int lsmFsWriteLog(FileSystem *pFS, i64 iOff, LsmString *pStr);
int lsmFsSyncLog(FileSystem *pFS);
int lsmFsReadLog(FileSystem *pFS, i64 iOff, int nRead, LsmString *pStr);
int lsmFsTruncateLog(FileSystem *pFS, i64 nByte);
int lsmFsTruncateDb(FileSystem *pFS, i64 nByte);
int lsmFsCloseAndDeleteLog(FileSystem *pFS);

void lsmFsDeferClose(FileSystem *pFS, LsmFile **pp);

/* And to sync the db file */
int lsmFsSyncDb(FileSystem *, int);

void lsmFsFlushWaiting(FileSystem *, int *);

/* Used by lsm_info(ARRAY_STRUCTURE) and lsm_config(MMAP) */
................................................................................
void lsmEnvShmUnmap(lsm_env *, lsm_file *, int);

void lsmEnvSleep(lsm_env *, int);

int lsmFsReadSyncedId(lsm_db *db, int, i64 *piVal);

int lsmFsSegmentContainsPg(FileSystem *pFS, Segment *, Pgno, int *);
Pgno lsmFsRedirectPage(FileSystem *, Redirect *, Pgno);

void lsmFsPurgeCache(FileSystem *);

/*
** End of functions from "lsm_file.c".
**************************************************************************/

int lsmFsWriteLog(FileSystem *pFS, i64 iOff, LsmString *pStr);
int lsmFsSyncLog(FileSystem *pFS);
int lsmFsReadLog(FileSystem *pFS, i64 iOff, int nRead, LsmString *pStr);
int lsmFsTruncateLog(FileSystem *pFS, i64 nByte);
int lsmFsTruncateDb(FileSystem *pFS, i64 nByte);
int lsmFsCloseAndDeleteLog(FileSystem *pFS);

LsmFile *lsmFsDeferClose(FileSystem *pFS);

/* And to sync the db file */
int lsmFsSyncDb(FileSystem *, int);

void lsmFsFlushWaiting(FileSystem *, int *);

/* Used by lsm_info(ARRAY_STRUCTURE) and lsm_config(MMAP) */
................................................................................
void lsmEnvShmUnmap(lsm_env *, lsm_file *, int);

void lsmEnvSleep(lsm_env *, int);

int lsmFsReadSyncedId(lsm_db *db, int, i64 *piVal);

int lsmFsSegmentContainsPg(FileSystem *pFS, Segment *, Pgno, int *);


void lsmFsPurgeCache(FileSystem *);

/*
** End of functions from "lsm_file.c".
**************************************************************************/

**   while writing to a meta page there is no risk of damage to the other
**   meta page or any other part of the database file. TODO: This may need
**   to be revisited.
**
** Blocks:
**
**   The database file is also divided into blocks. The default block size is
**   2MB. When writing to the database file, an attempt is made to write data
**   in contiguous block-sized chunks.
**
**   The first and last page on each block are special in that they are 4 
**   bytes smaller than all other pages. This is because the last four bytes 
**   of space on the first and last pages of each block are reserved for
**   pointers to other blocks (i.e. a 32-bit block number).
**
................................................................................
**   assigned page number (N+2). To avoid writing page (N+2) before page 
**   (N+1), the recently completed b-tree page is held in the singly linked
**   list headed by pWaiting until page (N+1) has been written. 
**
**   Function lsmFsFlushWaiting() is responsible for eventually writing 
**   waiting pages to disk.
**
**
** apHash/nHash:
**   Hash table used to store all Page objects that carry malloc'd arrays,
**   except those b-tree pages that have not yet been assigned page numbers.
**   Once they have been assigned page numbers - they are added to this
**   hash table.
**
**   Hash table overflow chains are connected using the Page.pHashNext
................................................................................
** Number of pgsz byte pages omitted from the start of block 1. The start
** of block 1 contains two 4096 byte meta pages (8192 bytes in total).
*/
#define BLOCK1_HDR_SIZE(pgsz)  LSM_MAX(1, 8192/(pgsz))

