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Overview
Comment: | Clarify some comments describing the WAL index file. No changes to code. |
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Downloads: | Tarball | ZIP archive |
Timelines: | family | ancestors | descendants | both | trunk |
Files: | files | file ages | folders |
SHA3-256: |
3be3aad9ecbe33060cfa9c6059b9206e |
User & Date: | drh 2017-10-30 20:44:36.285 |
Context
2017-10-30
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23:25 | Add the showshm utility program for printing out the shm header in a human-readable format. (check-in: f6304fd142 user: drh tags: trunk) | |
20:44 | Clarify some comments describing the WAL index file. No changes to code. (check-in: 3be3aad9ec user: drh tags: trunk) | |
18:49 | Improve the performance of the LIKE operator by using strcspn() to aid wildcard matching. (check-in: 37284d4e8f user: drh tags: trunk) | |
Changes
Changes to src/wal.c.
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127 128 129 130 131 132 133 134 135 136 137 138 139 140 | ** WAL-INDEX FORMAT ** ** Conceptually, the wal-index is shared memory, though VFS implementations ** might choose to implement the wal-index using a mmapped file. Because ** the wal-index is shared memory, SQLite does not support journal_mode=WAL ** on a network filesystem. All users of the database must be able to ** share memory. ** ** The wal-index is transient. After a crash, the wal-index can (and should ** be) reconstructed from the original WAL file. In fact, the VFS is required ** to either truncate or zero the header of the wal-index when the last ** connection to it closes. Because the wal-index is transient, it can ** use an architecture-specific format; it does not have to be cross-platform. ** Hence, unlike the database and WAL file formats which store all values | > > > > | 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 | ** WAL-INDEX FORMAT ** ** Conceptually, the wal-index is shared memory, though VFS implementations ** might choose to implement the wal-index using a mmapped file. Because ** the wal-index is shared memory, SQLite does not support journal_mode=WAL ** on a network filesystem. All users of the database must be able to ** share memory. ** ** In the default unix and windows implementation, the wal-index is a mmapped ** file whose name is the database name with a "-shm" suffix added. For that ** reason, the wal-index is sometimes called the "shm" file. ** ** The wal-index is transient. After a crash, the wal-index can (and should ** be) reconstructed from the original WAL file. In fact, the VFS is required ** to either truncate or zero the header of the wal-index when the last ** connection to it closes. Because the wal-index is transient, it can ** use an architecture-specific format; it does not have to be cross-platform. ** Hence, unlike the database and WAL file formats which store all values |
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267 268 269 270 271 272 273 | ** WALINDEX_MAX_VERSION, then no read-transaction is opened and SQLite ** returns SQLITE_CANTOPEN. */ #define WAL_MAX_VERSION 3007000 #define WALINDEX_MAX_VERSION 3007000 /* | | > > > > > > > > > | 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 | ** WALINDEX_MAX_VERSION, then no read-transaction is opened and SQLite ** returns SQLITE_CANTOPEN. */ #define WAL_MAX_VERSION 3007000 #define WALINDEX_MAX_VERSION 3007000 /* ** Index numbers for various locking bytes. WAL_NREADER is the number ** of available reader locks and should be at least 3. The default ** is SQLITE_SHM_NLOCK==8 and WAL_NREADER==5. ** ** Technically, the various VFSes are free to implement these locks however ** they see fit. However, compatibility is encouraged so that VFSes can ** interoperate. The standard implemention used on both unix and windows ** is for the index number to indicate a byte offset into the ** WalCkptInfo.aLock[] array in the wal-index header. In other words, all ** locks are on the shm file. The WALINDEX_LOCK_OFFSET constant (which ** should be 120) is the location in the shm file for the first locking ** byte. */ #define WAL_WRITE_LOCK 0 #define WAL_ALL_BUT_WRITE 1 #define WAL_CKPT_LOCK 1 #define WAL_RECOVER_LOCK 2 #define WAL_READ_LOCK(I) (3+(I)) #define WAL_NREADER (SQLITE_SHM_NLOCK-3) |
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393 394 395 396 397 398 399 | #define WALINDEX_LOCK_OFFSET (sizeof(WalIndexHdr)*2+offsetof(WalCkptInfo,aLock)) #define WALINDEX_HDR_SIZE (sizeof(WalIndexHdr)*2+sizeof(WalCkptInfo)) /* Size of header before each frame in wal */ #define WAL_FRAME_HDRSIZE 24 /* Size of write ahead log header, including checksum. */ | < | 406 407 408 409 410 411 412 413 414 415 416 417 418 419 | #define WALINDEX_LOCK_OFFSET (sizeof(WalIndexHdr)*2+offsetof(WalCkptInfo,aLock)) #define WALINDEX_HDR_SIZE (sizeof(WalIndexHdr)*2+sizeof(WalCkptInfo)) /* Size of header before each frame in wal */ #define WAL_FRAME_HDRSIZE 24 /* Size of write ahead log header, including checksum. */ #define WAL_HDRSIZE 32 /* WAL magic value. Either this value, or the same value with the least ** significant bit also set (WAL_MAGIC | 0x00000001) is stored in 32-bit ** big-endian format in the first 4 bytes of a WAL file. ** ** If the LSB is set, then the checksums for each frame within the WAL |
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