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Allow WAL record header to be split across pages.

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This saves a few bytes of WAL space, but the real motivation is to make it
predictable how much WAL space a record requires, as it no longer depends
on whether we need to waste the last few bytes at end of WAL page because
the header doesn't fit.

The total length field of WAL record, xl_tot_len, is moved to the beginning
of the WAL record header, so that it is still always found on the first page
where a WAL record begins.

Bump WAL version number again as this is an incompatible change.
This commit is contained in:
Heikki Linnakangas
2012-06-24 18:27:10 +03:00
parent 20ba5ca64c
commit 061e7efb1b
4 changed files with 207 additions and 155 deletions
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9
src/include/access/xlog.h
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@ -40,15 +40,16 @@
*/
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 */
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 */
/* 2 bytes of padding here, initialize to zero */
XLogRecPtr xl_prev; /* ptr to previous record in log */
pg_crc32 xl_crc; /* CRC for this record */
/* Depending on MAXALIGN, there are either 2 or 6 wasted bytes here */
/* If MAXALIGN==8, there are 4 wasted bytes here */
/* ACTUAL LOG DATA FOLLOWS AT END OF STRUCT */