Re: [COMMITTERS] pgsql: Move WAL continuation record information to WAL page header.

On 27.06.2012 17:53, Andres Freund wrote:
> I had noticed one thing when reviewing the patches before:
>
> @@ -717,6 +719,15 @@ XLogInsert(RmgrId rmid, uint8 info, XLogRecData *rdata)
> bool doPageWrites;
> bool isLogSwitch = (rmid == RM_XLOG_ID&& info == XLOG_SWITCH);
> uint8 info_orig = info;
> + static XLogRecord *rechdr;
> +
> + if (rechdr == NULL)
> + {
> + rechdr = malloc(SizeOfXLogRecord);
> + if (rechdr == NULL)
> + elog(ERROR, "out of memory");
> + MemSet(rechdr, 0, SizeOfXLogRecord);
> + }
>
> /* cross-check on whether we should be here or not */
> if (!XLogInsertAllowed())
>
> Why do you allocate this dynamically? XLogRecord is 32bytes, there doesn't
> seem to be much point in this?

On 64-bit architectures, the struct needs padding at the end to make the
size MAXALIGNed to 32 bytes; a simple "XLogRecord rechdr" local variable
would not include that. You could do something like:

union
{
XLogRecord rechdr;
char bytes[SizeOfXLogRecord];
}

but that's quite ugly too.

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