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<!-- $PostgreSQL: pgsql/doc/src/sgml/wal.sgml,v 1.29 2004/08/08 04:34:43 tgl Exp $ --> |
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<!-- $PostgreSQL: pgsql/doc/src/sgml/wal.sgml,v 1.30 2004/11/15 04:35:57 neilc Exp $ --> |
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<chapter id="wal"> |
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<title>Write-Ahead Logging (<acronym>WAL</acronym>)</title> |
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@ -104,17 +104,17 @@ |
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</para> |
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<para> |
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<firstterm>Checkpoints</firstterm><indexterm><primary>checkpoint</></> are points in the sequence of |
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transactions at which it is guaranteed that the data files have |
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been updated with all information logged before the checkpoint. At |
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checkpoint time, all dirty data pages are flushed to disk and a |
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special checkpoint record is written to the log file. As result, in |
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the event of a crash, the recoverer knows from what point in the |
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log (known as the redo record) it should start the REDO operation, |
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since any changes made to data files before that record are already |
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on disk. After a checkpoint has been made, any log segments written |
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before the redo record are no longer needed and can be recycled or |
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removed. (When <acronym>WAL</acronym> archiving is being done, the |
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<firstterm>Checkpoints</firstterm><indexterm><primary>checkpoint</></> |
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are points in the sequence of transactions at which it is guaranteed |
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that the data files have been updated with all information logged before |
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the checkpoint. At checkpoint time, all dirty data pages are flushed to |
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disk and a special checkpoint record is written to the log file. As a |
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result, in the event of a crash, the crash recovery procedure knows from |
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what point in the log (known as the redo record) it should start the |
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REDO operation, since any changes made to data files before that point |
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are already on disk. After a checkpoint has been made, any log segments |
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written before the redo record are no longer needed and can be recycled |
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or removed. (When <acronym>WAL</acronym> archiving is being done, the |
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log segments must be archived before being recycled or removed.) |
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</para> |
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