On 03.04.2013 18:58, Jeff Janes wrote:
On Tue, Apr 2, 2013 at 10:08 PM, Jeff Janes<jeff.ja...@gmail.com> wrote:
This commit introduced a problem with wal_keep_segments:
commit dfda6ebaec6763090fb78b458a979b558c50b39b
The problem seems to be that the underflow warned about is happening,
because the check to guard it was checking the wrong thing. However, I
don't really understand KeepLogSeg. It seems like segno, and hence recptr,
don't actually serve any purpose.
Hmm, the check is actually correct, but the assignment in the
else-branch isn't. The idea of KeepLogSeg is to calculate recptr -
wal_keep_segments, and assign that to *logSegNo. But only if *logSegNo
is not already < than the calculated value. Does the attached look
correct to you?
At some point when it is over-pruning and recycling, it recyles the log
files that are still needed for recovery, and if the database crashes at
that point it will not recover because it can't find either the primary
secondary checkpoint records.
So, KeepLogSeg incorrectly sets *logSegNo to 0, and CreateCheckPoint
decrements it, causing it to underflow to 2^64-1. Now RemoveOldXlogFiles
feels free to remove every WAL segment.
- Heikki
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 3227d4c..2f79af6 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -7518,9 +7518,9 @@ CreateRestartPoint(int flags)
}
/*
- * Calculate the last segment that we need to retain because of
- * wal_keep_segments, by subtracting wal_keep_segments from
- * the given xlog location, recptr.
+ * Retreat *logSegNo to the last segment that we need to retain because of
+ * wal_keep_segments. This is calculated by by subtracting wal_keep_segments
+ * from the given xlog location, recptr.
*/
static void
KeepLogSeg(XLogRecPtr recptr, XLogSegNo *logSegNo)
@@ -7536,7 +7536,7 @@ KeepLogSeg(XLogRecPtr recptr, XLogSegNo *logSegNo)
if (segno <= wal_keep_segments)
segno = 1;
else
- segno = *logSegNo - wal_keep_segments;
+ segno = segno - wal_keep_segments;
/* don't delete WAL segments newer than the calculated segment */
if (segno < *logSegNo)
--
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