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<title>Performance considerations for DDM</title>
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<!-- Java sync-link --><script language="Javascript" src="../rzahg/synch.js" type="text/javascript"></script>
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<h1 class="topictitle1">Performance considerations for DDM</h1>
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<div><p>These topics provide information to help you improve performance
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when using DDM and also provide some information about when to use something
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other than DDM to accomplish some functions. </p>
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<ul><li>When a DDM file is specified on the CPYF command, optimal performance
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is obtained if the following items are all true: <ul><li>The from-file is a logical or physical file and the to-file is a physical
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file.</li>
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<li>FMTOPT is *NONE, *NOCHK, or not specified.</li>
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<li>INCHAR, INCREL, ERRLVL, RCDDMT (*ALL), PRINT(*COPIED), PRINT(*EXCLD),
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SRCSEQ, TOKEY, SRCOPT, or FROMKEY parameter is not specified.</li>
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<li>The from-file is not overridden with the POS keyword, other than *NONE
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or *START.</li>
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<li>The to-file is not overridden with INHWRT(*YES).</li>
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</ul>
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</li>
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<li>The <span class="cmdname">Open Query File (OPNQRYF)</span> command uses <span class="keyword">System/38™</span> extensions to the DDM architecture.
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The <span class="keyword">System/38</span> DDM architecture
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extensions minimize DDM system processing time. These extensions are not used
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when: <ul><li>The source server is neither a <span class="keyword">System/38</span> nor
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an <span class="keyword">iSeries™ server</span></li>
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<li>The target server is neither a <span class="keyword">System/38</span> nor
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an <span class="keyword">iSeries server</span></li>
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</ul>
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</li>
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<li>You can greatly reduce the amount of data transferred between servers
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if you use query functions such as the <span class="keyword">iSeries</span> command
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OPNQRYF OPTIMIZE(*YES). However, for user-written applications, the amount
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of data exchanged between the servers is larger than that used to communicate
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using DDM with non-<span class="keyword">iSeries server</span>s.
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The additional data provides <span class="keyword">iSeries</span> extended
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DDM functions and also reduces source server DDM processing overhead. Using
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normal read, write, update, add, and delete operations as examples, consider
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the following items: <ul><li>Standard DDM architecture DDM overhead data includes such information
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as a file identification, operation code, and simple result information. A
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user program read-by-key operation uses approximately 40 characters of DDM
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information in addition to the length of the key data. Data returned from
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the remote server uses approximately 32 characters of DDM information plus
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the length of the data file record.</li>
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<li><span class="keyword">System/38</span> DDM architecture
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extensions cause additional data overhead such as record format identification
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and a major portion of the I/O feedback area information. A user program read-by-key
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operation uses approximately 60 characters of DDM information in addition
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to the length of the key data. Data returned from the remote server uses approximately
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80 characters of DDM information plus the length of the data file record.
