RE: Disable WAL logging to speed up data loading

From: Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com>
> We're emitting only redo logs. So I think theoretically we don't need
> anything other than the shutdown checkpoint record because we don't
> perform recovery and checkpoint record is required at startup.
>
> RM_XLOG_ID:
> XLOG_FPI_FOR_HINT - not needed?
> XLOG_FPI - not needed?
>
> XLOG_CHECKPOINT_SHUTDOWN - must have
>
> So how about the followings?
> XLOG_CHECKPOINT_ONLINE
> XLOG_NOOP
> XLOG_NEXTOID
> XLOG_SWITCH
> XLOG_BACKUP_END
> XLOG_PARAMETER_CHANGE
> XLOG_RESTORE_POINT
> XLOG_FPW_CHANGE
> XLOG_END_OF_RECOVERY
>
>
> RM_XACT_ID:
> XLOG_XACT_COMMIT
> XLOG_XACT_PREPARE
> XLOG_XACT_ABORT
> XLOG_XACT_COMMIT_PREPARED
> XLOG_XACT_ABORT_PREPARED
> XLOG_XACT_ASSIGNMENT
> XLOG_XACT_INVALIDATIONS
>
> Do we need all of these?

What need to be emitted even when wal_level = none are:

RM_XLOG_ID:
- XLOG_CHECKPOINT_SHUTDOWN
- XLOG_PARAMETER_CHANGE

RM_XACT_ID:
- XLOG_XACT_PREPARE

XLOG_PARAMETER_CHANGE is necessary to detect during archive recovery that wal_level was changed from >= replica to none, thus failing the archive recovery. This is for the fix discussed in this thread to change the error level from WARNING to FATAL.

> Yeah, although it's enough only to restrict non-harmful records
> practically, if we find that only a few kinds of records are needed,
> maybe it's cleaner to allow only required record type(s).
>
> Maybe it's right that if we can filter-out records looking only rmid,
> since the xlog facility doesn't need to know about record types of a
> resource manager. But if we need to finer-grained control on the
> record types, I'm afraid that that's wrong. However, if we need only
> the XLOG_CHECKPOINT_SHUTDOWN record, it might be better to let
> XLogInsert filter records rather than inserting that filtering code to
> all the caller sites.

Agreed. As the kind of WAL records to be emitted is very limited, I think XLogInsert() can filter them where the current patch does. (OTOH, the boot strap mode filters WAL coarsely as follows. I thought we may follow this coarse RMID-based filtering as a pessimistic safeguard against new kind of WAL records that are necessary even in wal_level = none.)

/*
* In bootstrap mode, we don't actually log anything but XLOG resources;
* return a phony record pointer.
*/
if (IsBootstrapProcessingMode() && rmid != RM_XLOG_ID)
{
XLogResetInsertion();
EndPos = SizeOfXLogLongPHD; /* start of 1st chkpt record */
return EndPos;
}

> I don't dislike "none" since it seems to me practically "none". It
> seems rather correct if we actually need only the shutdown checkpoint
> record.
>
> "unrecoverable" is apparently misleading. "crash_unsafe" is precise
> but seems somewhat alien being among "logical", "replica" and
> "minimal".

OK, I'm happy with "none" too. We can describe in the manual that some limited amount of WAL is emitted.

Regards
Takayuki Tsunakawa