Release 10.1C: OpenEdge Replication:
User Guide
Calculating current after-image volume
There are several ways to determine how much after-image data your database is currently generating. To determine the highest volume at any one time, you must sample at a small interval, such as one minute, for several days or over a month to arrive at average usage per month. Then, take the maximum value of your sample. If your network is close to capacity, the difference between your average load and your maximum load may be enough to saturate your network, causing additional network problems.
The following are four typical methods you can use to calculate after-image volume:
It is important to note that none of these methods take into account spikes in after-image activity.
Using after-image VST information
You can use VSTs to determine how many after-image blocks have been written. The VST table used for this purpose is _ActAILog. The fields with the appropriate information are _AiLog-TotWrites, and _AiLog-BytesWritn. The _AiLog-TotWrites field is in the form of after-image blocks.
Refer to your after-image block size to see how much data is involved. Take samples at the start of the time period and at the end of the time period. The difference between these fields results in the count of after-image blocks that have been written.
Using this method to determine after-image volume is preferable to other methods, as it is more flexible, easier to maintain, and less intrusive on database performance.
Using the RFUTIL command
To determine the after-image volume from the command line, you must run the following command at the start and at the end of your desired time frame:
The after-image extent size is shown in 1K blocks. Subtract the end value from the start value to determine how many blocks (count) have been written. Use this value to calculate the after-image generation rate per period depending on the network rate desired. It is important to note that emptying the after-image extents negates this method, as it changes the after-image block counts within the extent to zero.
Using the PROMON command
Start PROMON for the database that will become the source database by using the following command:
Once PROMON is running, do the following:
- Type R&D in the Enter your selection field, and press ENTER.
- Type 5 to select Adjust Monitor Options, and press ENTER.
- Type 3 to select Monitor sampling interval, and press ENTER.
- Type 3600 in the Enter sample interval <1 to 3600> field, and press ENTER twice.
- Type 2 to select Activity Displays, and press ENTER.
- Type 6 to select AI Log, and press ENTER.
The Activity: after-image Log appears, as shown:
Notice the highlighted elapsed time on the second line of the sample screen.
To gather a sample for 60 minutes, type S and press ENTER. The message “Sampling for 3600 . . . ” appears. When the 60 minutes have elapsed, the numbers shown on the rest of the screen reflect running totals for that elapsed time. Record the number from the Total after-image writes row in the Total column. This is the total number of after-image blocks written in one hour. You may repeat this process to gather additional samples. Calculating a realistic average number of blocks written in one hour requires several samples. The more samples gathered, the more accurate the average is.
Now divide the average number of blocks generated in one hour by after-image block size. The after-image block size can be found from the main menu of R&D by selecting option 1 (Status Displays) and then option 10 (after-image Log). The after-image block size is the second-to-last entry. Take this number and divide it by 1024 to arrive at the block size (8192 / 1024 = 8). Multiply the value from the activity menu by the value calculated. The result is the number of 1K after-image blocks written during a typical hour period.
Using before-image VST information
You can also use before-image VSTs to determine approximately how many after-image blocks will be written. The VST table used for this is _ActBILog. The fields with the appropriate information are _BiLog-TotWrites and _BiLog-BytesWritn. _BiLog-TotWrites is in the form of before-image blocks. Refer to your before-image block size to see how much data is involved. You must also take into account that the before-image block size must be the same as your after-image block size. Take samples at the start of the time period and at the end of the time period; the difference between these samples results in the count of after-image blocks that would have been written.
If you do not have after-imaging enabled against the database, this method is the only choice in calculating after-image volume.
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