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Overview
Comment:If a single page is written to the wal file more than once, have each subsequent copy overwrite the original frame.
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Timelines: family | ancestors | descendants | both | wal-overwrite-frames
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SHA1:5d113aef2c7d746e8eda88d4e36c04a39b0a11be
User & Date: dan 2016-01-09 16:39:29
Context
2016-01-09
18:57
If it is known that checksums will be recalculated on transaction commit, skip calculating checksums when appending frames to the wal file. When recalculating checksums, recalculate them starting with the first overwritten frame - not the first frame in the transaction. check-in: 16b34f25 user: dan tags: wal-overwrite-frames
16:39
If a single page is written to the wal file more than once, have each subsequent copy overwrite the original frame. check-in: 5d113aef user: dan tags: wal-overwrite-frames
2016-01-08
22:31
Typo fix in the previous commit. check-in: 52c16603 user: drh tags: trunk
Changes
Hide Diffs Unified Diffs Ignore Whitespace Patch

Changes to src/pager.c.

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                           pPager->pageSize, (u8*)pPager->pTmpSpace);
      pPager->pWal = 0;
      pagerFixMaplimit(pPager);
    }
  }
  return rc;
}








#ifdef SQLITE_ENABLE_SNAPSHOT
/*
** If this is a WAL database, obtain a snapshot handle for the snapshot
** currently open. Otherwise, return an error.
*/
int sqlite3PagerSnapshotGet(Pager *pPager, sqlite3_snapshot **ppSnapshot){







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                           pPager->pageSize, (u8*)pPager->pTmpSpace);
      pPager->pWal = 0;
      pagerFixMaplimit(pPager);
    }
  }
  return rc;
}

/*
** Return the number of open savepoints.
**/
int sqlite3PagerSavepointCount(Pager *pPager){
  return pPager->nSavepoint;
}

#ifdef SQLITE_ENABLE_SNAPSHOT
/*
** If this is a WAL database, obtain a snapshot handle for the snapshot
** currently open. Otherwise, return an error.
*/
int sqlite3PagerSnapshotGet(Pager *pPager, sqlite3_snapshot **ppSnapshot){

Changes to src/pager.h.

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#ifndef SQLITE_OMIT_WAL
  int sqlite3PagerCheckpoint(Pager *pPager, int, int*, int*);
  int sqlite3PagerWalSupported(Pager *pPager);
  int sqlite3PagerWalCallback(Pager *pPager);
  int sqlite3PagerOpenWal(Pager *pPager, int *pisOpen);
  int sqlite3PagerCloseWal(Pager *pPager);

# ifdef SQLITE_ENABLE_SNAPSHOT
  int sqlite3PagerSnapshotGet(Pager *pPager, sqlite3_snapshot **ppSnapshot);
  int sqlite3PagerSnapshotOpen(Pager *pPager, sqlite3_snapshot *pSnapshot);
# endif
#endif

#ifdef SQLITE_ENABLE_ZIPVFS







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#ifndef SQLITE_OMIT_WAL
  int sqlite3PagerCheckpoint(Pager *pPager, int, int*, int*);
  int sqlite3PagerWalSupported(Pager *pPager);
  int sqlite3PagerWalCallback(Pager *pPager);
  int sqlite3PagerOpenWal(Pager *pPager, int *pisOpen);
  int sqlite3PagerCloseWal(Pager *pPager);
  int sqlite3PagerSavepointCount(Pager *pPager);
# ifdef SQLITE_ENABLE_SNAPSHOT
  int sqlite3PagerSnapshotGet(Pager *pPager, sqlite3_snapshot **ppSnapshot);
  int sqlite3PagerSnapshotOpen(Pager *pPager, sqlite3_snapshot *pSnapshot);
# endif
#endif

#ifdef SQLITE_ENABLE_ZIPVFS

Changes to src/pcache.h.

