[back to "Optimizing server performance: Semaphores (Part 2)"]

How we used output from Show DBS (sidebar)
We converted a subset of the output from the Show DBS command from one of our R5 Domino servers into a table. Some of the columns of information helped detect bottleneck situations and helped us make optimization decisions before we made the final R5 release. So to understand the output in greater detail, understand that the database entries listed reflect which databases are requested for access; this list is not a superset or a subset of the entries in the Database Cache (see the output from the Show Directory command for the Database Cache entry list).

Name	Refs	Mod	FDs	LockWaits	Avg Waits	#Waiters	MaxWaiters
mail\cnoyes.nsf	3	N	1	0	0	0	0
Alice!!mail\hankins.nsf	2	N	0	0	0	0	0
mail\bhankins.nsf	4	Y	3	0	0	0	2
mail\rstephen.nsf	3	N	1	0	0	0	0
kerry\50Status.nsf	3	N	1	0	0	0	1
mail\mvincenz.nsf	3	N	1	0	0	0	0
StrHockey.nsf	3	Y	1	0	0	0	0
mail\rmaffa.nsf	7	N	1	0	0	0	1
clubusy.nsf	1	N	1	0	0	0	5
lotnames.nsf	73	N	20	0	0	0	4
notnames.nsf	79	N	17	0	0	0	1
names.nsf	128	N	17	0	0	0	3

Based on the table above, read the following way we used output from the Show DBS command to help detect semaphore issues:

• Monitor the columns LockWaits and Avg Waits for an indication of contention issues. In the example given above, there are no issues to report. But another command execution for another server may have shown numbers greater than zero for Avg Waits, which then provides a good opportunity to look at which file is the culprit. There is a good chance that file might be the Domino Directory (names.nsf), the Mail.Box file, or a unique database. If, for example, the Mail.Box has an Avg Waits time in the warning area, you may want to increase the number of Mail.Box files configured on your Domino server.
• If the Avg Waits time is in the warning area for a unique database (where “unique” really refers to a database that supports your applications), it is time to explore what application or agent is interacting with it and review the current procedure. A value in the "warning area" is really system-dependent when the value is above 0. Falling back on the original suggestion of getting a feel for your system, if you see a large change from the standard value reported, it is time to take proactive steps. So reviewing the output of the Show DBS command on a regular basis -- especially as it gives you a feel for activity on core databases -- is a good practice to follow.
• Consider the Refs count column, which represents the number of references to the database. You should consider this information within the big picture of understanding how your Domino server performs. Review the Refs count column in conjunction with the #Waiters column. Augment that information with the knowledge gathered about typical system behavior. For example, on a Domino mail server, you should expect the names.nsf file to have a high reference count. If #Waiters count is also growing, this is the point that you should determine that there is an issue that needs further investigation. So for another approach on making use of the available information, those databases that have a higher reference count are databases under contention. Their disk placement as well as view update rate needs to be considered individually.
• Review the #Waiters and MaxWaiters columns to see if there is a backlog of requests to gain access to the database, which can lead to a semaphore contention issue. If that information is steadily or actively growing, it could be an indication of a mounting or immediate problem (respectively).

For a practical observation, we used Show DBS output as part of our development efforts in fine tuning Domino Release 5. In particular, we reviewed Show DBS output to monitor the performance implications of multiple Mail.Box files. While running benchmark tests, we evaluated the impact of increasing the number of available Mail.Box files, going from 1 to 4, and saw the average wait time decrease. The table output given above illustrates an example of a "well-behaving" production system (that is, we did not observe any real problems).

For more information on the Show DBS command, see the topic "Improving database and Domino Directory performance" in Domino 5 Administration Help.