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1Fix the driver behind crashes, sound loss and screen glitches2Repair Windows errors before they cause bigger problems3Scan for outdated or missing drivers - takes under a minuteZFS ARC and the Linux page cache both keep recently used data in RAM, but they are separate cache systems. The ARC caches ZFS blocks; Linux’s Cached value reports file page cache plus tmpfs/shmem and is not an ARC meter. To decide whether a host has enough memory, read ARC statistics together with MemAvailable, file-cache counters, and evidence of workload pressure—not the size of either cache alone.
What ZFS ARC and the Linux page cache actually cache
ZFS ARC is a ZFS-managed block cache
OpenZFS describes the Adaptive Replacement Cache (ARC) as its in-memory cache. It stores ZFS data and metadata blocks in RAM so repeated reads can avoid storage access. ARC policy is adaptive: it balances recently used data with frequently used data rather than treating every cached block identically.
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ARC admission can be changed per dataset with the primarycache property:
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These settings change what ZFS places in ARC; they do not turn ARC into Linux’s page cache or make the two counters interchangeable.
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Linux page cache is the kernel’s file cache
Linux keeps file contents read from storage in the page cache. In /proc/meminfo, the Cached field includes that file cache together with tmpfs and shmem pages. It excludes SwapCached. A high value therefore says that Linux has many cacheable file-backed or tmpfs/shmem pages in memory; it does not say how much RAM ARC is using.
Both mechanisms can improve read performance, but they operate at different layers. ZFS sees blocks through its storage stack, while the Linux page cache represents file-backed pages at the kernel’s virtual-filesystem layer. On an OpenZFS Linux host, both may be active at once.
ARC versus Linux memory counters at a glance
| Question | ZFS ARC | Linux page-cache and memory view |
|---|---|---|
| What is cached? | ZFS-managed data and metadata blocks in an adaptive in-memory cache. | File pages read from storage; Cached also includes tmpfs/shmem and excludes SwapCached. |
| Where is it inspected? | /proc/spl/kstat/zfs/arcstats; release-specific ARC utilities such as zarcsummary or zarcstat. |
/proc/meminfo, especially Cached, Active(file), Inactive(file), and MemAvailable. |
| Does a large value prove a problem? | No. ARC size must be considered with its target, reclaim indicators, available memory, and workload behavior. | No. Linux normally reclaims cache when memory is needed, and the counters are not independent buckets. |
| What happens under pressure? | OpenZFS exposes a Linux shrinker; ARC can reduce its target. Optional controls govern interaction with page-cache reclaim. | The kernel reclaims file cache and other reclaimable memory as needed, subject to what is clean and reclaimable. |
Why “used RAM” and “Cached” can be misleading
Cached is not “all Linux cache” and not an ARC reading
The Linux documentation defines Cached narrowly: file cache plus tmpfs/shmem, without SwapCached. ARC is a separate ZFS subsystem, so reading Cached as though it were “ARC plus page cache” produces the wrong diagnosis. Conversely, treating a low Cached value as proof that ARC is small is also incorrect.
Exact placement of ZFS allocations in overall kernel accounting can vary with the OpenZFS and kernel implementation. The safe conclusion is that Cached is not an ARC meter; use ARC-specific statistics for ARC.
MemAvailable is the practical headroom estimate
MemAvailable is Linux’s estimate of how much memory can be given to new applications without swapping. Its calculation considers free memory, reclaimable slab, file-LRU pages, and zone watermarks. It also accounts for the fact that not every object labelled reclaimable can actually be reclaimed immediately.
That makes MemAvailable more useful for answering “can this host accommodate more work?” than subtracting one cache counter from total RAM. Linux documentation warns that non-overlapping counters may not add up to total memory usage and that the difference can be substantial for some workloads. Do not add every line in /proc/meminfo and expect a meaningful total.