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#define PGHDR_DIRTY           0x002  /* Page is on the PCache.pDirty list */
#define PGHDR_WRITEABLE       0x004  /* Journaled and ready to modify */
#define PGHDR_NEED_SYNC       0x008  /* Fsync the rollback journal before
                                     ** writing this page to the database */
#define PGHDR_NEED_READ       0x010  /* Content is unread */
#define PGHDR_DONT_WRITE      0x020  /* Do not write content to disk */
#define PGHDR_MMAP            0x040  /* This is an mmap page object */



/* Initialize and shutdown the page cache subsystem */
int sqlite3PcacheInitialize(void);
void sqlite3PcacheShutdown(void);

/* Page cache buffer management:
** These routines implement SQLITE_CONFIG_PAGECACHE.







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#define PGHDR_DIRTY           0x002  /* Page is on the PCache.pDirty list */
#define PGHDR_WRITEABLE       0x004  /* Journaled and ready to modify */
#define PGHDR_NEED_SYNC       0x008  /* Fsync the rollback journal before
                                     ** writing this page to the database */
#define PGHDR_NEED_READ       0x010  /* Content is unread */
#define PGHDR_DONT_WRITE      0x020  /* Do not write content to disk */
#define PGHDR_MMAP            0x040  /* This is an mmap page object */

#define PGHDR_WAL_APPEND      0x080  /* Appended to wal file */

/* Initialize and shutdown the page cache subsystem */
int sqlite3PcacheInitialize(void);
void sqlite3PcacheShutdown(void);

/* Page cache buffer management:
** These routines implement SQLITE_CONFIG_PAGECACHE.

Changes to src/wal.c.

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/*
** Candidate values for Wal.exclusiveMode.
*/
#define WAL_NORMAL_MODE     0
#define WAL_EXCLUSIVE_MODE  1     
#define WAL_HEAPMEMORY_MODE 2








/*
** Possible values for WAL.readOnly
*/
#define WAL_RDWR        0    /* Normal read/write connection */
#define WAL_RDONLY      1    /* The WAL file is readonly */
#define WAL_SHM_RDONLY  2    /* The SHM file is readonly */

................................................................................
  walEncodeFrame(p->pWal, pPage->pgno, nTruncate, pData, aFrame);
  rc = walWriteToLog(p, aFrame, sizeof(aFrame), iOffset);
  if( rc ) return rc;
  /* Write the page data */
  rc = walWriteToLog(p, pData, p->szPage, iOffset+sizeof(aFrame));
  return rc;
}























































/* 
** Write a set of frames to the log. The caller must hold the write-lock
** on the log file (obtained using sqlite3WalBeginWriteTransaction()).
*/
int sqlite3WalFrames(
  Wal *pWal,                      /* Wal handle to write to */
................................................................................
  u32 iFrame;                     /* Next frame address */
  PgHdr *p;                       /* Iterator to run through pList with. */
  PgHdr *pLast = 0;               /* Last frame in list */
  int nExtra = 0;                 /* Number of extra copies of last page */
  int szFrame;                    /* The size of a single frame */
  i64 iOffset;                    /* Next byte to write in WAL file */
  WalWriter w;                    /* The writer */



  assert( pList );
  assert( pWal->writeLock );

  /* If this frame set completes a transaction, then nTruncate>0.  If
  ** nTruncate==0 then this frame set does not complete the transaction. */
  assert( (isCommit!=0)==(nTruncate!=0) );
................................................................................

#if defined(SQLITE_TEST) && defined(SQLITE_DEBUG)
  { int cnt; for(cnt=0, p=pList; p; p=p->pDirty, cnt++){}
    WALTRACE(("WAL%p: frame write begin. %d frames. mxFrame=%d. %s\n",
              pWal, cnt, pWal->hdr.mxFrame, isCommit ? "Commit" : "Spill"));
  }
#endif








  /* See if it is possible to write these frames into the start of the
  ** log file, instead of appending to it at pWal->hdr.mxFrame.
  */
  if( SQLITE_OK!=(rc = walRestartLog(pWal)) ){
    return rc;
  }
................................................................................
  w.szPage = szPage;
  iOffset = walFrameOffset(iFrame+1, szPage);
  szFrame = szPage + WAL_FRAME_HDRSIZE;