/*
** If NDEBUG is not defined, set a breakpoint in function lsmIoerrBkpt()
** to catch IO errors. 
*/
#ifndef NDEBUG
static void lsmIoerrBkpt(){
  static int nErr = 0;
  nErr++;
}
static int IOERR_WRAPPER(int rc){
................................................................................
**     lsmEnvTruncate()
**     lsmEnvUnlink()
**     lsmEnvRemap()
*/
int lsmEnvOpen(lsm_env *pEnv, const char *zFile, int flags, lsm_file **ppNew){
  return pEnv->xOpen(pEnv, zFile, flags, ppNew);
}

static int lsmEnvRead(
  lsm_env *pEnv, 
  lsm_file *pFile, 
  lsm_i64 iOff, 
  void *pRead, 
  int nRead
){
  return IOERR_WRAPPER( pEnv->xRead(pFile, iOff, pRead, nRead) );
}

static int lsmEnvWrite(
  lsm_env *pEnv, 
  lsm_file *pFile, 
  lsm_i64 iOff, 
  const void *pWrite, 
  int nWrite
){
  return IOERR_WRAPPER( pEnv->xWrite(pFile, iOff, (void *)pWrite, nWrite) );
}

static int lsmEnvSync(lsm_env *pEnv, lsm_file *pFile){
  return IOERR_WRAPPER( pEnv->xSync(pFile) );
}

static int lsmEnvSectorSize(lsm_env *pEnv, lsm_file *pFile){
  return pEnv->xSectorSize(pFile);
}

int lsmEnvClose(lsm_env *pEnv, lsm_file *pFile){
  return IOERR_WRAPPER( pEnv->xClose(pFile) );
}

static int lsmEnvTruncate(lsm_env *pEnv, lsm_file *pFile, lsm_i64 nByte){
  return IOERR_WRAPPER( pEnv->xTruncate(pFile, nByte) );
}

static int lsmEnvUnlink(lsm_env *pEnv, const char *zDel){
  return IOERR_WRAPPER( pEnv->xUnlink(pEnv, zDel) );
}

static int lsmEnvRemap(
  lsm_env *pEnv, 
  lsm_file *pFile, 
  i64 szMin,
  void **ppMap,
  i64 *pszMap
){
................................................................................
    }
  }

  if( pbOpen ) *pbOpen = (pFS->fdLog!=0);
  return rc;
}




void lsmFsCloseLog(lsm_db *db){
  FileSystem *pFS = db->pFS;
  if( pFS->fdLog ){
    lsmEnvClose(pFS->pEnv, pFS->fdLog);
    pFS->fdLog = 0;
  }
}

/*
** Open a connection to a database stored within the file-system (the
** "system of files").
**
** If parameter bReadonly is true, then open a read-only file-descriptor
** on the database file. It is possible that bReadonly will be false even
** if the user requested that pDb be opened read-only. This is because the
** file-descriptor may later on be recycled by a read-write connection.
** If the db file can be opened for read-write access, it always is. Parameter
** bReadonly is only ever true if it has already been determined that the
** db can only be opened for read-only access.


*/
int lsmFsOpen(
  lsm_db *pDb,                    /* Database connection to open fd for */
  const char *zDb,                /* Full path to database file */
  int bReadonly                   /* True to open db file read-only */
){
  FileSystem *pFS;
................................................................................
      pFS->nMapLimit = 0;
    }else{
      pFS->pCompress = 0;
      if( db->iMmap==1 ){
        /* Unlimited */
        pFS->nMapLimit = (i64)1 << 60;
      }else{

        pFS->nMapLimit = (i64)db->iMmap * 1024;
      }
    }
  }

  return LSM_OK;
}
................................................................................
    lsmFree(pEnv, pFS->apHash);
    lsmFree(pEnv, pFS->aIBuffer);
    lsmFree(pEnv, pFS->aOBuffer);
    lsmFree(pEnv, pFS);
  }
}






















void lsmFsDeferClose(FileSystem *pFS, LsmFile **pp){
  LsmFile *p = pFS->pLsmFile;
  assert( p->pNext==0 );
  p->pFile = pFS->fdDb;
  pFS->fdDb = 0;
  pFS->pLsmFile = 0;
  *pp = p;
}

