Oracle10g 50M的online redo 归档为什么都小于50M ?

Subject: Archive logs are created with smaller, uneven size than the original redo logs. Why?
Doc ID: Note:388627.1 Type: HOWTO
Last Revision Date: 02-JUN-2007 Status: MODERATED

In this Document
Goal
Solution
References[@more@] http://www.itpub.net/viewthread.php?tid=921121&extra=&highlight=%2Btolywang&page=2





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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

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