How ARC and page-cache reclaim interact on Linux
OpenZFS for Linux provides an ARC shrinker so the kernel can ask ARC to release memory under pressure. The kernel’s page-cache LRU is reclaimed through a separate path. This means ARC and page cache can affect each other during a real memory shortage even though they are accounted for by different interfaces.
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OpenZFS documents zfs_arc_pc_percent for this pressure/reclaim interaction. The setting relates ARC reclaim to page-cache size; it is not a general-purpose “make ARC smaller” switch for every workload. The OpenZFS 2.4 zfs.4 documentation lists a default of 0, meaning the control is off by default in that release. Defaults and behavior are version-sensitive, so check the documentation for the OpenZFS release actually installed before changing it.
ARC statistics also expose reclaim state. In the documented arcstat fields, size is current ARC size, need represents reclaim need, and free and avail describe available memory from ARC’s perspective. The manual notes that the free estimate includes evictable page-cache memory and that avail is usually the more instructive value to observe. avail can temporarily become negative; when that happens, ARC reduces its target.
How to measure both caches correctly
1. Capture Linux’s related memory counters together
Start with the documented categories rather than a single headline number:
grep -E '^(MemAvailable|Cached|Active(file)|Inactive(file)|Dirty|Writeback):' /proc/meminfoRead the output as a set of related indicators. Active(file) and Inactive(file) describe file-backed pages on the active and inactive LRUs. Dirty and Writeback identify data that is being modified or written; dirty data is not equivalent to immediately discardable clean cache. None of these lines is an ARC-specific measurement.
2. Read ARC’s own statistics
Inspect the kernel statistics file:
/proc/spl/kstat/zfs/arcstatsUse the file directly or an installed utility. Current OpenZFS caching documentation names zarcsummary and zarcstat; older installations may provide commands named arc_summary or arcstat. Confirm the names and field layout supplied by your release instead of assuming that a command from another distribution is present.
When sampling ARC, pay particular attention to:
size: how much memory ARC currently occupies.need: whether ARC is being asked to reclaim memory.free: an estimate that includes evictable page-cache memory.avail: the more instructive availability estimate in the cited ARC documentation; a temporary negative value can trigger ARC target reduction.
3. Observe trends, not one sample
Take the Linux and ARC readings while the workload is idle, during its normal peak, and while a reported slowdown occurs. A single large ARC size or Cached value cannot distinguish a healthy warm cache from a system that is struggling to reclaim memory. Trends in MemAvailable, ARC need/avail, and application behavior provide that context.
4. Confirm the installed release before interpreting tunables
OpenZFS utility names, defaults, and Linux reclaim behavior have changed across releases. The cited OpenZFS 2.4 manual documents the current default for zfs_arc_pc_percent; historical tuning pages contain older entries. Record the installed OpenZFS and kernel versions alongside your measurements before comparing them with documentation or advice written for another release.
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How to interpret common memory situations
| What you observe | What it usually means | What to check next |
|---|---|---|
ARC is large, but MemAvailable remains healthy and the workload is responsive. |
RAM is being used for a useful ZFS cache. Size alone does not establish a leak. | Continue monitoring under peak load; verify that ARC hit behavior and application performance meet expectations. |
Cached is large while MemAvailable is still substantial. |
Linux has accumulated reclaimable file or tmpfs/shmem pages. The cache is not automatically a shortage. | Check whether availability falls during the actual workload and whether dirty/writeback activity is high. |
MemAvailable falls sharply, ARC shows reclaim need, or ARC avail becomes negative. |
The host is experiencing memory pressure and ARC is being pushed toward a smaller target. | Correlate the time with workload impact, swap activity, and version-specific reclaim settings before tuning. |
| A large number is visible in one counter, but other counters do not reconcile arithmetically. | Linux memory fields overlap or represent different classes of pages; they are not designed to be blindly summed. | Use the documented definitions and evaluate headroom through MemAvailable rather than forcing a hand-built total. |
Do ARC and page cache represent duplicate memory?
