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zfsslower(8) System Manager's Manual zfsslower(8)


zfsslower - Trace slow zfs file operations, with per-event details.


zfsslower [-h] [-j] [-p PID] [min_ms]


This tool traces common ZFS file operations: reads, writes, opens, and syncs. It measures the time spent in these operations, and prints details for each that exceeded a threshold.

WARNING: See the OVERHEAD section.

By default, a minimum millisecond threshold of 10 is used. If a threshold 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.

Since this uses BPF, only the root user can use this tool.


CONFIG_BPF and bcc.


-p PID Trace this PID only.

Minimum I/O latency (duration) to trace, in milliseconds. Default is 10 ms.


# zfsslower
# zfsslower 1
# zfsslower -j 1
# zfsslower 0
# zfsslower -p 181 1


Time of I/O completion since the first I/O seen, in seconds.
Process name.
Process ID.
Type of operation. R == read, W == write, O == open, S == fsync.
File offset for the I/O, in Kbytes.
Size of I/O, in bytes.
Latency (duration) of I/O, measured from when it was issued by VFS to the filesystem, to when it completed. This time is inclusive 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.
A cached kernel file name (comes from dentry->
Completion timestamp, microseconds (-j only).
File offset, bytes (-j only).
Latency (duration) of the I/O, in microseconds (-j only).


This adds low-overhead instrumentation to these ZFS operations, including 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).


This is from bcc.

Also look in the bcc distribution for a companion _examples.txt file containing example usage, output, and commentary for this tool.




Unstable - in development.


Brendan Gregg


biosnoop(8), funccount(8), fileslower(8)

2016-02-11 USER COMMANDS