writeback.bt(8)

📦
version 0.27.0 · 3 versions
  • 0.27.0 · alpine 3.24, arch rolling, centos-stream 10, centos-stream 9, debian trixie, fedora 43, fedora 44, nixos 26.05, nixpkgs unstable, opensuse-leap 16.0, ubuntu questing, ubuntu resolute, ubuntu stonking (latest)
  • 0.14.0 · ubuntu jammy
  • 0.20.2 · ubuntu noble
, in alpine 3.24, arch rolling, centos-stream 10, centos-stream 9, debian trixie, fedora 43, fedora 44, nixos 26.05, nixpkgs unstable, opensuse-leap 16.0, ubuntu questing, ubuntu resolute, ubuntu stonking

🎀 NAME 🎀

okay so the manual name is writeback.bt, and it's a bpftrace/eBPF tool that traces file system writeback events with details 🔥 it's giving observability bestie energy ✨

💖 SYNOPSIS 💖

writeback.bt

✨ DESCRIPTION ✨

okay bestie, this one traces when file system dirtied pages get flushed to disk by kernel writeback 💾 it prints details including when the event happened and how long it took ⏱️ this is lowkey so useful for matching those times up with other performance problems 📈 if they line up, that's a clue the problem might be coming from writeback 🕵️‍♀️

how fast the kernel does writeback can be tuned too 🎛️ check the kernel docs for stuff like vm.dirty_writeback_centisecs 🌸

this uses the tracepoint:writeback:writeback_start and tracepoint:writeback:writeback_written tracepoints 🧚

🙈 since this runs on BPF, only the root user can use this tool. no exceptions, no cap.

🦋 REQUIREMENTS 🦋

you'll need CONFIG_BPF and bpftrace 🛠️

💅 EXAMPLES 💅

trace all writeback events with timestamps and latency details:

# writeback.bt

📋 FIELDS 📋

  • TIME — the time the writeback event completed, in %H:%M:%S format ⏰
  • DEVICE — the device name, in major:minor number format 💿
  • PAGES — how many pages were written during writeback 📄
  • REASON — why the writeback event happened. this might be "background", "vmscan", "sync", "periodic", etc 🎯
  • ms — how long the writeback event lasted, in milliseconds ⚡

🌈 OVERHEAD 🌈

writeback events are supposed to be pretty rare (<10/sec), so the overhead of this tool is expected to be negligible, aka near 0% 🫶

🍓 SOURCE 🍓

this is from bpftrace 💜

https://github.com/bpftrace/bpftrace

also peek in the bpftrace distribution for a companion _examples.txt file. it has example usage, output, and commentary for this tool 📚

🧚 OS 🧚

Linux 🐧

⭐ STABILITY ⭐

⚠️ unstable, still in development. it's giving work in progress 🚧

👑 AUTHOR 👑

Brendan Gregg 🙌

🔗 SEE ALSO 🔗

biosnoop.bt(8) 🫧

that's the whole tea, bestie! 🌈 this is writeback.bt from the USER COMMANDS section, dated 2018-09-14. stay iconic 💖✨

📦 where this page comes from