified in the file. Do not convert a number to GiB until you have confirmed its unit. A small shell conversion is useful only after that check:
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awk '$1 ~ /^(size|c|c_max|avail|need|free)$/ {print}' /proc/spl/kstat/zfs/arcstatsFor historical comparisons, save complete snapshots with a timestamp. Comparing the same fields from two snapshots is more informative than copying one value into a dashboard without its target and memory-availability context.
Use interval statistics for trends
The OpenZFS interval utility produces vmstat-like samples. On releases that provide the older name, a ten-sample, one-second run is:
arcstat 1 10On OpenZFS 2.4.0 and later, use the installed zarcstat command instead:
zarcstat 1 10The v2.2 arcstat manual documents the interval and count arguments, along with useful output controls:
-adisplays all available statistics.-fselects particular fields, which keeps a terminal or log focused.-pprints unsuffixed numerical values, making output easier for scripts to parse.
Use the options supported by the utility installed on your host. For a focused interactive view, select the fields that your version exposes for ARC size, target, availability, and reclaim status. If a field name is rejected, run the command’s built-in help and use the spelling shown there; release changes can add, remove, or rename columns.
Record samples during normal activity and during the event that prompted the investigation: a database start, a build, a backup, or another known memory-demand spike. A trend that includes the workload’s timing lets you distinguish a cache that is merely large from one that fails to contract when the kernel needs memory.
Fields that answer the reclaimability question
Interpret related fields together. The following table describes the key concepts rather than promising identical column formatting in every release.
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|---|---|---|
size |
The ARC’s current occupied size. | Use it to see whether the cache grows or shrinks, but never treat it alone as application memory that cannot be reclaimed. |
c (current target) |
The ARC target it is trying to maintain. | Compare it with size and watch whether the target is reduced when memory becomes scarce. |
c_max |
The configured maximum/limit reported by the module. | It constrains the target; it is not a command to shrink the cache immediately. |
avail |
ARC’s estimate of memory available to ARC. | The OpenZFS manual says this is usually more instructive than free; follow its trend during pressure. |
free |
ARC’s estimate of free memory, including evictable memory in Linux’s page cache. | It is an ARC-specific estimate, not the same as application-available RAM reported by every Linux tool. |
need |
Indicates that ARC reclaim is needed. | Read it with avail, the target, and the direction of change rather than as an isolated alarm. |
According to the manual, avail is usually a better field to watch than free. Avail can briefly become negative; in that situation ARC reduces its target c. A transient negative value during an adjustment is therefore not, by itself, proof of a stuck cache. Persistent pressure, a continually asserted need, and a target that does not respond are more significant when they coincide with application allocation failures or reclaim latency.
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A repeatable test for whether ARC gives memory back
Use this procedure instead of diagnosing from one large number.
- Establish a baseline. Save the OpenZFS release, the relevant module parameters, and several minutes of interval output. Include
size,c,c_max,avail,free, andneedwhen your utility exposes them. - Observe representative work. Run the workload that normally matters on the host. Note whether ARC grows toward its target and whether the workload’s application memory remains available in Linux-wide metrics.
- Apply or capture real memory demand. Observe during the actual incident or a controlled, safe workload that allocates memory. Do not create an artificial stress test on a production system without an operational plan.
- Look for coordinated movement. As availability falls, check whether
availfalls, whetherneedindicates reclaim, and whether the targetcand actualsizemove downward. The sequence matters more than any one sample. - Correlate with symptoms. Compare the ARC trace with allocation failures, swap activity, latency, or cgroup pressure. If Linux reports pressure but ARC’s indicators never react, investigate the module, kernel, and workload rather than simply lowering a limit.
- Repeat after pressure ends. A cache may grow again once memory is available. Recovery toward a higher target is expected caching behavior, not evidence that the earlier reclaim was ineffective.
This test answers the operational question—whether ARC responds to demand—without assuming that every byte currently counted in size is permanently unavailable to applications.
Why free and avail are easy to misread
The free field is defined as “the ARC’s idea of how much free memory there is, which includes evictable memory in the page cache.” That definition is specific to ARC’s policy view. Linux may classify memory differently in other interfaces, and an application’s cgroup limit can make memory unavailable even when host-level estimates look comfortable.
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Understand what occupies ARC: data, metadata, and evictable categories
ARC accounting separates data and metadata. Depending on the release and the counters exposed, the implementation also describes categories such as evictable anonymous data and evictable metadata. The OpenZFS ARC implementation definitions show these accounting categories.
These counters help answer “what is making the ARC large?” For example, a metadata-heavy workload may have a different cache profile from a sequential data workload. Use the composition counters alongside total size, not instead of it. A category labelled evictable describes ARC’s accounting and eviction classification; it does not calculate total free RAM for every Linux process.
When comparing two hosts, keep the workload and observation window in view. A metadata percentage from a backup run cannot be used as a general-purpose ratio for an idle file server. Record the release, pool activity, and sampling period with any comparison.
