NAME

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

SYNOPSIS

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

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.

       -d DURATION
              Total duration of trace in seconds.

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

       Trace for 10 seconds only:
              # btrfsslower -d 10

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 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)