Monday, November 30, 2015


Having recently posted a blog entry on my use of output from the DB2 for z/OS -DISPLAY BUFFERPOOL command, it seems a good time to post a companion entry focused on the -DISPLAY GROUPBUFFERPOOL command. Obviously, this latter command is relevant to people who work with DB2 operating in data sharing mode on a Parallel Sysplex. If you are such a person, read on.

First, enter the command the right way

Actually, you'll want to enter the command in two forms. First, from any one member of the DB2 data sharing group of interest, issue the following:


More specifically, issue that command twice, one hour apart, and retain the output of the second issuance of the command. Why? Because that command's output will show activity, from a group perspective, for each group buffer pool for a one-hour period of time (i.e., for the period of time between the first and second issuances of the command). In the command output you'll see, for each group buffer pool (GBP), a DSNB782I message, the text of which will include the phrase "INCREMENTAL GROUP DETAIL STATISTICS SINCE timestamp-value." You can check that timestamp value to verify that the data in the second issuance of a pair of hour-separated -DISPLAY GROUPBUFFERPOOL command captures one hour of activity.

Consider issuing pairs of hour-separated -DISPLAY GROUPBUFFERPOOL commands, in the form shown above, at two different times of day -- maybe once during a busy online transaction time, and once during a period of heavy batch activity.

The other form of the command you'll want to use is as follows:


Since this command provides member-view information, you might want to issue it for more than one member of the data sharing group. As mentioned above for the GDETAIL form of the command, issue the MDETAIL form, on a given DB2 data sharing group member, twice, with an hour between each command issuance, so as to capture (in the output of the second issuance of the command) one hour of member-scope activity for each GBP (note that for the MDETAIL form of the command, the "STATISTICS SINCE timestamp-value" information is in a DSNB771I message, versus the DSNB782I message associated with the GDETAIL form of the command). Also as mentioned previously, consider issuing pairs of hour-separated commands at different times of the day (maybe heavy online and heavy batch).

If you or one of your colleagues has some REXX programming skills, you can do as some have done and have a program issue pairs of hour-separated -DISPLAY GROUPBUFFERPOOL commands, and then have a REXX routine parse the output of the second of those commands and insert useful information (as described below) into a DB2 table or maybe a file. If such a program and associated REXX routine are executed on a daily basis, you will have information that you can use to track, and adjust to, trends in GBP usage.

How I use information in the output of -DISPLAY GROUPBUFFERPOOL(*) TYPE(GCONN) GDETAIL(INTERVAL)
  • Look for the "double zeros" -- referring here to zero write failures due to lack of storage (see the DSNB786I message text that's part of the command output), and zero cross-invalidations due to directory entry reclaims (part of the DSNB788I message text in the command output), for each GBP. If GBP write failures occur due to lack or storage, more than likely the GBP is too small (or the ratio of directory entries to data entries for the GBP is too large -- more on this below). You don't want GBP write failures, because that can land pages on the logical page list (LPL), and that can lead to failures for programs that subsequently attempt to access those pages prior to their being recovered from the LPL. [Note that a DB2 11 enhancement called group buffer pool write-around was introduced to help reduce the incidence of GBP write failures due to lack of storage. You can read about this enhancement in section 5.1 of the IBM "redbook" titled DB2 11 for z/OS Technical Overview (downloadable from] Cross-invalidations due to directory entry reclaims occur when a directory entry (used to track inter-DB2 interest in table space and index pages) has to be stolen (because all are in use and a new page has to be registered). Because the directory entry reclaim results in the GBP losing the "pointer" to a page, from a GBP-dependent data set, that is cached locally in a buffer pool of at least one group member, the page has to be preemptively invalidated wherever it is locally cached. You'd rather avoid that situation, because when a page invalidated for this reason is next accessed by a DB2 member subsystem, it will have to be read back into memory from disk (or from the GBP, but a read "hit" in this case tends to be quite unlikely). The page read in from disk will probably look just like the previously cached page that was marked invalid (because the invalidation was necessitated by a directory entry reclaim, not by a change of the page by a program on another group member); thus, a disk read that would be otherwise unnecessary is driven by a directory entry reclaim. The key to avoiding directory entry reclaims is to have a number of directory entries in a GBP that is at least as large as the number of data entries in the GBP plus the number of buffers in the corresponding local buffer pools in each member DB2 subsystem (as described in further detail in an entry I posted to this blog a couple of years ago).
  • Check out the specifications of each GBP. In particular, look at the ratio of directory entries to data entries. The default for this ratio is 5:1. If you see a directory-to-data-entry ratio that is significantly higher than 5:1, check to see if ALLOWAUTOALT(YES) is specified for the GBP in the CFRM policy for the Sysplex. If the directory-to-data-entry ratio for a GBP has been automatically increased by the system in order to avoid directory entry reclaims, that's OK, but in my experience this kind of adjustment can be an indication that the GBP is smaller than it should be (I wrote about this in a blog entry I posted a few months ago). Note that if you decide to make a GBP larger, make sure that all structures defined for both coupling facilities used by a DB2 data sharing group can fit in one coupling facility LPAR (except for secondary group buffer pools, when GBPs are -- as they should be -- duplexed), in case the other CF LPAR is down for maintenance purposes or due to a failure (if you temporarily had all structures in one CF LPAR, you would not during that time be duplexing the GBPs).
How I use information in the output of -DISPLAY GROUPBUFFERPOOL(*) TYPE(MCONN) MDETAIL(INTERVAL)
  • Basically, I use the output of this command to get one item of information that is not available in the output of the GDETAIL form of the command: the XI GBP hit ratio. That ratio (described in more detail in the blog entry referenced in the item above about directory-to-data-entry ratios) is so named because it has to do with synchronous GBP read requests that are driven by cross-invalidations (or XIs, in DB2 data sharing shorthand). Why does this ratio merit attention? Because if there are no cross-invalidations due to directory entry reclaims (see the first item above pertaining to the output of the GDETAIL form of the -DISPLAY GROUPBUFFERPOOL command) then what cross-invalidations did occur pretty much had to be caused by data-change activity (when a program connected to member DB2A changes a page cached locally in a buffer pool of member DB2B, the DB2B copy of the page has to be invalidated so that DB2B will know to retrieve the current version of the page). Data-change activity associated with GBP-dependent data sets results in changed pages being written to the appropriate GBP, and if pages were written to a GBP and the GBP has enough data entries to keep changed pages cached for a while, it is reasonable to expect that a good percentage of GBP reads driven by data-change-caused cross-invalidations will result in GBP read "hits." The XI read hit ratio is easily calculated using the SYNCHRONOUS READS DUE TO BUFFER INVALIDATION information in the DSNB773I message that is part of the -DISPLAY GROUPBUFFERPOOL MDETAIL output for each GBP: just divide the number in the DATA RETURNED field by the sum of the DATA RETURNED and DATA NOT RETURNED fields. I regularly see this ratio in excess of 80% for well-sized GBPs. If you see an XI read hit ratio that is substantially below 80% for one of your GBPs, consider making that GBP larger (if available CF LPAR memory permits).
There you have it. I hope that this information will be of use to you.

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