They are different accounting views owned by different subsystems. A file read through a ZFS dataset can be relevant to both the filesystem’s block cache and Linux’s file-cache machinery, but the counters have different definitions and are not safe to add as if they were disjoint allocations. The practical rule is to use ARC statistics for ARC and /proc/meminfo for Linux’s memory view.
Do not attempt to calculate application-owned RAM by subtracting Cached and ARC size from total memory. Linux explicitly notes that memory fields overlap and that non-overlapping counters may still fail to add to total usage. Use MemAvailable as the headroom estimate, then validate it against application behavior and reclaim activity.
Common interpretation mistakes
- Calling every high cache value a leak. Both caches are designed to occupy otherwise useful RAM and release it when required.
- Reading
Cachedas an ARC gauge. It measures Linux file cache plus tmpfs/shmem, not ZFS ARC. - Adding all
/proc/meminfolines. Several fields overlap or describe different page states. - Ignoring dirty and writeback pages. Modified pages cannot be treated as instantly discardable clean cache.
- Copying a tunable from an old guide. OpenZFS defaults and utility names depend on the installed release.
- Judging success by a smaller cache. A smaller cache can simply mean more storage reads and worse latency; the goal is adequate headroom and reliable workload performance.
Why is ZFS using so much memory?
Usually, ARC is using otherwise idle RAM to avoid future storage reads. That is its purpose, and a high ARC size is expected on a host with a read-heavy workload and available memory. The meaningful question is whether the system can reclaim memory when applications need it and whether performance suffers.
Investigate further when high ARC coincides with falling MemAvailable, ARC reclaim need, negative or collapsing avail, swapping, allocation failures, or observable workload degradation. Those symptoms establish pressure; the ARC size by itself does not.
Is ZFS ARC included in Linux’s cache figure?
No. Linux’s documented Cached field covers file page cache plus tmpfs/shmem and excludes SwapCached; it is not a ZFS ARC counter. ARC must be inspected through OpenZFS statistics. The two systems can both consume RAM and can interact during reclaim, which is why a host may show substantial values in both places without either value being a duplicate measurement.
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Should you change primarycache?
primarycache is a workload-specific storage policy, not a general memory emergency switch. Use all when both data and metadata benefit from ARC, metadata when retaining directory and filesystem metadata is useful but caching file data would displace more valuable memory, and none when a dataset should bypass primary ARC caching. The documented default is all.
Changing the property can alter read performance and storage traffic. Establish which dataset is consuming memory, measure the workload before and after, and make the change only when the caching policy matches the access pattern. Do not infer that every large ARC requires a dataset-level restriction.
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When does zfs_arc_pc_percent make sense?
This control is relevant to Linux memory-pressure and reclaim behavior, not to routine cache grooming. Because the documented default is release-dependent—and OpenZFS 2.4 documents 0—first verify the installed release and establish that the host has genuine pressure. Then test any change against application latency, MemAvailable, ARC reclaim indicators, and storage I/O. A setting copied from an older tuning guide may not describe current behavior.
Why dropping caches is usually the wrong fix
Linux exposes a cache-drop interface, commonly invoked for testing with:
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echo 3 > /proc/sys/vm/drop_cachesThe kernel documentation says this releases clean page-cache pages and reclaimable slab objects. It does not free dirty objects, and rebuilding discarded cache can impose additional I/O and CPU work. The documentation explicitly says the interface is not a mechanism for controlling cache growth and that use outside testing or debugging is not recommended.
Dropping Linux caches also does not provide a principled way to size ARC. It can make a healthy system look slower while the caches warm again, without addressing the workload or reclaim policy that caused the concern. Diagnose pressure with MemAvailable, ARC statistics, and workload symptoms first.
A practical troubleshooting workflow
- Define the symptom. Record whether the issue is low application availability, allocation failures, swapping, latency, or simply a large cache number.
- Capture Linux counters. Save
MemAvailable,Cached, file-LRU counters,Dirty, andWritebackduring normal and peak periods. - Capture ARC counters. Read
/proc/spl/kstat/zfs/arcstatsor use the release’s ARC utility; tracksize,need,free, andavail. - Compare the timelines. Look for falling headroom and reclaim activity at the same time as the reported workload impact.
