writeback(8)

1writeback(8)                System Manager's Manual               writeback(8)
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NAME

6       writeback.bt  -  Trace  file system writeback events with details. Uses
7       bpftrace/eBPF.
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SYNOPSIS

10       writeback.bt
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DESCRIPTION

13       This traces when file system dirtied pages are flushed to disk by  ker‐
14       nel  writeback,  and  prints details including when the event occurred,
15       and the duration of the event. This can be useful for correlating these
16       times  with  other  performance  problems,  and if there is a match, it
17       would be a clue that the  problem  may  be  caused  by  writeback.  How
18       quickly  the  kernel  does writeback can be tuned: see the kernel docs,
19       eg, vm.dirty_writeback_centisecs.
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21       This   uses   the   tracepoint:writeback:writeback_start   and   trace‐
22       point:writeback:writeback_written tracepoints.
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24       Since this uses BPF, only the root user can use this tool.
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REQUIREMENTS

27       CONFIG_BPF and bpftrace.
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EXAMPLES

30       Trace all writeback events with timestamps and latency details:
31              # writeback.bt
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FIELDS

34       TIME   Time that the writeback event completed, in %H:%M:%S format.
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36       DEVICE Device name in major:minor number format.
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38       PAGES  Pages written during writeback.
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40       REASON Reason  for  the  writeback  event.  This  may  be "background",
41              "vmscan", "sync", "periodic", etc.
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43       ms     Duration of the writeback event in milliseconds.
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OVERHEAD

46       Since writeback events are expected to  be  infrequent  (<10/sec),  the
47       overhead of this tool is expected to be negligible (near 0%).
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SOURCE

50       This is from bpftrace.
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52              https://github.com/iovisor/bpftrace
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54       Also  look  in  the bpftrace distribution for a companion _examples.txt
55       file containing example usage, output, and commentary for this tool.
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OS

58       Linux
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STABILITY

61       Unstable - in development.
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AUTHOR

64       Brendan Gregg
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