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Normally, the additional length in data streams is not noticeable. However,
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as line activity increases, line utilization might peak sooner when using
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these extended data streams as opposed to standard DDM data streams.</li>
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</ul>
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</li>
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<li>The target DDM job priority is controlled by the job class specified by
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the associated subsystem description routing entry. The following routing
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entry is normally the one used for all target (program start request) jobs:
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<pre>ADDRTGE ... PGM(*RTGDTA) ... CMPVAL(PGMEVOKE 29)</pre>
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<p>The
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subsystems QBASE and QCMN, which are shipped with the <span class="keyword">iSeries server</span>,
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have this routing entry.</p>
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<div class="p">To have target DDM jobs in a subsystem run
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at a different priority than other APPC target jobs in the same subsystem,
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insert the following routing entry with the appropriate sequence number: <pre>ADDRTGE SBSD(xxx) SEQNBR(nnn) CMPVAL(QCNTEDDM 37)
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PGM(*RTGDTA) CLS(uuu)</pre>
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</div>
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<p>The class <em>uuu</em> is used
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to determine target job priority.</p>
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</li>
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<li>To display records in a remote file, display station pass-through should
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be used whenever possible. Otherwise, direct record positioning should be
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used with the <span class="cmdname">Display Physical File Member (DSPPFM)</span> command.</li>
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<li>If a DDM user exit security program is a CL program and it creates an <span class="keyword">i5/OS™</span> exception and an inquiry message
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that requires the target system operator to respond, both the user exit program
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and the source server job must wait for the response. Consider using the default
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system reply list by specifying INQMSGRPY(*SYSRPYL) for the TDDM job's description
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specified on the <span class="cmdname">Add Communications Entry (ADDCMNE)</span> command
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for that APPC remote location information. </li>
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<li>The WAIT and WAITFILE parameters, used on commands like <span class="cmdname">Allocate
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Object (ALCOBJ)</span> or <span class="cmdname">Receive Message (RCVMSG)</span>,
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have no effect on the source server program. These parameters function the
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same as they do when local files are accessed. The wait time values specified
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on commands run on the source server do not take effect on the source system;
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they affect only the target server and only if it is an <span class="keyword">iSeries server</span> or
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a <span class="keyword">System/38</span>. <div class="note"><span class="notetitle">Notes:</span> <ol><li>The WAITFILE parameter of the create or change <span class="keyword">i5/OS</span>-Intersystems
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Communications Function (ICF) file command determines how long the APPC support
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will wait for session resources to become available when doing an acquire
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operation or a start function. The WAITFILE value is not used for sessions
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where the connection to the adjacent server is over a switched connection.
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An example is an SDLC switched line, an X.25 SVC line, an Ethernet line, or
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a token-ring connection. Using a switched connection, acquire operations and
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start functions do not time out.</li>
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<li>Because APPN sessions might cross multiple servers and lines to reach
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the remote server, the WAITFILE timer should be adjusted to allow more time
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in these cases. You should not specify *IMMED for the WAITFILE parameter if
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your application is running in a network configured to use APPN functions.
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The value you specify for this parameter is dependent on the size and type
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of the network.</li>
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</ol>
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</div>
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</li>
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<li><p>As for any LU session type 6.2 data exchange, certain SNA parameters
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can affect performance. These parameters include the path information unit
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size (MAXFRAME), the request/response unit size (MAXLENRU), SNA pacing (INPACING,
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OUTPACING), and for X.25, packet size and window size. In general, the larger
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the value used, the better the realized performance.</p>
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</li>
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<li><strong>SNA path information unit size</strong> <p>The path information unit (PIU)
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is the size of the actual data transmission block between two servers. The
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MAXFRAME parameter on the Create Controller Description (APPC) (CRTCTLAPPC)
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or Create Controller Description (SNA Host) (CRTCTLHOST) command specifies
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the path information unit size the local server attempts to use. During session
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establishment, both servers determine which size is used, and it is always
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the smaller value. Other remote servers might have different PIU size considerations.</p>
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</li>
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<li><strong>SNA response/request unit size</strong> <p>The response/request unit (RU)
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size (CRTMODD MAXLENRU) controls the amount of server buffering before fitting
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that data into the path information unit that is actually transmitted. In
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APPC, the transmit and receive RU lengths are negotiated during session establishment.
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Again, the negotiation results in the smallest value being used. Other remote
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servers have different RU size considerations.</p>
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</li>
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<li><strong>SNA pacing values</strong> <p>The pacing value determines how many request/response
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units (RUs) can be received or sent before a response is required indicating
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buffer storage is available for more transmissions. During session establishment,
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both servers determine which size is used, and it is always the smaller value.</p>
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<p>In
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cases where both batch and interactive processing occur at the same time on
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the same communications line, <span class="keyword">iSeries</span> job
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priority might be used to favor interactive processing over batch processing.
