BTRFSSLOWER-BPFCC(8) - Linux man page online | Administration and privileged commands

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

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 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.


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: # 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


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 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. 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).


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 I/O).


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.


Brendan Gregg


btrfsdist(8), biosnoop(8), funccount(8), fileslower(8)
USER COMMANDS 2016-02-15 btrfsslower(8)
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