man vsl

       Backend_health - Backend health check
              The result of a backend health probe.

              The format is:

                 %s %s %s %u %u %u %f %f %s
                 |  |  |  |  |  |  |  |  |
                 |  |  |  |  |  |  |  |  +- Probe HTTP response
                 |  |  |  |  |  |  |  +---- Average response time
                 |  |  |  |  |  |  +------- Response time
                 |  |  |  |  |  +---------- Probe window size
                 |  |  |  |  +------------- Probe threshold level
                 |  |  |  +---------------- Number of good probes in window
                 |  |  +------------------- Probe window bits
                 |  +---------------------- Status message
                 +------------------------- Backend name