根據預寫式日誌WAL的定義，數據庫在修改磁盤上的元組前，必須先將日誌記錄刷回到磁盤。XLog的組織形式如下，依次爲日誌記錄頭，日誌類型（事務操作日誌、事務提交或終止日誌或是檢查點日誌等）、備份文件塊描述信息以及備份文件塊數據。數據庫系統把各種需要記錄日誌的數據分類，分配給與他們對應的資源管理器號，系統在恢復或者讀取日誌記錄時，能夠通過資源管理器號和日誌頭信息中的xl_info字段（高4位），知道數據庫對源數據做了哪種操作，從而正確的調用相應的響應函數（如redo函數等）。
PG_RMGR(RM_XLOG_ID, "XLOG", xlog_redo, xlog_desc, NULL, NULL)
PG_RMGR(RM_XACT_ID, "Transaction", xact_redo, xact_desc, NULL, NULL)
PG_RMGR(RM_SMGR_ID, "Storage", smgr_redo, smgr_desc, NULL, NULL)
PG_RMGR(RM_CLOG_ID, "CLOG", clog_redo, clog_desc, NULL, NULL)
PG_RMGR(RM_DBASE_ID, "Database", dbase_redo, dbase_desc, NULL, NULL)
PG_RMGR(RM_TBLSPC_ID, "Tablespace", tblspc_redo, tblspc_desc, NULL, NULL)
PG_RMGR(RM_MULTIXACT_ID, "MultiXact", multixact_redo, multixact_desc, NULL, NULL)
PG_RMGR(RM_RELMAP_ID, "RelMap", relmap_redo, relmap_desc, NULL, NULL)
PG_RMGR(RM_STANDBY_ID, "Standby", standby_redo, standby_desc, NULL, NULL)
PG_RMGR(RM_HEAP2_ID, "Heap2", heap2_redo, heap2_desc, NULL, NULL)
PG_RMGR(RM_HEAP_ID, "Heap", heap_redo, heap_desc, NULL, NULL)
PG_RMGR(RM_BTREE_ID, "Btree", btree_redo, btree_desc, NULL, NULL)
PG_RMGR(RM_HASH_ID, "Hash", hash_redo, hash_desc, NULL, NULL)
PG_RMGR(RM_GIN_ID, "Gin", gin_redo, gin_desc, gin_xlog_startup, gin_xlog_cleanup)
PG_RMGR(RM_GIST_ID, "Gist", gist_redo, gist_desc, gist_xlog_startup, gist_xlog_cleanup)
PG_RMGR(RM_SEQ_ID, "Sequence", seq_redo, seq_desc, NULL, NULL)
PG_RMGR(RM_SPGIST_ID, "SPGist", spg_redo, spg_desc, spg_xlog_startup, spg_xlog_cleanup)
日誌頭信息的描述結構體爲XLogRecord，其定義如下所示，xl_len記錄了RMGR數據的長度，xid記錄了修改事務ID，xl_tot_len記錄了整條日誌記錄的長度。單條XLOG日誌記錄支持最多備份4個文件塊，如果內存中的文件塊需要進行備份，每次就把xl_info低4位中的某一位依次置1，並將內存塊附在XLOG日誌記錄中。
typedef struct XLogRecord
{
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 */
/* If MAXALIGN==8, there are 4 wasted bytes here */
/* ACTUAL LOG DATA FOLLOWS AT END OF STRUCT */
} XLogRecord;typedef struct RmgrData
{
const char *rm_name;
void (*rm_redo) (XLogRecPtr lsn, struct XLogRecord *rptr);
void (*rm_desc) (StringInfo buf, uint8 xl_info, char *rec);
void (*rm_startup) (void);
void (*rm_cleanup) (void);
} RmgrData;typedef struct BkpBlock
{
RelFileNode node; /* relation containing block */
ForkNumber fork; /* fork within the relation */
BlockNumber block; /* block number */
uint16 hole_offset; /* number of bytes before "hole" */
uint16 hole_length; /* number of bytes in "hole" */
/* ACTUAL BLOCK DATA FOLLOWS AT END OF STRUCT */
} BkpBlock;XLOG日誌記錄在內存中以鏈表的形式組織，其描述結構如下，
typedef struct XLogRecData
{
char *data; /* start of rmgr data to include */
uint32 len; /* length of rmgr data to include */
Buffer buffer; /* buffer associated with data, if any */
bool buffer_std; /* buffer has standard pd_lower/pd_upper */
struct XLogRecData *next; /* next struct in chain, or NULL */
} XLogRecData;