/*
** Allocate a buffer and populate it with the output of the xFileid() 
** method of the database file handle. If successful, set *ppId to point 
** to the buffer and *pnId to the number of bytes in the buffer and return
** LSM_OK. Otherwise, set *ppId and *pnId to zero and return an LSM
................................................................................
*/
void lsmFsSetPageSize(FileSystem *pFS, int nPgsz){
  pFS->nPagesize = nPgsz;
  pFS->nCacheMax = 2048*1024 / pFS->nPagesize;
}

/*
** Configure the block-size used by this file-system. Actual pages may be 
** smaller or larger than this value.
*/
void lsmFsSetBlockSize(FileSystem *pFS, int nBlocksize){
  pFS->nBlocksize = nBlocksize;
}

/*
** Return the page number of the first page on block iBlock. Blocks are
................................................................................
  assert( !pFS->pCompress );
  return ( (iPg % nPagePerBlock)==1
        || (iPg<nPagePerBlock && iPg==fsFirstPageOnBlock(pFS, 1))
  );
}

/*
** Given a page reference, return a pointer to the in-memory buffer of the
** pages contents. If parameter pnData is not NULL, set *pnData to the size
** of the buffer in bytes before returning.
*/
u8 *lsmFsPageData(Page *pPage, int *pnData){
  if( pnData ){
    *pnData = pPage->nData;
  }
................................................................................
  }else{
    pFS->pLruFirst = pPg;
  }
  pFS->pLruLast = pPg;
}

/*
** Remove page pPg from the hash table.
*/
static void fsPageRemoveFromHash(FileSystem *pFS, Page *pPg){
  int iHash;
  Page **pp;

  iHash = fsHashKey(pFS->nHash, pPg->iPg);
  for(pp=&pFS->apHash[iHash]; *pp!=pPg; pp=&(*pp)->pHashNext);
  *pp = pPg->pHashNext;
  pPg->pHashNext = 0;
}




static void fsPageBufferFree(Page *pPg){
  pPg->pFS->nCacheAlloc--;
  lsmFree(pPg->pFS->pEnv, pPg->aData);
  lsmFree(pPg->pFS->pEnv, pPg);
}


................................................................................
static int fsPageBuffer(
  FileSystem *pFS, 
  Page **ppOut
){
  int rc = LSM_OK;
  Page *pPage = 0;
  if( pFS->pLruFirst==0 || pFS->nCacheAlloc<pFS->nCacheMax ){

    pPage = lsmMallocZero(pFS->pEnv, sizeof(Page));
    if( !pPage ){
      rc = LSM_NOMEM_BKPT;
    }else{
      pPage->aData = (u8 *)lsmMalloc(pFS->pEnv, pFS->nPagesize);
      if( !pPage->aData ){
        lsmFree(pFS->pEnv, pPage);
        rc = LSM_NOMEM_BKPT;
        pPage = 0;
      }
      pFS->nCacheAlloc++;
    }

  }else{

    u8 *aData;
    pPage = pFS->pLruFirst;
    aData = pPage->aData;
    fsPageRemoveFromLru(pFS, pPage);
    fsPageRemoveFromHash(pFS, pPage);

    memset(pPage, 0, sizeof(Page));
................................................................................
  if( pPage ){
    pPage->flags = PAGE_FREE;
  }
  *ppOut = pPage;
  return rc;
}













static void fsGrowMapping(
  FileSystem *pFS,
  i64 iSz,
  int *pRc


){
  /* This function won't work with compressed databases yet. */
  assert( pFS->pCompress==0 );
  assert( PAGE_HASPREV==4 );

  if( *pRc==LSM_OK && iSz>pFS->nMap ){
    int rc;
    u8 *aOld = pFS->pMap;
    rc = lsmEnvRemap(pFS->pEnv, pFS->fdDb, iSz, &pFS->pMap, &pFS->nMap);
................................................................................
      }
      lsmSortedRemap(pFS->pDb);
    }
    *pRc = rc;
  }
}


