Re: WAL Record Header Size Reduction

On Thu, 2007-01-25 at 14:04 +0000, Heikki Linnakangas wrote:
> 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.

Sure it would be active, but I meant it probably would not reduce the
overall WAL volume by very much. We'll see, I guess.

> > 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.

No escaping that though.

> 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*/

OK. I was assuming XLogRecPtr was 8-byte aligned, but its a struct with
2 4-byte aligned elements, so I thought I had to move it to get the
benefit. Seems not...

--  Simon Riggs              EnterpriseDB   http://www.enterprisedb.com