归档日志比在线日志小很多

Archived redolog is (significant) smaller than the redologfile. (Doc ID 1356604.1)

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In this Document Symptoms   Changes   Cause   Solution   References This document is being delivered to you via Oracle Support's Rapid Visibility (RaV) process and therefore has not been subject to an independent technical review. APPLIES TO: Oracle Database - Enterprise Edition - Version 11.2.0.1 and later Information in this document applies to any platform. SYMPTOMS The created archived redologs are (significant) smaller than the related online redologfile. CHANGES Bug 7016254 is included in 11.2 and makes a major change comparted or prior version in the 'log switch code' in kcrfwnf(). This document is based on the 11.2 behaviour.  CAUSE There are 2 possible causes for this : 1. Documented and designed behaviour due to explicit forcing an archive creation before the redolog file is full SQL> alter system switch logfile; SQL> alter system archive log current; RMAN> backup archivelog all; RMAN> backup database plus archivelog; ARCHIVE_LAG_TARGET : limits the amount of data that can be lost and effectively increases the availability of the standby database by forcing a log switch after the specified amount of time elapses. you can see this aswell in RAC with an idle/low-load instance. 2. Undocumented, but designed behaviour : BUG 9272059 - REDO LOG SWITCH AT 1/8 OF SIZE DUE TO CMT CPU'S BUG 10354739 - REDOLOGSIZE NOT COMPLETLY USED BUG 12317474 - FREQUENT REDO LOG SWITCHES GENERATING SMALL SIZED ARCHIVELOGS  BUG 5450861 - ARCHIVE LOGS ARE GENERATED WITH A SMALLER SIZE THAN THE REDO LOG FILES BUG 7016254 - DECREASE CONTROL FILE ENQUEUE WAIT AT LOG SWITCH Explanation : As per Bug: 5450861 (closed as 'Not a Bug'): * 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 was stopped, for a very good reason - in order to save disk space. * The log switch does not occur when a redo log file is 100% full. There is an internal algorithm that determines the log switch moment. This also has 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. There are a number of factors which combine to determine the log switch frequency. These are the most relevant factors in this case: a) RDBMS parameter LOG_BUFFER_SIZE If this is not explicitly set by the DBA then we use a default; at instance startup the RDBMS  calculates the number of shared redo strands as ncpus/16, and the size of each strand is 128Kb * ncpus (where ncpus is the number of CPUs in the system). The log buffer size is the number of stands multiplied by the strand size. The calculated or specified size is rounded up to a multiple of the granule size  of a memory segment in the SGA. For 11.2 if SGA size >= 128GB then granule size is 512MB 64GB <= SGA size < 128GB then granule size is 256MB 32GB <= SGA size < 64GB then granule size is 128MB 16GB <= SGA size < 32GB then granule size is 64MB 8GB <= SGA size < 16GB then granule size is 32MB 1GB <= SGA size < 8GB then granule size is 16MB SGA size < 1GB then granule size is 4MB There are some minimums and maximums enforced. b) System load Initially only one redo strand is used, ie the number of "active" redo strands is 1, and all the processes copy their redo into that one strand. When/if there is contention for that strand then the number of active redo strands is raised to 2. As contention for the active strands increases, the number of active strands increases. The maxmum possible number of active redo strands is the number of strands initially allocated in the log buffer. (This feature is called "dynamic strands", and there is a hidden parameter to disable it which then allows processes to use all the strands from the outset). c) Log file size This is the logfile size decided by the DBA when the logfiles are created. d) The logfile space reservation algorithm When the RDBMS switches into a new online redo logfile, all the log buffer redo strand memory is "mapped" to the logfile space. If the logfile is larger than the log buffer then each strand will map/reserve its strand size worth of logfile space, and the remaining logfile space (the "log residue") is still available. If the logfile is smaller than the log buffer, then the whole logfile space is divided/mapped/reserved equally among all the strands, and there is no unreserved space (ie no log residue). When any process fills a strand such that all the reserved underlying logfile space for that strand is used, AND there is no log residue, then a log switch is scheduled. Example : 128 CPU's so the RDBMS allocates a log_buffer of size 128Mb containing 8 shared strands of size 16Mb. It may be a bit larger than 128Mb as it rounds up to an SGA granule boundary. The logfiles are 100Mb, so when the RDBMS switches into a new online redo logfile each strand reserves 100Mb/8 = 25600 blocks and there is no log residue. If there is low system load, only one of the redo strands will be active/used and when 25600 blocks of that strand are filled then a log switch will be scheduled - the created archive logs have a size around 25600 blocks. With everything else staying the same (128 cpu's and low load), using a larger logfile would not really reduce the amount of unfilled space when the log switches are requested, but it would make that unfilled space less significant as a percentage of the total logfile space, eg - with a 100Mb logfile, the log switch happens with 7 x 16Mb logfile space unfilled (ie the logfile is 10% full when the log switch is requested) - with a 1Gb logfile, the log switch would happen with 7 x 16Mb logfile space unfilled (ie the logfile is 90% full when the log switch is requested) With a high CPU_COUNT, a low load and a redo log file size smaller than  the redolog buffer, you may see small archived log files because of log switches at about 1/8 of the size of the define log file size. This is because CPU_COUNT defines the number of redo strands (ncpus/16). With a low load only a single strand may be used. With redo log file size smaller than the redolog buffer, the log file space is divided over the available strands. When for instance only a single active strand is used, a log switch can already occur when that strand is filled. SOLUTION Check if the above is matching the behaviour you are seeing, then it is expected behaviour. If the behaviour is different than open a Service Request to Oracle Support for futher analysis