/*
** fsync() the database file.
*/
int lsmFsSyncDb(FileSystem *pFS, int nBlock){
#if 0
  if( nBlock && pFS->bUseMmap ){
    int rc = LSM_OK;
    i64 nMin = (i64)nBlock * (i64)pFS->nBlocksize;
    fsGrowMapping(pFS, nMin, &rc);
    if( rc!=LSM_OK ) return rc;
  }
#endif
  return lsmEnvSync(pFS->pEnv, pFS->fdDb);
}

static int fsPageGet(FileSystem *, Segment *, Pgno, int, Page **, int *);




static int fsRedirectBlock(Redirect *p, int iBlk){
  if( p ){
    int i;
    for(i=0; i<p->n; i++){
      if( iBlk==p->a[i].iFrom ) return p->a[i].iTo;
    }
  }
  assert( iBlk!=0 );
  return iBlk;
}






Pgno lsmFsRedirectPage(FileSystem *pFS, Redirect *pRedir, Pgno iPg){
  Pgno iReal = iPg;

  if( pRedir ){
    const int nPagePerBlock = (
        pFS->pCompress ? pFS->nBlocksize : (pFS->nBlocksize / pFS->nPagesize)
    );
    int iBlk = fsPageToBlock(pFS, iPg);
................................................................................
    }
  }

  assert( iReal!=0 );
  return iReal;
}




/*
** Parameter iBlock is a database file block. This function reads the value 
** stored in the blocks "next block" pointer and stores it in *piNext.
** LSM_OK is returned if everything is successful, or an LSM error code
** otherwise.
*/
static int fsBlockNext(
................................................................................
** Return the page number of the last page on the same block as page iPg.
*/
Pgno fsLastPageOnPagesBlock(FileSystem *pFS, Pgno iPg){
  return fsLastPageOnBlock(pFS, fsPageToBlock(pFS, iPg));
}

/*







** This function is only called in compressed database mode.
*/
static int fsReadData(
  FileSystem *pFS,                /* File-system handle */
  Segment *pSeg,                  /* Block redirection */
  i64 iOff,                       /* Read data from this offset */
  u8 *aData,                      /* Buffer to read data into */
................................................................................
  nByte  = (aBuf[0] & 0x7F) << 14;
  nByte += (aBuf[1] & 0x7F) << 7;
  nByte += (aBuf[2] & 0x7F);
  *pbFree = !(aBuf[1] & 0x80);
  return nByte;
}











static int fsSubtractOffset(
  FileSystem *pFS, 
  Segment *pSeg,
  i64 iOff, 
  int iSub, 
  i64 *piRes
){
................................................................................
  }

  rc = fsBlockPrev(pFS, pSeg, fsPageToBlock(pFS, iOff), &iBlk);
  *piRes = fsLastPageOnBlock(pFS, iBlk) - iSub + (iOff - iStart + 1);
  return rc;
}











static int fsAddOffset(
  FileSystem *pFS, 
  Segment *pSeg,
  i64 iOff, 
  int iAdd, 
  i64 *piRes
){
................................................................................
  }

  rc = fsBlockNext(pFS, pSeg, fsPageToBlock(pFS, iOff), &iBlk);
  *piRes = fsFirstPageOnBlock(pFS, iBlk) + iAdd - (iEob - iOff + 1);
  return rc;
}






static int fsAllocateBuffer(FileSystem *pFS, int bWrite){
  u8 **pp;                        /* Pointer to either aIBuffer or aOBuffer */

  assert( pFS->pCompress );

  /* If neither buffer has been allocated, figure out how large they
  ** should be. Store this value in FileSystem.nBuffer.  */
................................................................................
  Page *p;
  int iHash;
  int rc = LSM_OK;

  /* In most cases iReal is the same as iPg. Except, if pSeg->pRedirect is 
  ** not NULL, and the block containing iPg has been redirected, then iReal
  ** is the page number after redirection.  */
  Pgno iReal = lsmFsRedirectPage(pFS, (pSeg ? pSeg->pRedirect : 0), iPg);

  assert_lists_are_ok(pFS);
  assert( iPg>=fsFirstPageOnBlock(pFS, 1) );
  assert( iReal>=fsFirstPageOnBlock(pFS, 1) );
  *ppPg = 0;


................................................................................

