On 01/22/2015 10:34 PM, Jim Nasby wrote:
On 1/22/15 2:19 PM, Heikki Linnakangas wrote:
On 01/22/2015 09:20 PM, Bruce Momjian wrote:
One question I have is whether hint bits are set by read-only
transactions on standby servers.
No. See comments in MarkBufferDirtyHint:
/*
* If we need to protect hint bit updates from torn writes, WAL-log a
* full page image of the page. This full page image is only necessary
* if the hint bit update is the first change to the page since the
* last checkpoint.
*
* We don't check full_page_writes here because that logic is included
* when we call XLogInsert() since the value changes dynamically.
*/
if (XLogHintBitIsNeeded() && (bufHdr->flags & BM_PERMANENT))
{
/*
* If we're in recovery we cannot dirty a page because of a hint.
* We can set the hint, just not dirty the page as a result so the
* hint is lost when we evict the page or shutdown.
*
* See src/backend/storage/page/README for longer discussion.
*/
if (RecoveryInProgress())
return;
What if XLogHintBitIsNeeded is false? That would be the case if we're not wall
logging hints *on the standby*.
Then the page will be updated without writing a WAL record. Just like in
the master, if wal_log_hints is off. wal_log_hints works the same on the
master or the standby.
- Heikki
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