mdflush.bt - Man Page

Trace md flush events. Uses bpftrace/eBPF.

This tool traces flush events by md, the Linux multiple device driver (software RAID). The timestamp and md device for the flush are printed. Knowing when these flushes happen can be useful for correlation with unexplained spikes in disk latency.

This works by tracing the kernel md_flush_request() function using kernel dynamic tracing, and will need updating to match any changes to this function.

Note that the flushes themselves are likely to originate from higher in the I/O stack, such as from the file systems.

Since this uses BPF, only the root user can use this tool.

Requirements

CONFIG_BPF and bpftrace.

Examples

Trace md flush events:: # mdflush.bt

Fields

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