zfsslower(8) System Manager's Manual zfsslower(8)
NAME
zfsslower - Trace slow zfs file operations, with per-event details.
SYNOPSIS
zfsslower [-h] [-j] [-p PID] [min_ms]
DESCRIPTION
This tool traces common ZFS file operations: reads, writes, opens, and
syncs. It measures the time spent in these operations, and prints de-
tails for each that exceeded a threshold.
WARNING: See the OVERHEAD section.
By default, a minimum millisecond threshold of 10 is used. If a thresh-
old of 0 is used, all events are printed (warning: verbose).
This uses kernel dynamic tracing of the ZPL interface (ZFS POSIX
Layer), and will need updates to match any changes to this interface.
This is intended to work with the ZFS on Linux project:
http://zfsonlinux.org
Since this uses BPF, only the root user can use this tool.
REQUIREMENTS
CONFIG_BPF and bcc.
OPTIONS
-p PID Trace this PID only.
min_ms Minimum I/O latency (duration) to trace, in milliseconds. De-
fault is 10 ms.
EXAMPLES
Trace synchronous file reads and writes slower than 10 ms:
# zfsslower
Trace slower than 1 ms:
# zfsslower 1
Trace slower than 1 ms, and output just the fields in parsable format
(csv):
# zfsslower -j 1
Trace all file reads and writes (warning: the output will be verbose):
# zfsslower 0
Trace slower than 1 ms, for PID 181 only:
# zfsslower -p 181 1
FIELDS
TIME(s)
Time of I/O completion since the first I/O seen, in seconds.
COMM Process name.
PID Process ID.
T Type of operation. R == read, W == write, O == open, S == fsync.
OFF_KB File offset for the I/O, in Kbytes.
BYTES Size of I/O, in bytes.
LAT(ms)
Latency (duration) of I/O, measured from when it was issued by
VFS to the filesystem, to when it completed. This time is inclu-
sive of block device I/O, file system CPU cycles, file system
locks, run queue latency, etc. It's a more accurate measure of
the latency suffered by applications performing file system I/O,
than to measure this down at the block device interface.
FILENAME
A cached kernel file name (comes from dentry->d_name.name).
ENDTIME_us
Completion timestamp, microseconds (-j only).
OFFSET_b
File offset, bytes (-j only).
LATENCY_us
Latency (duration) of the I/O, in microseconds (-j only).
OVERHEAD
This adds low-overhead instrumentation to these ZFS operations, includ-
ing reads and writes from the file system cache. Such reads and writes
can be very frequent (depending on the workload; eg, 1M/sec), at which
point the overhead of this tool (even if it prints no "slower" events)
can begin to become significant. Measure and quantify before use. If
this continues to be a problem, consider switching to a tool that
prints in-kernel summaries only.
Note that the overhead of this tool should be less than fileslower(8),
as this tool targets zfs functions only, and not all file read/write
paths (which can include socket I/O).
SOURCE
This is from bcc.
https://github.com/iovisor/bcc
Also look in the bcc distribution for a companion _examples.txt file
containing example usage, output, and commentary for this tool.
OS
Linux
STABILITY
Unstable - in development.
AUTHOR
Brendan Gregg
SEE ALSO
biosnoop(8), funccount(8), fileslower(8)
USER COMMANDS 2016-02-11 zfsslower(8)
Generated by dwww version 1.16 on Tue Dec 16 17:08:20 CET 2025.