#ifndef NDEBUG
/*
** Return true if page iPg, which is a part of segment p, lies on
** a redirected block. 
*/
static int fsPageRedirects(FileSystem *pFS, Segment *p, Pgno iPg){
  return (iPg!=0 && iPg!=lsmFsRedirectPage(pFS, p->pRedirect, iPg));
}

/*
** Return true if the second argument is not NULL and any of the first
** last or root pages lie on a redirected block. 
*/
static int fsSegmentRedirects(FileSystem *pFS, Segment *p){

**   while writing to a meta page there is no risk of damage to the other
**   meta page or any other part of the database file. TODO: This may need
**   to be revisited.
**
** Blocks:
**
**   The database file is also divided into blocks. The default block size is
**   1MB. When writing to the database file, an attempt is made to write data
**   in contiguous block-sized chunks.
**
**   The first and last page on each block are special in that they are 4 
**   bytes smaller than all other pages. This is because the last four bytes 
**   of space on the first and last pages of each block are reserved for
**   pointers to other blocks (i.e. a 32-bit block number).
**
................................................................................
**   assigned page number (N+2). To avoid writing page (N+2) before page 
**   (N+1), the recently completed b-tree page is held in the singly linked
**   list headed by pWaiting until page (N+1) has been written. 
**
**   Function lsmFsFlushWaiting() is responsible for eventually writing 
**   waiting pages to disk.
**

** apHash/nHash:
**   Hash table used to store all Page objects that carry malloc'd arrays,
**   except those b-tree pages that have not yet been assigned page numbers.
**   Once they have been assigned page numbers - they are added to this
**   hash table.
**
**   Hash table overflow chains are connected using the Page.pHashNext
................................................................................
** Number of pgsz byte pages omitted from the start of block 1. The start
** of block 1 contains two 4096 byte meta pages (8192 bytes in total).
*/
#define BLOCK1_HDR_SIZE(pgsz)  LSM_MAX(1, 8192/(pgsz))

/*
** If NDEBUG is not defined, set a breakpoint in function lsmIoerrBkpt()
** to catch IO errors (any error returned by a VFS method). 
*/
#ifndef NDEBUG
static void lsmIoerrBkpt(){
  static int nErr = 0;
  nErr++;
}
static int IOERR_WRAPPER(int rc){
................................................................................
**     lsmEnvTruncate()
**     lsmEnvUnlink()
**     lsmEnvRemap()
*/
int lsmEnvOpen(lsm_env *pEnv, const char *zFile, int flags, lsm_file **ppNew){
  return pEnv->xOpen(pEnv, zFile, flags, ppNew);
}

static int lsmEnvRead(
  lsm_env *pEnv, 
  lsm_file *pFile, 
  lsm_i64 iOff, 
  void *pRead, 
  int nRead
){
  return IOERR_WRAPPER( pEnv->xRead(pFile, iOff, pRead, nRead) );
}

static int lsmEnvWrite(
  lsm_env *pEnv, 
  lsm_file *pFile, 
  lsm_i64 iOff, 
  const void *pWrite, 
  int nWrite
){
  return IOERR_WRAPPER( pEnv->xWrite(pFile, iOff, (void *)pWrite, nWrite) );
}

static int lsmEnvSync(lsm_env *pEnv, lsm_file *pFile){
  return IOERR_WRAPPER( pEnv->xSync(pFile) );
}

static int lsmEnvSectorSize(lsm_env *pEnv, lsm_file *pFile){
  return pEnv->xSectorSize(pFile);
}

int lsmEnvClose(lsm_env *pEnv, lsm_file *pFile){
  return IOERR_WRAPPER( pEnv->xClose(pFile) );
}

static int lsmEnvTruncate(lsm_env *pEnv, lsm_file *pFile, lsm_i64 nByte){
  return IOERR_WRAPPER( pEnv->xTruncate(pFile, nByte) );
}

static int lsmEnvUnlink(lsm_env *pEnv, const char *zDel){
  return IOERR_WRAPPER( pEnv->xUnlink(pEnv, zDel) );
}

static int lsmEnvRemap(
  lsm_env *pEnv, 
  lsm_file *pFile, 
  i64 szMin,
  void **ppMap,
  i64 *pszMap
){
................................................................................
    }
  }

  if( pbOpen ) *pbOpen = (pFS->fdLog!=0);
  return rc;
}

/*
** Close the log file, if it is open.
*/
void lsmFsCloseLog(lsm_db *db){
  FileSystem *pFS = db->pFS;
  if( pFS->fdLog ){
    lsmEnvClose(pFS->pEnv, pFS->fdLog);
    pFS->fdLog = 0;
  }
}

/*
** Open a connection to a database stored within the file-system.

