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_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, 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)