Log data set definitions

Before setting up the log data sets, review the following section to decide on the most appropriate configuration for your system.

 

Should your installation use single or dual logging?

If your DASD type is 3390 or similar, you are recommended to use dual logging to ensure that you have an alternative backup source in the event of losing a data set. You should also use dual BSDSs and dual archiving to ensure adequate provision for data recovery.

If you use devices with in-built data redundancy (for example, Redundant Array of Independent Disks (RAID) devices) you might consider using single active logging. If you use persistent messages, single logging can increase maximum capacity by 10-30% and can also improve response times.

Dual active logging adds a small performance overhead. You might want to specify it on a test system used for benchmarking, in addition to a production system.

 

How many active log data sets do we need?

You should have sufficient active logs to ensure that your system is not impacted in the event of an archive being delayed.

In practice, you should have at least three active log data sets but it is preferable to define more. For example, if the time taken to fill a log is likely to approach the time taken to archive a log during peak load, you should define more logs. You are also recommended to define more logs to offset possible delays in log archiving. If you use archive logs on tape, allow for the time required to mount the tape.

It is possible to dynamically define new active log data sets as a way of minimizing the effect of archive delays or problems. New data sets can be rapidly brought online, in order to avoid queue manager 'stall' due to lack of space in the active log.

 

How large should the active logs be?

Your logs should be large enough so that it takes at least 30 minutes to fill a single log during peak message loads. If you are archiving to tape, you are advised to make the logs large enough to fill one tape cartridge, or a number of tape cartridges. (For example, a log size of 1000 cylinders on 3390 DASD fits onto a 3490E non-compacted tape with space to spare.)

Note:

When archiving to tape, a copy of the BSDS is also written to the tape. When archiving to DASD, a separate data set is created for the BSDS copy.

If the logs are small (for example, 10 cylinders) it is likely that they will fill up frequently, which could result in performance degradation. In addition, you might find that the large number of archive logs required is difficult to manage.

If the logs are very large, and you are archiving to DASD, we need a corresponding amount of space reserved on DASD for SMS retrieval of migrated archive logs, which might cause space management problems. In addition, the time taken to restart might increase because one or more of the logs has to be read sequentially at startup.

 

Active log placement

Ideally, you should allocate each active log on separate, low-usage DASD volumes. As a minimum, no two adjacent logs should be on the same volume.

When an active log fills, the next log in the ring is used and the previous log data set is copied to the archive data set. If these two active data sets are on the same volume, contention will result, because one data set is read while the other is written to. For example, if you have three active logs and use dual logging, we need six DASD volumes because each log is adjacent to both of the two other logs. Alternatively, if you have four active logs and you want to conserve DASD space, by allocating logs 1 and 3 on one volume and logs 2 and 4 on another, we need four DASD volumes only.

In addition, you should ensure that primary and secondary logs are on separate physical units. If you use 3390 DASD, be aware that each head disk assembly contains two logical volumes. The physical layout of other DASD subsystems such as RAMAC arrays should also be taken into account.

It is recommended that logs are pre-formatted before first use using the CSQJUFMT utility.