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zfs_arc_max sets a maximum or target boundary for ARC behavior. Lowering it below the current ARC size does not force an immediate shrink when the system is otherwise comfortable. ARC contracts in response to memory pressure and its reclaim mechanisms; changing the value alone is not a synchronous eviction command.
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The module-parameter documentation explains that the parameter can be changed dynamically, but it cannot be set back to zero while the system is running. If ARC competes too heavily with other applications, the documentation gives reducing the limit as a possible configuration response; if ZFS is the primary application and can use more RAM, increasing it may be appropriate. Those are workload-dependent choices, not a universal tuning recipe.
Verify the effective limit through c_max in the procfs statistics:
grep '^c_max' /proc/spl/kstat/zfs/arcstatsAfter a configuration change, continue sampling. A lower c_max should influence the target over time, but the observed size may remain above it until pressure causes eviction. Do not report the unchanged size immediately after a change as proof that the parameter was ignored.
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Version-specific automatic limits
Always include the OpenZFS release when explaining a host’s default. Since OpenZFS 2.3.0, when zfs_arc_max is zero, every platform uses the larger of:
- all system memory minus 1 GiB; or
- five-eighths (5/8) of all system memory.
The official wording is: “Since v2.3.0 every platform uses the larger of all_system_memory - 1 GiB and 5/8 x all_system_memory.” Before 2.3.0, Linux used half of system memory for the automatic rule, while FreeBSD used the larger formula. Applying the newer calculation to an older Linux installation will produce the wrong expectation.
The automatic calculation is a starting policy, not a promise that ARC will consume that amount. Actual size depends on workload, memory pressure, reclaim activity, and other limits. Confirm the running value and behavior on the host rather than inferring them from installed RAM alone.
Other reclaim-related context: zfs_arc_sys_free and shrinkers
zfs_arc_sys_free represents a target amount of system free memory that ARC should leave available. When the parameter is zero, its documented equivalent is the larger of 512 KiB and one sixty-fourth (1/64) of total system memory. This is policy context for interpreting ARC’s behavior, not a recommendation to change it for every server.
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Linux ARC shrinker parameters govern eviction in response to allocation pressure. The OpenZFS zfs.4 manual documents those parameters and their relationship to Linux memory reclaim. If you are investigating a kernel or distribution-specific reclaim problem, capture the relevant parameter values and kernel logs before changing them.
Changing several reclaim tunables at once removes your ability to identify the cause of an improvement or regression. For monitoring work, first measure; tune only when the workload, memory budget, and operational objective justify a change.
Common observations and what they mean
“ARC uses most of RAM while the server is idle”
That is normally consistent with a cache doing useful work. Check whether avail remains healthy, need is not persistently asserting reclaim, and the ARC contracts when another workload needs memory. Idle occupancy alone is not evidence of an application-facing shortage.
“I lowered zfs_arc_max, but size did not fall”
The limit is not an immediate shrink command. Confirm the new c_max, then observe under normal or actual memory pressure. ARC may remain above the new target until eviction is required.
“free says there is memory, but the application is failing”
ARC’s free includes evictable page-cache memory and is not a complete view of process, cgroup, swap, or kernel allocations. Check Linux-wide memory and cgroup statistics, then compare them with ARC’s avail and need.
“avail briefly goes negative”
The manual allows this during adjustment; ARC responds by reducing target c. Determine whether the value recovers and whether size follows the target. Persistent negative availability with application symptoms deserves further investigation; a short transition does not by itself establish failure.
“The command shown in a guide does not exist”
Check the release and installed binaries. Use arcstat before OpenZFS 2.4.0 and zarcstat starting with 2.4.0. If neither is installed, read /proc/spl/kstat/zfs/arcstats or install the package supplied for your distribution and release.
Comparing two Linux ZFS hosts
Make comparisons meaningful by collecting the same evidence from both systems:
- OpenZFS release and kernel version.
- ARC
size, current targetc, andc_max. avail,free, andneedover the same sampling interval.- Data, metadata, and exposed evictable-category counters.
- Relevant ARC parameters, including
zfs_arc_maxandzfs_arc_sys_free. - The workload, system RAM, cgroup limits, swap policy, and time of day.
Comparing only the percentage of RAM shown as ARC can invert the conclusion: one host may have a larger healthy cache, while another has a smaller cache but persistent reclaim pressure. Compare behavior under the same demand, not just occupancy at rest.
Quick Recap
A compact monitoring checklist
- Identify whether the host uses OpenZFS before or after 2.4.0.
- Run
cat /proc/spl/kstat/zfs/arcstatsand save a baseline. - Use
arcstatorzarcstatwith an interval and count for trend data. - Watch
size,c,c_max,avail,free, andneedtogether. - Interpret
freeas ARC’s estimate that includes evictable page cache; preferavailfor ARC headroom. - Test during representative memory demand and correlate with Linux-wide and application metrics.
- Check version-specific defaults before explaining or changing
zfs_arc_max. - Use data and metadata counters to explain composition, not to estimate total application headroom.
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