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xlogdefs.h
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1 /*
2  * xlogdefs.h
3  *
4  * Postgres write-ahead log manager record pointer and
5  * timeline number definitions
6  *
7  * Portions Copyright (c) 1996-2019, PostgreSQL Global Development Group
8  * Portions Copyright (c) 1994, Regents of the University of California
9  *
10  * src/include/access/xlogdefs.h
11  */
12 #ifndef XLOG_DEFS_H
13 #define XLOG_DEFS_H
14 
15 #include <fcntl.h> /* need open() flags */
16 
17 /*
18  * Pointer to a location in the XLOG. These pointers are 64 bits wide,
19  * because we don't want them ever to overflow.
20  */
21 typedef uint64 XLogRecPtr;
22 
23 /*
24  * Zero is used indicate an invalid pointer. Bootstrap skips the first possible
25  * WAL segment, initializing the first WAL page at WAL segment size, so no XLOG
26  * record can begin at zero.
27  */
28 #define InvalidXLogRecPtr 0
29 #define XLogRecPtrIsInvalid(r) ((r) == InvalidXLogRecPtr)
30 
31 /*
32  * First LSN to use for "fake" LSNs.
33  *
34  * Values smaller than this can be used for special per-AM purposes.
35  */
36 #define FirstNormalUnloggedLSN ((XLogRecPtr) 1000)
37 
38 /*
39  * XLogSegNo - physical log file sequence number.
40  */
41 typedef uint64 XLogSegNo;
42 
43 /*
44  * TimeLineID (TLI) - identifies different database histories to prevent
45  * confusion after restoring a prior state of a database installation.
46  * TLI does not change in a normal stop/restart of the database (including
47  * crash-and-recover cases); but we must assign a new TLI after doing
48  * a recovery to a prior state, a/k/a point-in-time recovery. This makes
49  * the new WAL logfile sequence we generate distinguishable from the
50  * sequence that was generated in the previous incarnation.
51  */
53 
54 /*
55  * Replication origin id - this is located in this file to avoid having to
56  * include origin.h in a bunch of xlog related places.
57  */
59 
60 /*
61  * Because O_DIRECT bypasses the kernel buffers, and because we never
62  * read those buffers except during crash recovery or if wal_level != minimal,
63  * it is a win to use it in all cases where we sync on each write(). We could
64  * allow O_DIRECT with fsync(), but it is unclear if fsync() could process
65  * writes not buffered in the kernel. Also, O_DIRECT is never enough to force
66  * data to the drives, it merely tries to bypass the kernel cache, so we still
67  * need O_SYNC/O_DSYNC.
68  */
69 #ifdef O_DIRECT
70 #define PG_O_DIRECT O_DIRECT
71 #else
72 #define PG_O_DIRECT 0
73 #endif
74 
75 /*
76  * This chunk of hackery attempts to determine which file sync methods
77  * are available on the current platform, and to choose an appropriate
78  * default method. We assume that fsync() is always available, and that
79  * configure determined whether fdatasync() is.
80  */
81 #if defined(O_SYNC)
82 #define OPEN_SYNC_FLAG O_SYNC
83 #elif defined(O_FSYNC)
84 #define OPEN_SYNC_FLAG O_FSYNC
85 #endif
86 
87 #if defined(O_DSYNC)
88 #if defined(OPEN_SYNC_FLAG)
89 /* O_DSYNC is distinct? */
90 #if O_DSYNC != OPEN_SYNC_FLAG
91 #define OPEN_DATASYNC_FLAG O_DSYNC
92 #endif
93 #else /* !defined(OPEN_SYNC_FLAG) */
94 /* Win32 only has O_DSYNC */
95 #define OPEN_DATASYNC_FLAG O_DSYNC
96 #endif
97 #endif
98 
99 #if defined(PLATFORM_DEFAULT_SYNC_METHOD)
100 #define DEFAULT_SYNC_METHOD PLATFORM_DEFAULT_SYNC_METHOD
101 #elif defined(OPEN_DATASYNC_FLAG)
102 #define DEFAULT_SYNC_METHOD SYNC_METHOD_OPEN_DSYNC
103 #elif defined(HAVE_FDATASYNC)
104 #define DEFAULT_SYNC_METHOD SYNC_METHOD_FDATASYNC
105 #else
106 #define DEFAULT_SYNC_METHOD SYNC_METHOD_FSYNC
107 #endif
108 
109 #endif /* XLOG_DEFS_H */
uint32 TimeLineID
Definition: xlogdefs.h:52
uint16 RepOriginId
Definition: xlogdefs.h:58
unsigned short uint16
Definition: c.h:358
uint64 XLogSegNo
Definition: xlogdefs.h:41
unsigned int uint32
Definition: c.h:359
uint64 XLogRecPtr
Definition: xlogdefs.h:21