NAME

       btrfsslower - Trace slow btrfs file operations, with per-event details.

SYNOPSIS

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

DESCRIPTION

       This  tool  traces  common  btrfs  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).

       Since this works by tracing the btrfs_file_operations interface functions,  it  will  need
       updating to match any changes to these functions.

       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:
              # btrfsslower

       Trace slower than 1 ms:
              # btrfsslower 1

       Trace slower than 1 ms, and output just the fields in parsable format (csv):
              # btrfsslower -j 1

       Trace all file reads and writes (warning: the output will be verbose):
              # btrfsslower 0

       Trace slower than 1 ms, for PID 181 only:
              # btrfsslower -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 btrfs writes and fsyncs, as well as all system
       reads  and  opens  (due  to  the  current  implementation  of  the   btrfs_file_operations
       interface). Particularly, all reads and writes from the file system cache will incur extra
       overhead while tracing. Such reads and writes can  be  very  frequent  (depending  on  the
       workload;  eg,  1M/sec),  at  which point the overhead of this tool may become noticeable.
       Measure and quantify before use. If this continues to be a problem, consider switching  to
       a tool that prints in-kernel summaries only, such as btrfsdist(8).

       Note  that  the  overhead  of  this  tool  should be less than fileslower(8), as this tool
       targets btrfs 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

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