**
** If parameter bReadonly is true, then open a read-only file-descriptor
** on the database file. It is possible that bReadonly will be false even
** if the user requested that pDb be opened read-only. This is because the
** file-descriptor may later on be recycled by a read-write connection.
** If the db file can be opened for read-write access, it always is. Parameter
** bReadonly is only ever true if it has already been determined that the
** db can only be opened for read-only access.
**
** Return LSM_OK if successful or an lsm error code otherwise.
*/
int lsmFsOpen(
  lsm_db *pDb,                    /* Database connection to open fd for */
  const char *zDb,                /* Full path to database file */
  int bReadonly                   /* True to open db file read-only */
){
  FileSystem *pFS;
................................................................................
      pFS->nMapLimit = 0;
    }else{
      pFS->pCompress = 0;
      if( db->iMmap==1 ){
        /* Unlimited */
        pFS->nMapLimit = (i64)1 << 60;
      }else{
        /* iMmap is a limit in KB. Set nMapLimit to the same value in bytes. */
        pFS->nMapLimit = (i64)db->iMmap * 1024;
      }
    }
  }

  return LSM_OK;
}
................................................................................
    lsmFree(pEnv, pFS->apHash);
    lsmFree(pEnv, pFS->aIBuffer);
    lsmFree(pEnv, pFS->aOBuffer);
    lsmFree(pEnv, pFS);
  }
}

/*
** This function is called when closing a database handle (i.e. lsm_close()) 
** if there exist other connections to the same database within this process.
** In that case the file-descriptor open on the database file is not closed
** when the FileSystem object is destroyed, as this would cause any POSIX
** locks held by the other connections to be silently dropped (see "man close"
** for details). Instead, the file-descriptor is stored in a list by the
** lsm_shared.c module until it is either closed or reused.
**
** This function returns a pointer to an object that can be linked into
** the list described above. The returned object now 'owns' the database
** file descriptr, so that when the FileSystem object is destroyed, it
** will not be closed. 
**
** This function may be called at most once in the life-time of a 
** FileSystem object. The results of any operations involving the database 
** file descriptor are undefined once this function has been called.
**
** None of this is necessary on non-POSIX systems. But we do it anyway in
** the name of using as similar code as possible on all platforms.
*/
LsmFile *lsmFsDeferClose(FileSystem *pFS){
  LsmFile *p = pFS->pLsmFile;
  assert( p->pNext==0 );
  p->pFile = pFS->fdDb;
  pFS->fdDb = 0;
  pFS->pLsmFile = 0;
  return p;
}

/*
** Allocate a buffer and populate it with the output of the xFileid() 
** method of the database file handle. If successful, set *ppId to point 
** to the buffer and *pnId to the number of bytes in the buffer and return
** LSM_OK. Otherwise, set *ppId and *pnId to zero and return an LSM
................................................................................
*/
void lsmFsSetPageSize(FileSystem *pFS, int nPgsz){
  pFS->nPagesize = nPgsz;
  pFS->nCacheMax = 2048*1024 / pFS->nPagesize;
}

/*
** Configure the block-size used by this file-system. 

