| Previous | Contents | Index |
Whenever OpenVMS deletes a file, the various fragments of that file are placed into the ACP extent cache. The purpose of the extent cache is to speed up file allocations. Later, when a file is extended (or initially allocated), OpenVMS first scans the extent cache to fulfill the extension request. Since the extent cache contains random pieces of previously deleted files---the newly extended file becomes very fragmented!
In the event that the ACP extent cache doesn't contain enough disk fragments to fulfill the extension request, the ACP bitmap cache is scanned. This cache also contains random pieces of disk space.
If the ACP bitmap cache also does not contain enough disk space to fulfill the extension request, the entire BITMAP.SYS is scanned. This scan starts wherever the previous scan left off. The effect of the BITMAP.SYS file scan is again---a very fragmented file.
Fragmentation prevention intercepts file open, creation and extension requests. The parameters given in each request are examined. If OpenVMS system default values are given, and fragmentation prevention determines that the default values would cause the file to be fragmented, fragmentation prevention modifies the values. The values are modified by requesting contiguous best try file allocation, and by requesting an optimized file extension size. On a typical OpenVMS system, fragmentation prevention will intercept and optimize thousands of file create and extend requests per day!
To measure the effects of the fragmentation prevention feature, see Chapter 19, Benchmarking the Fragmentation Prevention Feature. |
Each time fragmentation prevention intercepts an XQP request, the request is examined. If needed, the request is modified to:
By default, the fragmentation prevention feature is enabled for all processes. In some cases you may wish to enable or disable the fragmentation prevention feature on a per-process basis.
The TTI_DLB:CDCONTROL procedure enables or disables fragmentation prevention on a per-process basis.
For example, to disable fragmentation prevention for your process, the
following command would be used:
$ @TTI_DLB:CDCONTROL DISABLE
The CDCONTROL procedure uses the CDCONTROL image. In order to execute this image, you must have the following privileges: NETMBX, TMPMBX |
$ @TTI_DLB:CDCONTROL ENABLE
$ @TTI_DLB:CDCONTROL DISABLE
$ @TTI_DLB:CDCONTROL SHOW
|
A successful ENABLE or DISABLE will display a message on the terminal indicating the success of the requested operation.
| Example 18-1 Using the SHOW Command |
|---|
$ @TTI_DLB:CDCONTROL SHOW Dynamic Load Balancer Plus (Version 4.1) - CDcontrol Copyright (c) 1991-1996 Touch Technologies, Inc. CD is ENABLED for your process. ...Modified file allocation requests: 13 |
| Example 18-2 Using the SHOW/ALL Command |
|---|
$ @TTI_DLB:CDCONTROL SHOW/ALL
Dynamic Load Balancer Plus (Version 4.1) - CDcontrol
Copyright (c) 1991-1996 Touch Technologies, Inc.
Modified
Pid Process Name User Name CD status Requests
26400081 SWAPPER not enabled
26400086 CONFIGURE SYSTEM enabled 0
26400087 IPCACP SYSTEM enabled 0
26400089 ERRFMT SYSTEM enabled 102
2640008A CACHE_SERVER SYSTEM enabled 0
2640008B CLUSTER_SERVER SYSTEM enabled 0
26400098 --RTclient-- SYSTEM enabled 0
2640009B XXX_09B_2 SMITH enabled 213
Modified file requests since CD was started: 486 (92%)
|
The last line of the display shows the total number of file create and extend requests that fragmentation prevention has optimized since it was started on this node. In order to measure the impact of fragmentation prevention on your system, you can check the percentage of all requests that fragmentation prevention modified.
In the above example, 92% of the file create and extend requests were found to be nonoptimal! Fragmentation prevention modified these nonoptimal requests, for a total of 486 modifications.
Without fragmentation prevention, these requests would have resulted in fragmented files, higher I/O overhead and poorer response times.
The fragmentation prevention Benchmark procedure requires 3000 blocks of disk space on the disk pointed to by SYS$SCRATCH. When the benchmark has completed, the disk space can be reclaimed. |
Included with fragmentation prevention, is a Benchmark procedure. By using the supplied procedure, you can measure the effectiveness of fragmentation prevention on your system. Also, the CDCONTROL SHOW/ALL command can be used to show the effects of fragmentation prevention system-wide (see Section 18.2.1, Showing Fragmentation Prevention Usage System-wide).
