C.8. Log tracepoints

The tracepoints in this subsystem track blocks being added to and removed from the journal (gfs2_pin), as well as the time taken to commit the transactions to the log (gfs2_log_flush). This can be very useful when trying to debug journaling performance issues.

The gfs2_log_blocks tracepoint keeps track of the reserved blocks in the log, which can help show if the log is too small for the workload, for example.

The gfs2_ail_flush tracepoint (Red Hat Enterprise Linux 6.2 and later) is similar to the gfs2_log_flush tracepoint in that it keeps track of the start and end of flushes of the AIL list. The AIL list contains buffers which have been through the log, but have not yet been written back in place and this is periodically flushed in order to release more log space for use by the filesystem, or when a process requests a sync or fsync.