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 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.

       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. Default 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 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.

       FILENAME
              A cached kernel file name (comes from dentry->d_iname).

       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, 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).

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)