Benchmark symbols are used to perform specific steps in the benchmarking procedure.
| Symbol | Description | Argument |
|---|---|---|
| CRFILE | Creates an empty temporary file | none |
| EXFILE | Extends the temporary file by a number of blocks | Number of blocks to extend by |
| FRAGS | Displays file fragments | filename---defaults to the last temporary file created or extended |
$ @TTI_DLB:CD_BENCHSET |
After executing the Benchmark set up routine, you can create a file and extend it several times. Then, you can see how many fragments the file contains by running the FRAGS report. Each retrieval pointer in the FRAGS report represents one fragment.
These are the symbols you will enter to create and extend a file several times and run the FRAGS report:
The following example shows the results of a benchmark run.
| Example 19-1 Creating and Extending without fragmentation prevention Enabled |
|---|
$ @tti_dlb:cd_benchset
**********************************************
** CD Benchmark environment being set up...
Benchmark cleanup done
CleanDisk DISABLED
Fragmenting SYS$SCRATCH: by creating/deleting files
creating the temporary files...
deleting half of the temporary files...
$ crfile == "@tti_dlb:cd_do crfile "
$ exfile == "@tti_dlb:cd_do exfile "
$ frags == "@tti_dlb:cd_do frags "
** CD benchmark environment set up completed.
**********************************************
$
$ crfile
sys$scratch:tti_cd999.tmp created
$ exfile 100
Extending sys$scratch:tti_cd999.tmp by 100 blocks
sys$scratch:tti_cd999.tmp extended
$
$ exfile 100
Extending sys$scratch:tti_cd999.tmp by 100 blocks
sys$scratch:tti_cd999.tmp extended
$
$ exfile 100
Extending sys$scratch:tti_cd999.tmp by 100 blocks
sys$scratch:tti_cd999.tmp extended
$
$ exfile 100
Extending sys$scratch:tti_cd999.tmp by 100 blocks
sys$scratch:tti_cd999.tmp extended
$
$ exfile 100
Extending sys$scratch:tti_cd999.tmp by 100 blocks
sys$scratch:tti_cd999.tmp extended
$
$ frags
File name: TTI_CD999.TMP;1
Retrieval pointers
Count: 3 LBN: 411
Count: 6 LBN: 18294
Count: 3 LBN: 48459
Count: 3 LBN: 83100
Count: 3 LBN: 89787
.
.
.
Count: 3 LBN: 938211
Count: 6 LBN: 945297
Count: 6 LBN: 972132
Count: 90 LBN: 1027965
Count: 51 LBN: 1028226
$
|
You can now enable fragmentation prevention using the command:
$ @TTI_DLB:CDCONTROL ENABLE |
Then, create another file, extend it several times and run the FRAGS report to see how many fragments the file contains. You can use the following symbols:
The following example shows the results of a benchmark run with fragmentation prevention enabled.
| Example 19-2 Creating and Extending with fragmentation prevention Enabled |
|---|
$ @tti_dlb:cdcontrol enable
Dynamic Load Balancer Plus (Version 4.1) - CDcontrol
Copyright (c) 1991-1996 Touch Technologies, Inc.
CleanDisk ENABLED
$ crfile
sys$scratch:tti_cd998.tmp created
$ exfile 100
Extending sys$scratch:tti_cd998.tmp by 100 blocks
sys$scratch:tti_cd998.tmp extended
$
$ exfile 100
Extending sys$scratch:tti_cd998.tmp by 100 blocks
sys$scratch:tti_cd998.tmp extended
$
$ exfile 100
Extending sys$scratch:tti_cd998.tmp by 100 blocks
sys$scratch:tti_cd998.tmp extended
$
$ exfile 100
Extending sys$scratch:tti_cd998.tmp by 100 blocks
sys$scratch:tti_cd998.tmp extended
$
$ exfile 100
Extending sys$scratch:tti_cd998.tmp by 100 blocks
sys$scratch:tti_cd998.tmp extended
$
$ frags
File name: TTI_CD998.TMP;1
Retrieval pointers
Count: 90 LBN: 1033836
Count: 102 LBN: 1033929
Count: 309 LBN: 1034061
$
|
You will notice that with fragmentation prevention enabled, the created and extended file contains far fewer fragments than the file created without using fragmentation prevention. Fewer fragments means fewer I/Os and fewer window-turns. In most cases, fragmentation prevention prevents 90 percent or more of file fragmentation. The reduced fragmentation, in turn, reduces file I/Os.
