Re: InvalidXLogRecPtr in docs

On 10/06/10 11:37, Fujii Masao wrote:
> On Thu, Jun 10, 2010 at 5:04 PM, Heikki Linnakangas
> <heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:
>>>> I believe that starting from the beginning of the WAL segment is just
>>>> paranoia, to avoid creating a WAL file that's missing some data from the
>>>> beginning. Right?
>>>
>>> Only when the recovery starting record (i.e., the record at the checkpoint
>>> redo location) is not found, we need to start replication from the
>>> beginning
>>> of the segment, I think. That is, fetching_ckpt = true case in the
>>> following
>>> code.
>>>
>>>> if (PrimaryConnInfo)
>>>> {
>>>> RequestXLogStreaming(
>>>> fetching_ckpt ? RedoStartLSN : *RecPtr,
>>>> PrimaryConnInfo);
>>>> continue;
>>>> }
>>
>> Even then, we wouldn't need to start from the beginning of the WAL segment
>> AFAICS. The point is to start from the Redo pointer, not from the checkpoint
>> record, because as soon as we read the checkpoint record we'll need to start
>> applying WAL from the Redo pointer, which is earlier. The WAL file
>> boundaries don't come into play there.
>
> You mean that the WAL file containing the Redo pointer is guaranteed to exist
> if we could read the checkpoint record, so we don't need to start from the
> beginning of the segment? This is probably true. But what if we could not read
> the checkpoint record? In this case, the WAL file containing the Redo pointer
> also might not exist.

Oh, I think I understand the issue now: we need the header in the
beginning of the WAL segment to be valid, even if the first record we're
interested in is in the middle of the file. I missed that.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com