FCM Tuning
With DB2
version 7, the DBA has to collect the snapshot statistics and adjust the parameters
accordingly. Fortunately, in DB2 version 8.1, most of the FCM configuration parameters
are self-configurable besides FCM buffer parameter.
FCM_NUM_BUFFERS
FCM resources
can be monitored via database snapshot monitoring by frequently taking
snapshots and storing the output in a log file. When the number of free buffers
falls below 25% of free FCM buffers, adding new FCM buffers is urgently needed.
My
opinion in this is in direct opposition to the IBM documentation, which states to
add additional FCM buffers at a a low limit of 15%. Because a shortage of FCM
buffers can cause database crashes and serious performance degradation, it is my
opinion that additional FCM buffers should be added if the free buffers fall
below 25%. IBM documentation, for example does not mention the possibility of a
database crash due to the lack of FCM buffers. In reality this is happening.
Let's look at a DBM
configuration with 204.1 buffers, where the level of 512 free buffers would signal
the necessity of making a change.
Online, the FCM
buffers extension for DB2 version 8.1:
db2 => update dbm cfg using FCM_NUM_BUFFERS 4096
DB20000I The UPDATE DATABASE MANAGER CONFIGURATION command completed successfully.
Changing the FCM_NUM_BUFFERS parameter
in DB2 version 7 is similar, with only one exception. We need to restart DBM
for changes to become active.
FCM_NUM_RQB
The following is an additional hint for DB2 version 7. The
Parameter fcm_num_rqb has to be configured to support the maximum number of connections.
Listing DBM parameters related to the maximum connections:
$ db2 get dbm cfg |grep AGENTS
Max number of existing agents (MAXAGENTS) = 400
Agent pool size (NUM_POOLAGENTS) = 40 (calculated)
Initial number of agents in pool (NUM_INITAGENTS) = 0
Max number of coordinating agents (MAX_COORDAGENTS) = (MAXAGENTS - NUM_INITAGENTS)
Max no. of concurrent coordinating agents (MAXCAGENTS) = MAX_COORDAGENTS
Max number of logical agents (MAX_LOGICAGENTS) = MAX_COORDAGENTS
Max number of DARI processes (MAXDARI) = MAX_COORDAGENTS
According
to the actual DBM setting, the calculation is next:
MAX_COORDAGENTS= MAXAGENTS-NUM_INITAGENTS = 400 0 = 400
My test
system is configured to support the maximum number of 400 connections.
fcm_num_rqb >= max_coordagents*2,5= 400*2,5=1000
The current
setting is sufficient for
DBM defined agents.
# db2 get dbm cfg | grep FCM_NUM_RQB
Number of FCM request blocks (FCM_NUM_RQB) = 2048
Conclusion
With this article, Fast Communication Manager has been fully
explained.
There are some differences in FCM behavior between AIX and SUN platform in
memory allocation. Intentionally I did not mix AIX into my article, leaving IBM
to cover this area with some document in the future. Some things that every DBA
should know before starting with any kind of tuning:
-
current instance and database settings
-
design/implementation structure
- number
of database partitions
- number
of active instance users
- level
of application code complexity
Once
armed with the above information, a DBA's view becomes sharpened and he can
predict potentional hot spots in self-tuned and high intelligence DB2 databases.
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