btrfsslower(8) System Manager's Manual btrfsslower(8)
btrfsslower - Trace slow btrfs file operations, with per-event details.
btrfsslower [-h] [-j] [-p PID] [min_ms]
This tool traces common btrfs file operations: reads, writes, opens, and syncs. It mea‐
sures the time spent in these operations, and prints details for each that exceeded a
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.
CONFIG_BPF and bcc.
-p PID Trace this PID only.
min_ms Minimum I/O latency (duration) to trace, in milliseconds. Default is 10 ms.
Trace synchronous file reads and writes slower than 10 ms:
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
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.
Latency (duration) of I/O, measured from when it was issued by VFS to the filesys‐
tem, 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->d_iname).
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 btrfs writes and fsyncs, as well as all system
reads and opens (due to the current implementation of the btrfs_file_operations inter‐
face). 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 work‐
load; eg, 1M/sec), at which point the overhead of this tool may become noticeable. Mea‐
sure 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 tar‐
gets btrfs functions only, and not all file read/write paths (which can include socket
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.
btrfsdist(8), biosnoop(8), funccount(8), fileslower(8)
USER COMMANDS 2016-02-15 btrfsslower(8)