An example of a statistics report and the meaning
of the values.
- seconds_active
- The attribute seconds_active is a single value attribute (see Table 1) and simply
gives the seconds the FCP device has been active.
- requests
- The attribute requests is a three-valued attribute that provides
the number of requests that are processed since FCP
device activation split into the areas of (in that order):
The following example shows that three input, ten output, and
five control requests were issued since FCP device activation:
[root]# cat /sys/class/scsi_host/host0/requests
3 10 5
[root]#
- megabytes
- The attribute megabytes is a two-valued attribute that provides
the number of megabytes transferred in and out.
The following example shows that 3 MB were received and 6 MB were
sent out since FCP device activation:
[root@T6360007 host0]# cat /sys/class/scsi_host/host0/megabytes
3 6
[root@T6360007 host0]#
- utilization
- The attribute utilization is a three-valued attribute, showing
the utilization of the processor, bus, and FCP channel over
the last two seconds. The FCP channel continuously
refreshes the values covering the utilization of the individual sections
over the past two seconds. These values cannot be reset manually.
- cmd_latency, read_latency, and write_latency
- Each latency provides seven values as follows:
- value, minimum fabric latencies [microseconds]
- value, maximum fabric latencies [microseconds]
- value, summarized fabric latencies [microseconds]
- value, minimum channel latencies [microseconds]
- value, maximum channel latencies [microseconds]
- value, summarized channel latencies [microseconds]
- value, amount of requests
No interpretation or modification of the values is done by the zfcp
device driver. The individual values are summed up during normal operation
of the FCP device. An overrun of the variables
is neither detected nor treated. You must read the latency twice to
make a meaningful statement, because only the difference between the
values of the two reads can be used. Example: After reading the
file twice, you have the following values:
Type |
1st read |
2nd read |
Difference |
Fabric |
403 |
821 |
418 |
Channel |
115 |
163 |
48 |
Count |
21 |
23 |
2 |
The average fabric latency (see
Figure 1) over two readings is
418/2 = 209 microseconds
The
results for the other values can be calculated accordingly.
Figure 1. SCSI latency
breakdown