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In addition, reducing the value of pacing for a batch application and raising
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it for an interactive application might be necessary to provide a level of
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line activity priority for the interactive application.</p>
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<p>On an <span class="keyword">iSeries server</span>, different pacing values can
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be specified through the creation of different MODES (Create Mode Description
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[CRTMODD] command) to the different applications. Other remote
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systems have different SNA pacing value considerations.</p>
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</li>
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<li><strong>X.25 packet</strong> <p>An X.25 packet smaller than the MAXFRAME value
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adds data transmission time over a non-X.25 data link. In general, for X.25,
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the longer the MAXFRAME and the actual amount of data being transmitted, the
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greater this difference is. In the case of DDM, which adds DDM control information
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to the normal file record data, the packet size has an additional effect on
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the difference between local and remote file processing and between non-X.25
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and X.25 data links.</p>
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<p>In cases of many deblocked DDM operations, the
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number of packets required to transmit data might become so high that packet
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processing overhead within the X.25 adapter affects performance significantly.
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Use the largest X.25 packet window size supported by the network and communicating
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products to maximize performance.</p>
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<p>When X.25 must be used to access remote
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files, successive transmission of many small deblocked records, such as less
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than 80 character records, might cause the X.25 adapter to expend a disproportionate
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amount of time processing X.25 packet characters as opposed to transmission
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of user data.</p>
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<p>In general, the overhead in processing X.25 packets results
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in less throughput than the use of a conventional line when identical line
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speeds are used and data transfer is in only one direction. When data is transferred
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at the same time in both directions, the advantages of X.25 duplex support
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is realized. On the <span class="keyword">System/38</span>,
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the overall processing effect is minimal, because the overhead in processing
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the packets is done within the Integrated X.25 Adapter.</p>
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</li>
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</ul>
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<p>In general, the processing of remote files by using DDM is transparent
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to an application program or utility function, such as that provided by the
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Copy File (CPYF) command. However, additional time is required when accessing
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remote files by using a communications line. The performance difference between
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local file and remote file processing is proportional to the number of accesses
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to remote files, the data record length, and the line speed during a unit
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of performance measurement.</p>
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<p>An additional difference between local and remote file processing is that
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the input or output operation to a local file might not result in an immediate
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physical disk operation because the server transfers blocks of data from the
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disk and writes blocks of data to the disk. There are times, then, that the
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user program accesses data within main storage and the physical I/O occurs
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at a different time. Therefore, to minimize the difference between local file
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and remote file performance, it is essential that knowledge of an application
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design and the amount and type of accesses to files be considered when determining
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which files are to be accessed remotely using DDM.</p>
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<p>The additional time for each remote access is comprised of: </p>
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<ul><li>Additional system processing to convert local server file interfaces to
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the DDM architecture interfaces</li>
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<li>Amount of data transmitted over the communications line</li>
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<li>Amount of remote system processing of the file operations</li>
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<li>Speed of the communications line</li>
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</ul>
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<p>The communications line time accounts for most of the additional time,
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though the actual time is dependent on line speed and the amount of line activity
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during the DDM function.</p>
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<p>As is true in non-DDM cases, local and remote server job priorities have
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the most significant effect on performance. On an <span class="keyword">iSeries server</span>,
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the PRIORITY and TIME SLICE values of the class being used control job priority.
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The SDDM runs under the source job, and the TDDM runs under the class assigned
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to the APPC routing entry of the target server's subsystem. In applications
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that access multiple files, the best results are achieved when the most heavily
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accessed files are on the same server as the program that is running and the
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less heavily accessed files are on a remote server. Key considerations regarding
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the placement of files and application programs follow: </p>
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<ul><li>The system having primary responsibility for file maintenance needs to
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be identified. In all cases of multiple servers applications, the best performance
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results if only one server is responsible for file maintenance. If an application
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program maintains the file through exclusive (nonshared) processing, best
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performance can be realized when the application program resides on the system
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with the file. <div class="p">In some cases, transmitting the file back to the local
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server might require: <ul><li>An APPC program.</li>
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<li>A program using remote DDM files.</li>
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<li>The Copy File (CPYF) command by using DDM.</li>
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<li>Object distribution SNDNETF and RCVNETF operations. In interactive applications,
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display station pass-through should be considered when the amount of display
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data transferred is significantly less than the amount of database file data
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that would be sent by using DDM.</li>
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</ul>
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</div>
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</li>
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<li>In cases where file placement requires movement of application processing
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to a remote server for best performance results, use of the Submit Remote
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Command (SBMRMTCMD) command should be considered. This works best in a batch
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processing input stream where each program waits for the preceding program
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to complete. The use of the SBMRMTCMD command is valid only when the source
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and target servers are <span class="keyword">iSeries server</span>s
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or Systems/38s. For example, assume that program A accesses local files. Program
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A would run on a local server. Program B accesses remote files. You can use
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the SBMRMTCMD command to run program B on the remote server.</li>
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<li>In cases where file maintenance is shared across servers, the best performance
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can be obtained if the file is placed on the server with the largest percentage
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of file update, add, and delete operations. <p>In certain cases, a pair of
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source and target APPC programs can provide performance improvements over
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DDM. For example, assume 10 records are to be retrieved from the remote server.
