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Clarify operation of online checkpoints.

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Previous comments left, but were too obscure
for such an important aspect of the system.
This commit is contained in:
Simon Riggs 2012-12-02 13:09:55 +00:00
parent 53edb8dc02
commit 9f98704b82
1 changed files with 12 additions and 0 deletions
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12
src/backend/access/transam/xlog.c
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@ -6847,6 +6847,18 @@ LogCheckpointEnd(bool restartpoint)
* Note: flags contains other bits, of interest here only for logging purposes.
* In particular note that this routine is synchronous and does not pay
* attention to CHECKPOINT_WAIT.
*
* If !shutdown then we are writing an online checkpoint. This is a very special
* kind of operation and WAL record because the checkpoint action occurs over
* a period of time yet logically occurs at just a single LSN. The logical
* position of the WAL record (redo ptr) is the same or earlier than the
* physical position. When we replay WAL we locate the checkpoint via its
* physical position then read the redo ptr and actually start replay at the
* earlier logical position. Note that we don't write *anything* to WAL at
* the logical position, so that location could be any other kind of WAL record.
* All of this mechanism allows us to continue working while we checkpoint.
* As a result, timing of actions is critical here and be careful to note that
* this function will likely take minutes to execute on a busy system.
*/
void
CreateCheckPoint(int flags)