Troubleshooting
Problem
This document describes parameters on save commands that will affect performance when saving to tape media.
Resolving The Problem
Use Optimum Block Size (USEOPTBLK)
The USEOPTBLK parameter is used to send a larger block of data to backup devices that can take advantage of the larger block size. Every block of data that is sent has a certain amount of overhead that goes with it. This overhead includes block transfer time, IOA overhead, and backup device overhead. The block size does not change the IOA overhead and backup device overhead; however, the number of blocks does. For example, sending eight small blocks will result in eight times as much IOA overhead and backup device overhead. This allows the actual transfer time of the data to become the gating factor. In this example, eight software operations with eight hardware operations essentially become one software operation with one hardware operation when USEOPTBLK(*YES) is specified. The usual results are significantly lower CPU utilization, and the backup device will perform more efficiently.
Note: Specifying USEOPTBLK(*YES) may result in a tape that can be duplicated only to a device that supports the same block size.
When USEOPTBLK is set to *YES, the following will apply:
Data Compression (DTACPR)
Data compression is the ability to compress strings of identical characters and mark the beginning of the compressed string with a control byte. Strings of blanks from 2 to 63 bytes are compressed to a single byte. Strings of identical characters between 3 and 63 bytes are compressed to 2 bytes. If a string cannot be compressed, a control character is still added which will actually expand the data. This parameter is usually used to conserve storage media. If the backup device does not support data compaction, the System i software can be used to compress the data. This situation can require a considerable amount of CPU. When *yes is specified, software data compression is used. If the save is running while other jobs on the system are active and software compression is used, the overall system performance may be affected.
Notes:
1. If *DEV is specified for both this parameter and the Data compaction (COMPACT) parameter, only device data compaction is performed if device data compaction is supported on the device. Otherwise, data compression is performed.
2. If *YES is specified for this parameter and *DEV is specified for the COMPACT parameter, both device data compaction and device data compression are performed if supported on the device.
Data Compaction (COMPACT)
Data compaction is the same concept as software compression but available at the hardware level. If you wish to use data compaction, the backup device you choose must support it.
Notes:
1. If *DEV is specified for both the Data compression (DTACPR) parameter and this parameter, only device data compaction is performed if device data compaction is supported on the device. Otherwise, data compression is performed if supported on the device.
2. If *YES is specified for the DTACPR parameter and *DEV is specified for this parameter, both device data compaction and device data compression are performed if supported on the device.
3. For optimum performance, USEOPTBLK(*YES) DTACPR(*DEV) COMPACT(*DEV) is recommended on all native OS and BRMS save commands.
The USEOPTBLK parameter is used to send a larger block of data to backup devices that can take advantage of the larger block size. Every block of data that is sent has a certain amount of overhead that goes with it. This overhead includes block transfer time, IOA overhead, and backup device overhead. The block size does not change the IOA overhead and backup device overhead; however, the number of blocks does. For example, sending eight small blocks will result in eight times as much IOA overhead and backup device overhead. This allows the actual transfer time of the data to become the gating factor. In this example, eight software operations with eight hardware operations essentially become one software operation with one hardware operation when USEOPTBLK(*YES) is specified. The usual results are significantly lower CPU utilization, and the backup device will perform more efficiently.
Note: Specifying USEOPTBLK(*YES) may result in a tape that can be duplicated only to a device that supports the same block size.
When USEOPTBLK is set to *YES, the following will apply:
o | Performance may improve. |
o | The tape file that is created is only compatible with a device that supports the block size used. Commands such as Duplicate Tape (DUPTAP) do not duplicate files unless the files are being duplicated to a device which supports the same block size that was used. |
o | The value for the DTACPR parameter is ignored. |
Data Compression (DTACPR)
Data compression is the ability to compress strings of identical characters and mark the beginning of the compressed string with a control byte. Strings of blanks from 2 to 63 bytes are compressed to a single byte. Strings of identical characters between 3 and 63 bytes are compressed to 2 bytes. If a string cannot be compressed, a control character is still added which will actually expand the data. This parameter is usually used to conserve storage media. If the backup device does not support data compaction, the System i software can be used to compress the data. This situation can require a considerable amount of CPU. When *yes is specified, software data compression is used. If the save is running while other jobs on the system are active and software compression is used, the overall system performance may be affected.
Notes:
1. If *DEV is specified for both this parameter and the Data compaction (COMPACT) parameter, only device data compaction is performed if device data compaction is supported on the device. Otherwise, data compression is performed.
2. If *YES is specified for this parameter and *DEV is specified for the COMPACT parameter, both device data compaction and device data compression are performed if supported on the device.
Data Compaction (COMPACT)
Data compaction is the same concept as software compression but available at the hardware level. If you wish to use data compaction, the backup device you choose must support it.
Notes:
1. If *DEV is specified for both the Data compression (DTACPR) parameter and this parameter, only device data compaction is performed if device data compaction is supported on the device. Otherwise, data compression is performed if supported on the device.
2. If *YES is specified for the DTACPR parameter and *DEV is specified for this parameter, both device data compaction and device data compression are performed if supported on the device.
3. For optimum performance, USEOPTBLK(*YES) DTACPR(*DEV) COMPACT(*DEV) is recommended on all native OS and BRMS save commands.
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Historical Number
514445373
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Document Information
More support for:
IBM i
Component:
Save Restore
Software version:
All Versions
Operating system(s):
IBM i
Document number:
634953
Modified date:
19 December 2024
UID
nas8N1013198
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