Case Study : Db2 Reorg takes longer in GDPC environment as opposed to a regular purescale environment

Body

Came across a behavior where Db2 Reorg was taking atleast 3X times longer in GDPC environment as opposed to a regular purescale environment.
 

We gathered various performance monitoring and diagnostic data (eg db2mon reports, db2trc etc) and reviewed the data /repro in lab. From the db2mon data, we can see that WAR (writing a new version of the page, which goes to both cf's) and RAR (read and register page) calls from a member to the CF on the other site take longer in a GDPC env.
For example, the avg time taken for WriteAndRegister (Write and Register command) from member 0 to cf 129 is higher than from member 0 to cf 128. And vice versa for WriteAndRegister calls for member 1. In a regular Purescale env, the average times for these calls from a member to both cf will be relatively closer.
 

Also, in a GPDC env, each IO to a filesystem will perform IO to both replicas. This will result in some overhead for write activity in the GDPC env as opposed to the regular purescale env.

The above factors will affect the time taken to complete a reorg operation in a GDPC env as opposed to a regular purescale env.
Given the metrics below, this behavior regarding the time was working as per design.
 

===================================================================
Round-trip CF command execution counts and average response times
===================================================================
db2mon.10230845 :

MEMBER ID     CF_CMD_NAME                    TOTAL_CF_REQUESTS    AVG_CF_REQUEST_TIME_MICRO
------ ------ ------------------------------ -------------------- -------------------------
     0    128 WriteAndRegister                               1128                     21.60
     0    129 WriteAndRegister                               1128                    739.55  
     1    128 WriteAndRegister                              12883                    747.36
     1    129 WriteAndRegister                              12884                    751.68
     0    128 ReadAndRegister                                1770                    121.51
     0    129 ReadAndRegister                                1791                   1291.98
     1    128 ReadAndRegister                                1813                    766.60
     1    129 ReadAndRegister                                3021                    652.21
     0    129 ReadAndRegisterMultiple                       26                  17559.50
     1    129 ReadAndRegisterMultiple                   18353                   4852.85

Thanks,
Shashank Kharche
IBM DB2 LUW Lab