归档日志大小之谜-优快云博客

今天在oracle交流群里，有朋友问为什么归档日志大小一直都比在线redo log小，查看了下我们的正式库和测试库，发现的确是这种情况，我们的oracle版本是10.2.0.5.4，已经是oracle 10的最终版本了，按道理这些BUG应该都已经修复了，最后查询官方文档发现，对此问题的描述如下：

Applies to:
Oracle Server - Enterprise Edition - Version: 8.1.7.4 to 11.1
Information in this document applies to any platform.

Goal
Archive logs are created with smaller, uneven size than the original redo logs.
Commands like:
ALTER SYSTEM SWITCH LOGFILE
or
ALTER SYSTEM ARCHIVE LOG ...
are not used to generate archive or switch the logfile. As well, there is no ARCHIVE_LAG_TARGET parameter set.
What else could cause such a behaviour?
Solution
According to:
Bug 5450861 : ARCHIVE LOGS ARE GENERATED WITH A SMALLER SIZE THAN THE REDO LOG FILES
the explanation for this situation has 2 main reasons:
1. the archive logs do not have to be even in size. This was decided a very long time ago, when blank padding the archive logs has stopped, for a very good reason - in order to save disk space.

2. the log switch does not occur when a redo log file is 100% full. There is an internal algorythm that determines the log switch moment. This has also a very good reason - doing the log switch at the last moment could incur performance problems (for various reasons, out of the scope of this note).
As a result, after the log switch occurs the archivers are copying only the actual information from the redo log files. Since the redo logs are not 100% full after the log switch and the archive logs are not blank padded after the copy operation has finished, this results in uneven, smaller files than the original redo log files.
This is very conspicuous for very small redo log files (smaller than 10 MB); as such, the 2.5 MB archive logs generated from 5 MB redo logs are very visible.
Just note that at this moment, the default redo log files are 100 MB in size. If the resulting archive log files would be between 98 and 100 MB nobody would notice.
The main concern that one should have for the archive log files is an eventual corruption. This can be very easily checked by trying a test recovery. When this is ok, the uneven archive log size should be of no concern, as it's expected.
References
Bug 5450861 - ARCHIVE LOGS ARE GENERATED WITH A SMALLER SIZE THAN THE REDO LOG FILES