.\" SPDX-License-Identifier: CDDL-1.0
.\"
.\" This file and its contents are supplied under the terms of the
.\" Common Development and Distribution License ("CDDL"), version 1.0.
.\" You may only use this file in accordance with the terms of version
.\" 1.0 of the CDDL.
.\"
.\" A full copy of the text of the CDDL should have accompanied this
.\" source.  A copy of the CDDL is also available via the Internet at
.\" http://www.illumos.org/license/CDDL.
.\"
.\" Copyright (c) 2026 Christos Longros.  All rights reserved.
.\"
.Dd March 14, 2026
.Dt ZARCSUMMARY 1
.Os
.
.Sh NAME
.Nm zarcsummary
.Nd print ZFS ARC and subsystem statistics
.Sh SYNOPSIS
.Nm
.Op Fl adghr
.Op Fl p Ar page
.Op Fl s Ar section
.
.Sh DESCRIPTION
.Nm
collects statistics from the running kernel and prints a human-readable
report on the state of the ZFS Adaptive Replacement Cache
.Pq ARC
and several related subsystems.
The output is a snapshot; for periodic sampling, use
.Xr zarcstat 1
instead.
.Pp
When invoked without arguments,
.Nm
prints all sections in sequence.
A single section may be selected with the
.Fl s
option.
.Pp
The report is organized into the following sections:
.Bl -tag -offset Ds -width "tunables"
.It Sy arc
Current and target ARC sizes, the split between the Most Recently Used
.Pq MRU
and Most Frequently Used
.Pq MFU
lists, overall hit and miss ratios, compression savings, and whether the
ARC is experiencing memory pressure.
.It Sy archits
Breakdown of ARC hits and misses by demand reads versus prefetches
and data versus metadata.
.It Sy dmu
Data Management Unit predictive prefetcher statistics, including stream
hits, misses, and I/O issued ahead of and behind the current stream.
.It Sy l2arc
Level-2 ARC statistics for any cache devices attached to imported pools.
This section only appears when an L2ARC device is present and covers
bytes read and written, hit rates, and eviction activity.
.It Sy spl
Solaris Porting Layer memory allocator statistics.
This section is only available on Linux.
.It Sy tunables
Current values of kernel tunables that govern ARC behavior, including
.Va zfs_arc_max
and
.Va zfs_arc_min .
Combining this section with
.Fl d
adds a brief explanation of each tunable.
.It Sy zil
ZFS Intent Log statistics: committed transaction counts, flush requests
to stable storage, and the byte-level split between normal vdevs and
any separate log
.Pq SLOG
device.
.El
.
.Sh OPTIONS
.Bl -tag -width "-s"
.It Fl a , -alternate
Use an alternate layout for the tunables and SPL sections.
.It Fl d , -description
Print a short description next to each tunable and SPL entry.
.It Fl h , -help
Display a help message.
.It Fl g , -graph
Print an ASCII graph of current ARC use and exit.
.It Fl p Ar page , Fl -page Ar page
Print a section by its legacy page number.
.Sy Deprecated ;
use
.Fl s
instead.
.It Fl r , -raw
Dump all available data with minimal formatting.
.It Fl s Ar section , Fl -section Ar section
Print only the named section.
See
.Sx DESCRIPTION
for the list of valid section names.
.El
.
.Sh ENVIRONMENT
.Bl -tag -width "FreeBSD"
.It Sy Linux
Statistics are read from
.Pa /proc/spl/kstat/zfs/ .
.It Fx
Statistics are read via
.Xr sysctl 3
from the
.Va vfs.zfs
and
.Va kstat.zfs.misc
trees.
The
.Pa py-sysctl
package must be installed for
.Nm
to function.
.El
.
.Sh EXIT STATUS
.Ex -std
.
.Sh EXAMPLES
Print the full report:
.Dl # zarcsummary
.Pp
Display only ARC size and hit rate information:
.Dl # zarcsummary -s arc
.Pp
Show a visual overview of ARC health:
.Dl # zarcsummary -g
.Pp
Search for all statistics containing
.Dq hit :
.Dl # zarcsummary -r | grep hit
.Pp
Review all tunables with their descriptions:
.Dl # zarcsummary -d -s tunables
.
.Sh SEE ALSO
.Xr zarcstat 1 ,
.Xr zpool-status 8
.
.Sh HISTORY
.Nm
originated as a Perl script called
.Nm arc_summary ,
written by
.An Ben Rockwood
in 2008 for Solaris.
.An Martin Matuska
ported it to
.Fx
in 2010, and
.An Jason J. Hellenthal
took over maintenance.
.An Scot W. Stevenson
contributed a full rewrite in Python 3 in 2017, introducing the
.Fl g ,
.Fl r ,
and
.Fl s
options along with the section-based output layout.
The tool was renamed from
.Nm arc_summary
to
.Nm zarcsummary
in 2025 to align with the OpenZFS command naming convention.
.
.Sh AUTHORS
This manual page was written by
.An Christos Longros Aq Mt chris.longros@gmail.com .