On 10/06/10 10:43, Fujii Masao wrote:
On Thu, Jun 10, 2010 at 4:07 PM, Heikki Linnakangas
<[email protected]> wrote:
BTW, the docs claim about pg_last_xlog_location() that "While streaming
replication is in progress this will increase monotonically." That's a bit
misleading: when the replication connection is broken for some reason and we
restart it, we begin streaming from the beginning of the last WAL segment.
So at that moment, pg_last_xlog_location() moves backwards to the beginning
of the WAL segment.
Should we:
1. Just document that,
2. Change pg_last_xlog_location() to not move backwards in that case, or
3. Change the behavior so that we start streaming at the exact byte location
where we left off?
I'm for 2 as follows.
diff --git a/src/backend/replication/walreceiver.c
b/src/backend/replication/walreceiver.c
index 26aeca6..f0fd813 100644
--- a/src/backend/replication/walreceiver.c
+++ b/src/backend/replication/walreceiver.c
@@ -524,7 +524,8 @@ XLogWalRcvFlush(void)
/* Update shared-memory status */
SpinLockAcquire(&walrcv->mutex);
- walrcv->receivedUpto = LogstreamResult.Flush;
+ if (XLByteLT(walrcv->receivedUpto, LogstreamResult.Flush))
+ walrcv->receivedUpto = LogstreamResult.Flush;
SpinLockRelease(&walrcv->mutex);
That's not enough, because we set receivedUpto in RequestXlogStreaming()
already.
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.
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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