diff options
Diffstat (limited to 'src/pager.c')
| -rw-r--r-- | src/pager.c | 96 |
1 files changed, 38 insertions, 58 deletions
diff --git a/src/pager.c b/src/pager.c index 4ca2b20a1..0b8ac722e 100644 --- a/src/pager.c +++ b/src/pager.c @@ -1,33 +1,24 @@ /* -** Copyright (c) 2001 D. Richard Hipp +** 2001 September 15 ** -** This program is free software; you can redistribute it and/or -** modify it under the terms of the GNU General Public -** License as published by the Free Software Foundation; either -** version 2 of the License, or (at your option) any later version. +** The author disclaims copyright to this source code. In place of +** a legal notice, here is a blessing: ** -** This program is distributed in the hope that it will be useful, -** but WITHOUT ANY WARRANTY; without even the implied warranty of -** MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU -** General Public License for more details. -** -** You should have received a copy of the GNU General Public -** License along with this library; if not, write to the -** Free Software Foundation, Inc., 59 Temple Place - Suite 330, -** Boston, MA 02111-1307, USA. -** -** Author contact information: -** drh@hwaci.com -** http://www.hwaci.com/drh/ +** May you do good and not evil. +** May you find forgiveness for yourself and forgive others. +** May you share freely, never taking more than you give. ** ************************************************************************* -** This is the implementation of the page cache subsystem. +** This is the implementation of the page cache subsystem or "pager". ** -** The page cache is used to access a database file. The pager journals -** all writes in order to support rollback. Locking is used to limit -** access to one or more reader or to one writer. +** The pager is used to access a database disk file. It implements +** atomic commit and rollback through the use of a journal file that +** is separate from the database file. The pager also implements file +** locking to prevent two processes from writing the same database +** file simultaneously, or one process from reading the database while +** another is writing. ** -** @(#) $Id: pager.c,v 1.19 2001/09/15 00:57:29 drh Exp $ +** @(#) $Id: pager.c,v 1.20 2001/09/16 00:13:27 drh Exp $ */ #include "sqliteInt.h" #include "pager.h" @@ -103,7 +94,7 @@ struct PgHdr { ** How big to make the hash table used for locating in-memory pages ** by page number. Knuth says this should be a prime number. */ -#define N_PG_HASH 907 +#define N_PG_HASH 373 /* ** A open page cache is an instance of the following structure. @@ -684,7 +675,16 @@ int sqlitepager_ref(void *pData){ } /* -** Sync the journal and write all free dirty pages to the database file. +** Sync the journal and then write all free dirty pages to the database +** file. +** +** Writing all free dirty pages to the database after the sync is a +** non-obvious optimization. fsync() is an expensive operation so we +** want to minimize the number that occur. So after an fsync() is forced +** and we are free to write dirty pages back to the database, it is best +** to go ahead and do as much of that as possible to minimize the chance +** of having to do another fsync() later on. Writing dirty free pages +** in this way make database operations go up to 10 times faster. */ static int syncAllPages(Pager *pPager){ PgHdr *pPg; @@ -818,6 +818,19 @@ int sqlitepager_get(Pager *pPager, Pgno pgno, void **ppPage){ while( pPg->dirty && 0<cnt-- && pPg->pNextFree ){ pPg = pPg->pNextFree; } + + /* If we could not find a page that has not been used recently + ** and which is not dirty, then sync the journal and write all + ** dirty free pages into the database file, thus making them + ** clean pages and available for recycling. + ** + ** We have to sync the journal before writing a page to the main + ** database. But syncing is a very slow operation. So after a + ** sync, it is best to write everything we can back to the main + ** database to minimize the risk of having to sync again in the + ** near future. That is way we write all dirty pages after a + ** sync. + */ if( pPg==0 || pPg->dirty ){ int rc = syncAllPages(pPager); if( rc!=0 ){ @@ -828,39 +841,6 @@ int sqlitepager_get(Pager *pPager, Pgno pgno, void **ppPage){ pPg = pPager->pFirst; } assert( pPg->nRef==0 ); - - -#if 0 - /**** Since putting in the call to syncAllPages() above, this code - ** is no longer used. I've kept it here for historical reference - ** only. - */ - /* If the page to be recycled is dirty, sync the journal and write - ** the old page into the database. */ - if( pPg->dirty ){ - int rc; - assert( pPg->inJournal==1 ); - assert( pPager->state==SQLITE_WRITELOCK ); - if( pPager->needSync ){ - rc = fsync(pPager->jfd); - if( rc!=0 ){ - rc = sqlitepager_rollback(pPager); - *ppPage = 0; - if( rc==SQLITE_OK ) rc = SQLITE_IOERR; - return rc; - } - pPager->needSync = 0; - } - pager_seek(pPager->fd, (pPg->pgno-1)*SQLITE_PAGE_SIZE); - rc = pager_write(pPager->fd, PGHDR_TO_DATA(pPg), SQLITE_PAGE_SIZE); - if( rc!=SQLITE_OK ){ - rc = sqlitepager_rollback(pPager); - *ppPage = 0; - if( rc==SQLITE_OK ) rc = SQLITE_FULL; - return rc; - } - } -#endif assert( pPg->dirty==0 ); /* Unlink the old page from the free list and the hash table |