  /* Write all frames into the log file exactly once */
  for(p=pList; p; p=p->pDirty){
    int nDbSize;   /* 0 normally.  Positive == commit flag */



















    iFrame++;
    assert( iOffset==walFrameOffset(iFrame, szPage) );
    nDbSize = (isCommit && p->pDirty==0) ? nTruncate : 0;
    rc = walWriteOneFrame(&w, p, nDbSize, iOffset);
    if( rc ) return rc;
    pLast = p;
    iOffset += szFrame;







  }

  /* If this is the end of a transaction, then we might need to pad
  ** the transaction and/or sync the WAL file.
  **
  ** Padding and syncing only occur if this set of frames complete a
  ** transaction and if PRAGMA synchronous=FULL.  If synchronous==NORMAL
................................................................................
  /* Append data to the wal-index. It is not necessary to lock the 
  ** wal-index to do this as the SQLITE_SHM_WRITE lock held on the wal-index
  ** guarantees that there are no other writers, and no data that may
  ** be in use by existing readers is being overwritten.
  */
  iFrame = pWal->hdr.mxFrame;
  for(p=pList; p && rc==SQLITE_OK; p=p->pDirty){

    iFrame++;
    rc = walIndexAppend(pWal, iFrame, p->pgno);
  }
  while( rc==SQLITE_OK && nExtra>0 ){
    iFrame++;
    nExtra--;
    rc = walIndexAppend(pWal, iFrame, pLast->pgno);
................................................................................
    if( isChanged && pWal->pDbFd->pMethods->iVersion>=3 ){
      sqlite3OsUnfetch(pWal->pDbFd, 0, 0);
    }
  }

  /* Copy data from the log to the database file. */
  if( rc==SQLITE_OK ){

    if( pWal->hdr.mxFrame && walPagesize(pWal)!=nBuf ){
      rc = SQLITE_CORRUPT_BKPT;
    }else{
      rc = walCheckpoint(pWal, eMode2, xBusy2, pBusyArg, sync_flags, zBuf);
    }

    /* If no error occurred, set the output variables. */







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/*
** Candidate values for Wal.exclusiveMode.
*/
#define WAL_NORMAL_MODE     0
#define WAL_EXCLUSIVE_MODE  1     
#define WAL_HEAPMEMORY_MODE 2

/* 
** Values for Wal.writeLock.
*/
#define WAL_WRITELOCK_UNLOCKED 0
#define WAL_WRITELOCK_LOCKED   1
#define WAL_WRITELOCK_RECKSUM  2

/*
** Possible values for WAL.readOnly
*/
#define WAL_RDWR        0    /* Normal read/write connection */
#define WAL_RDONLY      1    /* The WAL file is readonly */
#define WAL_SHM_RDONLY  2    /* The SHM file is readonly */

................................................................................
  walEncodeFrame(p->pWal, pPage->pgno, nTruncate, pData, aFrame);
  rc = walWriteToLog(p, aFrame, sizeof(aFrame), iOffset);
  if( rc ) return rc;
  /* Write the page data */
  rc = walWriteToLog(p, pData, p->szPage, iOffset+sizeof(aFrame));
  return rc;
}

/*
** This function is called as part of committing a transaction within which
** one or more frames have been overwritten. It updates the checksums for
** all frames written to the wal file by the current transaction.
**
** Argument pLive is a pointer to the first wal-index header in shared 
** memory (the copy readers will see if they open a read-transaction now,
** before the current commit is finished). This is safe to use because the
** caller holds the WRITER lock. The first frame to update the checksum
** for is (pLive->mxFrame+1). The last is argument iLast.
**
** SQLITE_OK is returned if successful, or an SQLite error code otherwise.
*/
static int walRewriteChecksums(Wal *pWal, WalIndexHdr *pLive, u32 iLast){
  const int szPage = pWal->szPage;/* Database page size */
  int rc = SQLITE_OK;             /* Return code */
  u8 *aBuf;                       /* Buffer to load data from wal file into */
  u8 aFrame[WAL_FRAME_HDRSIZE];   /* Buffer to assemble frame-headers in */
  u32 iRead;                      /* Next frame to read from wal file */

  aBuf = sqlite3_malloc(szPage + WAL_FRAME_HDRSIZE);
  if( aBuf==0 ) return SQLITE_NOMEM;

