On Wed, Jan 4, 2012 at 3:49 AM, Robert Haas <robertmhaas@gmail.com> wrote:
> On Fri, Dec 30, 2011 at 11:58 AM, Jeff Janes <jeff.janes@gmail.com> wrote:
>> On 12/29/11, Ants Aasma <ants.aasma@eesti.ee> wrote:
>>> Unless I'm missing something, double-writes are needed for all writes,
>>> not only the first page after a checkpoint. Consider this sequence of
>>> events:
>>>
>>> 1. Checkpoint
>>> 2. Double-write of page A (DW buffer write, sync, heap write)
>>> 3. Sync of heap, releasing DW buffer for new writes.
>>> ... some time goes by
>>> 4. Regular write of page A
>>> 5. OS writes one part of page A
>>> 6. Crash!
>>>
>>> Now recovery comes along, page A is broken in the heap with no
>>> double-write buffer backup nor anything to recover it by in the WAL.
>>
>> Isn't 3 the very definition of a checkpoint, meaning that 4 is not
>> really a regular write as it is the first one after a checkpoint?
>
> I think you nailed it.
No, I should have explicitly stated that no checkpoint happens in
between. I think the confusion here is because I assumed Kevin
described a fixed size d-w buffer in this message:
On Thu, Dec 29, 2011 at 6:44 PM, Kevin Grittner
<Kevin.Grittner@wicourts.gov> wrote:
> ... The file is fsync'd (like I said,
> hopefully to BBU cache), then each page in the double-write buffer is
> written to the normal page location, and that is fsync'd. Once that
> is done, the database writes have no risk of being torn, and the
> double-write buffer is marked as empty. ...
If the double-write buffer survives until the next checkpoint,
double-writing only the first write should work just fine. The
advantage over current full-page writes is that the write is not into
the WAL stream and is done (hopefully) by the bgwriter/checkpointer in
the background.
--
Ants Aasma
