Re: WAL Record Header Size Reduction

Simon Riggs wrote:
> Current WAL Header uses 32 bytes on a 64-bit CPU. It seems possible to
> reduce this to 24 bytes, without reducing resilience, when
> full_page_writes = off. This will reduce overall WAL volumes by around
> 5-15%, depending upon the application with performance gains in various
> ways.

Actually, it would help even when full_page_writes=on, because even then
most xlog records don't have backup blocks attached to them.

> xlog.h shows this definition currently:
>
> typedef struct XLogRecord
> {
> pg_crc32 xl_crc; /* CRC for this record */
> XLogRecPtr xl_prev; /* ptr to previous record in log */
> TransactionId xl_xid; /* xact id */
> uint32 xl_tot_len; /* total len of entire record */
> uint32 xl_len; /* total len of rmgr data */
> uint8 xl_info; /* flag bits, see below */
> RmgrId xl_rmid; /* resource manager for this record */
>
> /* Depending on MAXALIGN, there are either 2 or 6 wasted bytes here */
>
> I propose to rearrange the XLogRecord structure to this:

I think you got your alignment wrong:

> pg_crc32 xl_crc; /* CRC for this record */
> uint8 xl_info; /* flag bits, see below */
> RmgrId xl_rmid; /* resource manager for this record */

Because of xl_prev below which is two uint32 fields, there will be 2
bytes of wasted space in here.

> XLogRecPtr xl_prev; /* ptr to previous record in log */
> TransactionId xl_xid; /* xact id */
> uint32 xl_tot_len; /* total len of entire record */

ISTM that we would get the effect your looking for by just moving the
xl_tot_len field to the end, and only storing it for records with backup
blocks:

> pg_crc32 xl_crc; /* CRC for this record */
> XLogRecPtr xl_prev; /* ptr to previous record in log */
> TransactionId xl_xid; /* xact id */
> uint32 xl_len; /* total len of rmgr data */
> uint8 xl_info; /* flag bits, see below */
> RmgrId xl_rmid; /* resource manager for this record */
>> uint32 xl_tot_len; /* total len of entire record, if backup blocks
indicated in xl_info*/

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