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Add a comment about why we always replay backup blocks from WAL.

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This commit is contained in:
Tom Lane 2004-08-08 03:22:08 +00:00
parent fd62065fa6
commit 7dca975c5d
1 changed files with 10 additions and 1 deletions
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11
src/backend/access/transam/xlog.c
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@ -7,7 +7,7 @@
* Portions Copyright (c) 1996-2003, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* $PostgreSQL: pgsql/src/backend/access/transam/xlog.c,v 1.156 2004/08/04 21:33:46 tgl Exp $
* $PostgreSQL: pgsql/src/backend/access/transam/xlog.c,v 1.157 2004/08/08 03:22:08 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -2187,6 +2187,15 @@ MoveOfflineLogs(uint32 log, uint32 seg, XLogRecPtr endptr)
* Restore the backup blocks present in an XLOG record, if any.
*
* We assume all of the record has been read into memory at *record.
*
* Note: when a backup block is available in XLOG, we restore it
* unconditionally, even if the page in the database appears newer.
* This is to protect ourselves against database pages that were partially
* or incorrectly written during a crash. We assume that the XLOG data
* must be good because it has passed a CRC check, while the database
* page might not be. This will force us to replay all subsequent
* modifications of the page that appear in XLOG, rather than possibly
* ignoring them as already applied, but that's not a huge drawback.
*/
static void
RestoreBkpBlocks(XLogRecord *record, XLogRecPtr lsn)