  /* Find the checksum values to use as input for the checksum of the
  ** first frame written by this transaction. If that frame is frame 1
  ** (implying that the current transaction restarted the wal file), 
  ** these values must be read from the wal-file header. If the first
  ** frame to update the checksum of is not frame 1, then the initial
  ** checksum values can be copied from pLive.  */
  if( pLive->mxFrame==0 ){
    rc = sqlite3OsRead(pWal->pWalFd, aBuf, sizeof(u32)*2, 24);
    pWal->hdr.aFrameCksum[0] = sqlite3Get4byte(aBuf);
    pWal->hdr.aFrameCksum[1] = sqlite3Get4byte(&aBuf[sizeof(u32)]);
  }else{
    memcpy(pWal->hdr.aFrameCksum, pLive->aFrameCksum, sizeof(u32)*2);
  }

  for(iRead=pLive->mxFrame+1; rc==SQLITE_OK && iRead<=iLast; iRead++){
    i64 iOff = walFrameOffset(iRead, szPage);
    rc = sqlite3OsRead(pWal->pWalFd, aBuf, szPage+WAL_FRAME_HDRSIZE, iOff);
    if( rc==SQLITE_OK ){
      u32 iPgno, nDbSize;
      iPgno = sqlite3Get4byte(aBuf);
      nDbSize = sqlite3Get4byte(&aBuf[4]);

      walEncodeFrame(pWal, iPgno, nDbSize, &aBuf[WAL_FRAME_HDRSIZE], aFrame);
      rc = sqlite3OsWrite(pWal->pWalFd, aFrame, sizeof(aFrame), iOff);
    }
  }

  sqlite3_free(aBuf);
  return rc;
}

/* 
** Write a set of frames to the log. The caller must hold the write-lock
** on the log file (obtained using sqlite3WalBeginWriteTransaction()).
*/
int sqlite3WalFrames(
  Wal *pWal,                      /* Wal handle to write to */
................................................................................
  u32 iFrame;                     /* Next frame address */
  PgHdr *p;                       /* Iterator to run through pList with. */
  PgHdr *pLast = 0;               /* Last frame in list */
  int nExtra = 0;                 /* Number of extra copies of last page */
  int szFrame;                    /* The size of a single frame */
  i64 iOffset;                    /* Next byte to write in WAL file */
  WalWriter w;                    /* The writer */
  u32 iFirst = 0;                 /* First frame that may be overwritten */
  WalIndexHdr *pLive;             /* Pointer to shared header */

  assert( pList );
  assert( pWal->writeLock );

  /* If this frame set completes a transaction, then nTruncate>0.  If
  ** nTruncate==0 then this frame set does not complete the transaction. */
  assert( (isCommit!=0)==(nTruncate!=0) );
................................................................................

#if defined(SQLITE_TEST) && defined(SQLITE_DEBUG)
  { int cnt; for(cnt=0, p=pList; p; p=p->pDirty, cnt++){}
    WALTRACE(("WAL%p: frame write begin. %d frames. mxFrame=%d. %s\n",
              pWal, cnt, pWal->hdr.mxFrame, isCommit ? "Commit" : "Spill"));
  }
#endif

  pLive = (WalIndexHdr*)walIndexHdr(pWal);
  if( memcmp(&pWal->hdr, (void *)pLive, sizeof(WalIndexHdr))!=0
   && (isCommit || sqlite3PagerSavepointCount(pList->pPager)==0)
  ){
    iFirst = pLive->mxFrame+1;
  }

  /* See if it is possible to write these frames into the start of the
  ** log file, instead of appending to it at pWal->hdr.mxFrame.
  */
  if( SQLITE_OK!=(rc = walRestartLog(pWal)) ){
    return rc;
  }
................................................................................
  w.szPage = szPage;
  iOffset = walFrameOffset(iFrame+1, szPage);
  szFrame = szPage + WAL_FRAME_HDRSIZE;

  /* Write all frames into the log file exactly once */
  for(p=pList; p; p=p->pDirty){
    int nDbSize;   /* 0 normally.  Positive == commit flag */