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When DDM is used and record blocking cannot be used (for example, user program
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random input operation, sequential input for change, or use of the OVRDBF
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SEQONLY[*NO] command), assume 10 data transmissions are sent and 10 are received,
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|
|
for a total of 20 transmissions. User-written APPC programs can build additional
|
||
|
|
intelligence into the data stream such that request for the data and receipt
|
||
|
|
of the data can be done in two data transmissions instead of 20, one request
|
||
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|
for all records of customer 00010 and one response containing 10 records for
|
||
|
|
customer 00010.</p>
|
||
|
|
</li>
|
||
|
|
</ul>
|
||
|
|
<p>Consider two sample application processing techniques, batch file processing
|
||
|
|
and interactive file processing.</p>
|
||
|
|
</div>
|
||
|
|
<div>
|
||
|
|
<ul class="ullinks">
|
||
|
|
<li class="ulchildlink"><strong><a href="rbae5batchproc.htm">Batch file processing with DDM</a></strong><br />
|
||
|
|
Consider these items when using batch file processing with DDM.</li>
|
||
|
|
<li class="ulchildlink"><strong><a href="rbae5interactivefp.htm">Interactive file processing with DDM</a></strong><br />
|
||
|
|
Consider these items when using interactive file processing with DDM.</li>
|
||
|
|
<li class="ulchildlink"><strong><a href="rbae5length.htm">DDM conversation length considerations</a></strong><br />
|
||
|
|
Consider these items regarding the length of conversations when using DDM.</li>
|
||
|
|
</ul>
|
||
|
|
|
||
|
|
<div class="familylinks">
|
||
|
|
<div class="parentlink"><strong>Parent topic:</strong> <a href="rbae5ddmop.htm" title="This topic provides task-oriented information and examples that describe various aspects of DDM operation considerations.">Operating considerations for DDM</a></div>
|
||
|
|
</div>
|
||
|
|
<div class="relconcepts"><strong>Related concepts</strong><br />
|
||
|
|
<div><a href="rbae5ddm1.htm" title="This topic describes how iSeries security relates to DDM, and how it can limit access to the data resources of a target server by source server programs and users.">Security</a></div>
|
||
|
|
<div><a href="../books/sc415406.pdf" target="_blank">Communications Management PDF</a></div>
|
||
|
|
<div><a href="../rzahj/rzahjovr.htm">APPC, APPN, and HPR</a></div>
|
||
|
|
<div><a href="../books/sc415404.pdf " target="_blank">LAN, Frame-Relay, and ATM Support PDF</a></div>
|
||
|
|
</div>
|
||
|
|
<div class="relref"><strong>Related reference</strong><br />
|
||
|
|
<div><a href="rbae5brwrcds.htm" title="The Display Physical File Member (DSPPFM) command can be used to display a remote file on a target server.">Display DDM remote file records</a></div>
|
||
|
|
<div><a href="rbae5uepcsd.htm" title="There are some considerations that you should understand before using user exit programs for DDM.">User exit program considerations for DDM</a></div>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
</body>
|
||
|
|
</html>
|