- Check versions and defaults. Verify the installed kernel and OpenZFS releases before interpreting shrinker behavior or
zfs_arc_pc_percent. - Choose the narrowest intervention. Adjust a dataset’s
primarycacheonly for a demonstrated workload reason; consider reclaim controls only after confirming pressure and release-specific behavior. - Re-measure after any change. Keep the same sampling points and compare application performance, availability, ARC reclaim, and I/O rather than judging by cache size alone.
What to record during an incident
Capture a short time series rather than a single command result. Record timestamps, workload activity, MemAvailable, Cached, Active(file), Inactive(file), Dirty, and Writeback. At the same times, record ARC size, need, free, and avail. Note application latency, allocation failures, swapping, and storage I/O as separate symptoms.
This record helps distinguish three cases: a warm cache with healthy headroom, a reclaim event that briefly lowers cache sizes, and sustained pressure that affects the workload. It also prevents a common troubleshooting error—changing a setting after seeing one alarming number and then having no comparable baseline for the result.
Version and scope limits
This comparison applies to Linux systems using OpenZFS. The OpenZFS 2.4 zfs.4 documentation gives zfs_arc_pc_percent=0 as its default, but other releases may differ. The Linux /proc/meminfo definitions cited here come from kernel documentation for version 6.17, while the cache-drop behavior cited comes from documentation for version 5.17. Those references explain the accounting concepts and operational warning; they should not be treated as a promise that every implementation detail is identical on every kernel or OpenZFS release.
The bottom line
ARC and page cache are complementary but independently accounted-for caches. Use ARC statistics to understand ZFS, use MemAvailable and Linux file-cache counters to understand system headroom, and judge pressure by reclaim activity and workload impact. A large cache is normally a performance resource; it becomes a problem only when the host cannot reclaim enough memory or the workload shows measurable distress.
Frequently Asked Questions
How do I tell whether Linux has enough memory?
Check MemAvailable together with workload behavior, swap activity, file-cache counters, and ARC reclaim indicators. Do not decide from total used RAM or Cached alone.
Is a large ZFS ARC a memory leak?
Not by itself. ARC is intended to use available RAM. A problem is more likely when MemAvailable falls, ARC reports reclaim need, avail becomes negative or collapses, swapping occurs, or applications slow down.
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Is ZFS ARC included in Linux’s Cached value?
No. Linux defines Cached as file page cache plus tmpfs/shmem, excluding SwapCached. Inspect ARC through /proc/spl/kstat/zfs/arcstats or the utility supplied by your OpenZFS release.
Should I reduce ARC whenever Linux shows high cache use?
No. High cache use is normally useful and reclaimable. First confirm real pressure with MemAvailable, ARC need/avail, and workload symptoms.
What does primarycache=metadata do?
It allows dataset metadata into ARC while preventing ordinary file data from entering the primary cache. It is a workload-specific policy; the documented default is all, and changing it can affect read performance and storage traffic.
What is the documented default for zfs_arc_pc_percent?
OpenZFS 2.4 documents a default of 0, meaning the control is off in that release. Defaults and behavior are version-sensitive, so verify the installed OpenZFS documentation.
Does echo 3 > /proc/sys/vm/drop_caches solve memory pressure?
It is intended for testing or debugging, not routine cache management. It drops clean page cache and reclaimable slab objects, does not free dirty objects, and can add I/O and CPU cost while caches rebuild.
Why do Linux memory counters not add up?
The kernel documents overlapping and differently defined fields. Non-overlapping counters may still fail to equal total memory for some workloads, so use their definitions and MemAvailable instead of constructing a total by blindly adding lines.
The Bottom Line
ARC and Linux page cache use RAM for different purposes and expose different accounting. Read ARC statistics, MemAvailable, file-cache counters, and workload symptoms together; never treat a large cache number alone as proof of a memory problem.
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