    /* Check if this page has already been written into the wal file by
    ** the current transaction. If so, overwrite the existing frame and
    ** set Wal.writeLock to WAL_WRITELOCK_RECKSUM - indicating that 
    ** checksums must be recomputed when the transaction is committed.  */
    if( iFirst && (p->pDirty || isCommit==0) ){
      u32 iWrite = 0;
      rc = sqlite3WalFindFrame(pWal, p->pgno, &iWrite);
      if( rc ) return rc;
      if( iWrite>=iFirst ){
        i64 iOff = walFrameOffset(iWrite, szPage) + WAL_FRAME_HDRSIZE;
        pWal->writeLock = WAL_WRITELOCK_RECKSUM;
        rc = sqlite3OsWrite(pWal->pWalFd, p->pData, szPage, iOff);
        if( rc ) return rc;
        p->flags &= ~PGHDR_WAL_APPEND;
        continue;
      }
    }

    iFrame++;
    assert( iOffset==walFrameOffset(iFrame, szPage) );
    nDbSize = (isCommit && p->pDirty==0) ? nTruncate : 0;
    rc = walWriteOneFrame(&w, p, nDbSize, iOffset);
    if( rc ) return rc;
    pLast = p;
    iOffset += szFrame;
    p->flags |= PGHDR_WAL_APPEND;
  }

  /* Recalculate checksums within the wal file if required. */
  if( isCommit && pWal->writeLock==WAL_WRITELOCK_RECKSUM ){
    rc = walRewriteChecksums(pWal, pLive, iFrame);
    if( rc ) return rc;
  }

  /* If this is the end of a transaction, then we might need to pad
  ** the transaction and/or sync the WAL file.
  **
  ** Padding and syncing only occur if this set of frames complete a
  ** transaction and if PRAGMA synchronous=FULL.  If synchronous==NORMAL
................................................................................
  /* Append data to the wal-index. It is not necessary to lock the 
  ** wal-index to do this as the SQLITE_SHM_WRITE lock held on the wal-index
  ** guarantees that there are no other writers, and no data that may
  ** be in use by existing readers is being overwritten.
  */
  iFrame = pWal->hdr.mxFrame;
  for(p=pList; p && rc==SQLITE_OK; p=p->pDirty){
    if( (p->flags & PGHDR_WAL_APPEND)==0 ) continue;
    iFrame++;
    rc = walIndexAppend(pWal, iFrame, p->pgno);
  }
  while( rc==SQLITE_OK && nExtra>0 ){
    iFrame++;
    nExtra--;
    rc = walIndexAppend(pWal, iFrame, pLast->pgno);
................................................................................
    if( isChanged && pWal->pDbFd->pMethods->iVersion>=3 ){
      sqlite3OsUnfetch(pWal->pDbFd, 0, 0);
    }
  }

  /* Copy data from the log to the database file. */
  if( rc==SQLITE_OK ){

    if( pWal->hdr.mxFrame && walPagesize(pWal)!=nBuf ){
      rc = SQLITE_CORRUPT_BKPT;
    }else{
      rc = walCheckpoint(pWal, eMode2, xBusy2, pBusyArg, sync_flags, zBuf);
    }

    /* If no error occurred, set the output variables. */

Changes to test/wal.test.

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    SELECT count(*) FROM t1;
    PRAGMA integrity_check;
  }
} {16 ok}
do_test wal-11.6 {
  execsql COMMIT
  list [expr [file size test.db]/1024] [file size test.db-wal]
} [list 3 [wal_file_size 41 1024]]
do_test wal-11.7 {
  execsql { 
    SELECT count(*) FROM t1;
    PRAGMA integrity_check;
  }
} {16 ok}
do_test wal-11.8 {
  execsql { PRAGMA wal_checkpoint }
  list [expr [file size test.db]/1024] [file size test.db-wal]
} [list 37 [wal_file_size 41 1024]]
do_test wal-11.9 {
  db close
  list [expr [file size test.db]/1024] [log_deleted test.db-wal]
} {37 1}
sqlite3_wal db test.db
set nWal 39
if {[permutation]!="mmap"} {set nWal 37}







