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