If you do not see the results shown in this benchmark example, please contact our Technical Support staff for assistance. (The phone number is located in the preface of this manual.) |
$ @TTI_DLB:CD_DO CLEANUP |
This section describes the DLB Plus menus and the various items on each of the menus.
| Example A-1 DLB Plus Master Menu |
|---|
DLB Plus Dynamic Load Balancer PLUS 01-Sep-1995 +-------------------------------Master Menu-------------------------------+ | | | Dynamic Load Balancing Reports and Graphs | | DISPLAY DLB Display Utility SREPORTS Summary Reports... | | STATUS DLB Status Utility DREPORTS Detail Reports... | | GRAPHS Analysis Graphs... | | I/O Sampling SCAN Batch I/O Scan | | SAMPLE Sample I/Os and Locks | | CONS Consolidate Sample Data Batch Procedures | | SELECT Select a Data Sample BATCH Set Up Batch Stream | | LIVE Live Lock Analysis... SUBMIT Submit Batch Stream | | AUTO Automatic Sampling CANCEL Cancel Batch Set Up | | | | Miscellaneous | | SYSTEM VMS System Commands | | EXIT EXIT DLB Plus | +-------------------------------------------------------------------------+ EXIT = Exit \ = Back HELP = Help |
The summary reports on the Summary Analysis Reports menu are created from the I/O and lock information collected when the selected node was last sampled.
The Summary Analysis Reports menu appears as follows:
| Example A-2 Summary Analysis Reports Menu |
|---|
DLB Plus Dynamic Load Balancer PLUS 01-Sep-1995
+-----------Summary Analysis Reports------------+
| |
| PERFORMANCE REPORTS |
| RSHOT Hot File Analysis (High I/Os) |
| RSCACHE Suggested Files for Data Caching |
| RSFRAG File Fragmentation Analysis |
| RSDEVICE I/O Analysis Summary by Device |
| |
| PLANNING REPORTS |
| RSIMAGE I/O Analysis Summary by Image |
| RSUSER I/O Analysis Summary by User |
| RSPID I/O Analysis Summary by PID |
+-----------------------------------------------+
EXIT = Exit \ = Back HELP = Help
|
The reports on the Detail Analysis Reports menu are created from the I/O and lock information collected when the selected node was last sampled.
The Detail Analysis Reports menu appears as follows:
| Example A-3 Detail Analysis Reports Menu |
|---|
DLB Plus Dynamic Load Balancer PLUS 01-Sep-1995
+----------Detail Analysis Reports-----------+
| |
| DETAIL REPORTS |
| RDIMAGE I/O Analysis Detail by Image |
| RDUSER I/O Analysis Detail by User |
| RDDEVICE I/O Analysis Detail by Device |
| RDPID I/O Analysis Detail by PID |
| RDFILE I/O Analysis Detail by File |
+--------------------------------------------+
EXIT = Exit \ = Back HELP = Help
|
The graphs on the Analysis Graphs menu are created from the I/O information collected when the selected node was last sampled.
The Analysis Graphs menu appears as follows:
| Example A-4 Analysis Graphs Menu |
|---|
DLB Plus Dynamic Load Balancer PLUS 01-Sep-1995
+-----------------Analysis Graphs-----------------+
| |
| GRAPHS OF I/O COUNTS |
| GUSER I/O Analysis Summary by User |
| GIMAGE I/O Analysis Summary by Image |
| |
| GRAPHS OF I/O RATES |
| GRUSER I/O Rate Analysis Summary by User |
| GRIMAGE I/O Rate Analysis Summary by Image |
+-------------------------------------------------+
EXIT = Exit \ = Back HELP = Help
|
| Previous | Next | Contents | Index |