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    SELECT count(*) FROM t1;
    PRAGMA integrity_check;
  }
} {16 ok}
do_test wal-11.6 {
  execsql COMMIT
  list [expr [file size test.db]/1024] [file size test.db-wal]
} [list 3 [wal_file_size 40 1024]]
do_test wal-11.7 {
  execsql { 
    SELECT count(*) FROM t1;
    PRAGMA integrity_check;
  }
} {16 ok}
do_test wal-11.8 {
  execsql { PRAGMA wal_checkpoint }
  list [expr [file size test.db]/1024] [file size test.db-wal]
} [list 37 [wal_file_size 40 1024]]
do_test wal-11.9 {
  db close
  list [expr [file size test.db]/1024] [log_deleted test.db-wal]
} {37 1}
sqlite3_wal db test.db
set nWal 39
if {[permutation]!="mmap"} {set nWal 37}

Added test/waloverwrite.test.

















































































































































































































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# 2010 May 5
#
# The author disclaims copyright to this source code.  In place of
# a legal notice, here is a blessing:
#
#    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 file implements regression tests for SQLite library.  The
# focus of this file is testing the operation of the library in
# "PRAGMA journal_mode=WAL" mode.
#

set testdir [file dirname $argv0]
source $testdir/tester.tcl
source $testdir/wal_common.tcl
set testprefix waloverwrite

ifcapable !wal {finish_test ; return }

# Simple test:
#
#   1. Create a database of blobs roughly 50 pages in size.
#
#   2. Set the db cache size to something much smaller than this (5 pages)
#
#   3. Within a transaction, loop through the set of blobs 5 times. Update
#      each blob as it is visited.
#
#   4. Test that the wal file is roughly 50 pages in size - even though many
#      database pages have been written to it multiple times.
#
#   5. Take a copy of the database and wal file. Test that recovery can
#      be run on it.
#
# The above is run twice - once where the wal file is empty at the start of
# step 3 (tn==1) and once where it already contains a transaction (tn==2).
#
foreach {tn xtra} {
  1 {}
  2 { UPDATE t1 SET y = randomblob(799) WHERE x=4 }
} {
  reset_db
  do_execsql_test 1.$tn.0 {
    CREATE TABLE t1(x, y);
    CREATE INDEX i1y ON t1(y);
  
    WITH cnt(i) AS (
      SELECT 1 UNION ALL SELECT i+1 FROM cnt WHERE i<20
    )
    INSERT INTO t1 SELECT i, randomblob(800) FROM cnt;
  } {}
  
  do_test 1.$tn.1 {
    set nPg [db one { PRAGMA page_count } ]
    expr $nPg>40 && $nPg<50
  } {1}
  
  do_test 1.$tn.2 {
    db close
    sqlite3 db test.db
  
    execsql {PRAGMA journal_mode = wal}
    execsql {PRAGMA cache_size = 5}
    execsql $xtra
  
    db transaction {
      for {set i 0} {$i < 5} {incr i} {
        foreach x [db eval {SELECT x FROM t1}] {
          execsql { UPDATE t1 SET y = randomblob(799) WHERE x=$x }
        }
      }
    }
  
    set nPg [wal_frame_count test.db-wal 1024]
    expr $nPg>40 && $nPg<60
  } {1}
  
  do_execsql_test 1.$tn.3 { PRAGMA integrity_check } ok
  
  do_test 1.$tn.4 {
    forcedelete test.db2 test.db2-wal
    forcecopy test.db test.db2
    sqlite3 db2 test.db2
    execsql { SELECT sum(length(y)) FROM t1 } db2
  } [expr 20*800]
  
  do_test 1.$tn.5 {
    db2 close
    forcecopy test.db test.db2
    forcecopy test.db-wal test.db2-wal
    sqlite3 db2 test.db2
    execsql { SELECT sum(length(y)) FROM t1 } db2
  } [expr 20*799]
  
  do_test 1.$tn.6 {
    execsql { PRAGMA integrity_check } db2
  } ok
}

finish_test