On 10/06/10 11:37, Fujii Masao wrote:
On Thu, Jun 10, 2010 at 5:04 PM, Heikki Linnakangas
<heikki.linnakan...@enterprisedb.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
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