mdflush(8)

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

6       mdflush.bt - Trace md flush events. Uses bpftrace/eBPF.
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SYNOPSIS

9       mdflush.bt
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DESCRIPTION

12       This  tool  traces flush events by md, the Linux multiple device driver
13       (software RAID). The timestamp and md device for the flush are printed.
14       Knowing  when  these  flushes happen can be useful for correlation with
15       unexplained spikes in disk latency.
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17       This works by tracing the kernel md_flush_request() function using ker‐
18       nel  dynamic  tracing,  and  will need updating to match any changes to
19       this function.
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21       Note that the flushes themselves are likely to originate from higher in
22       the I/O stack, such as from the file systems.
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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 md flush events:
31              # mdflush.bt
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FIELDS

34       TIME   Time of the flush event (HH:MM:SS).
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36       PID    The  process  ID that was on-CPU when the event was issued. This
37              may identify the cause of the flush (eg,  the  "sync"  command),
38              but will often identify a kernel worker thread that was managing
39              I/O.
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41       COMM   The command name for the PID.
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43       DEVICE The md device name.
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OVERHEAD

46       Expected to be negligible.
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SOURCE

49       This is from bpftrace.
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51              https://github.com/iovisor/bpftrace
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53       Also look in the bpftrace distribution for  a  companion  _examples.txt
54       file containing example usage, output, and commentary for this tool.
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56       This  is  a  bpftrace version of the bcc tool of the same name. The bcc
57       tool may provide more options and customizations.
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59              https://github.com/iovisor/bcc
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OS

62       Linux
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STABILITY

65       Unstable - in development.
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AUTHOR

68       Brendan Gregg
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