*/
void lsmFsSetBlockSize(FileSystem *pFS, int nBlocksize){
  pFS->nBlocksize = nBlocksize;
}

/*
** Return the page number of the first page on block iBlock. Blocks are
................................................................................
  assert( !pFS->pCompress );
  return ( (iPg % nPagePerBlock)==1
        || (iPg<nPagePerBlock && iPg==fsFirstPageOnBlock(pFS, 1))
  );
}

/*
** Given a page reference, return a pointer to the buffer containing the 
** pages contents. If parameter pnData is not NULL, set *pnData to the size
** of the buffer in bytes before returning.
*/
u8 *lsmFsPageData(Page *pPage, int *pnData){
  if( pnData ){
    *pnData = pPage->nData;
  }
................................................................................
  }else{
    pFS->pLruFirst = pPg;
  }
  pFS->pLruLast = pPg;
}

/*
** Page pPg is currently stored in the apHash/nHash hash table. Remove it.
*/
static void fsPageRemoveFromHash(FileSystem *pFS, Page *pPg){
  int iHash;
  Page **pp;

  iHash = fsHashKey(pFS->nHash, pPg->iPg);
  for(pp=&pFS->apHash[iHash]; *pp!=pPg; pp=&(*pp)->pHashNext);
  *pp = pPg->pHashNext;
  pPg->pHashNext = 0;
}

/*
** Free a Page object allocated by fsPageBuffer().
*/
static void fsPageBufferFree(Page *pPg){
  pPg->pFS->nCacheAlloc--;
  lsmFree(pPg->pFS->pEnv, pPg->aData);
  lsmFree(pPg->pFS->pEnv, pPg);
}


................................................................................
static int fsPageBuffer(
  FileSystem *pFS, 
  Page **ppOut
){
  int rc = LSM_OK;
  Page *pPage = 0;
  if( pFS->pLruFirst==0 || pFS->nCacheAlloc<pFS->nCacheMax ){
    /* Allocate a new Page object */
    pPage = lsmMallocZero(pFS->pEnv, sizeof(Page));
    if( !pPage ){
      rc = LSM_NOMEM_BKPT;
    }else{
      pPage->aData = (u8 *)lsmMalloc(pFS->pEnv, pFS->nPagesize);
      if( !pPage->aData ){
        lsmFree(pFS->pEnv, pPage);
        rc = LSM_NOMEM_BKPT;
        pPage = 0;
      }else{
        pFS->nCacheAlloc++;
      }
    }
  }else{
    /* Reuse an existing Page object */
    u8 *aData;
    pPage = pFS->pLruFirst;
    aData = pPage->aData;
    fsPageRemoveFromLru(pFS, pPage);
    fsPageRemoveFromHash(pFS, pPage);

    memset(pPage, 0, sizeof(Page));
................................................................................
  if( pPage ){
    pPage->flags = PAGE_FREE;
  }
  *ppOut = pPage;
  return rc;
}

/*
** Assuming *pRc is initially LSM_OK, attempt to ensure that the 
** memory-mapped region is at least iSz bytes in size. If it is not already,
** iSz bytes in size, extend it and update the pointers associated with any
** outstanding Page objects.
**
** If *pRc is not LSM_OK when this function is called, it is a no-op. 
** Otherwise, *pRc is set to an lsm error code if an error occurs, or
** left unmodified otherwise.
**
** This function is never called in compressed database mode.
*/
static void fsGrowMapping(
  FileSystem *pFS,                /* File system object */


  i64 iSz,                        /* Minimum size to extend mapping to */
  int *pRc                        /* IN/OUT: Error code */
){

  assert( pFS->pCompress==0 );
  assert( PAGE_HASPREV==4 );

  if( *pRc==LSM_OK && iSz>pFS->nMap ){
    int rc;
    u8 *aOld = pFS->pMap;
    rc = lsmEnvRemap(pFS->pEnv, pFS->fdDb, iSz, &pFS->pMap, &pFS->nMap);
................................................................................
      }
      lsmSortedRemap(pFS->pDb);
    }
    *pRc = rc;
  }
}


/*
** fsync() the database file.
*/
int lsmFsSyncDb(FileSystem *pFS, int nBlock){








  return lsmEnvSync(pFS->pEnv, pFS->fdDb);
}

/*
** If block iBlk has been redirected according to the redirections in the
** object passed as the first argument, return the destination block to
** which it is redirected. Otherwise, return a copy of iBlk.
*/
static int fsRedirectBlock(Redirect *p, int iBlk){
  if( p ){
    int i;
    for(i=0; i<p->n; i++){
      if( iBlk==p->a[i].iFrom ) return p->a[i].iTo;
    }
  }
  assert( iBlk!=0 );
  return iBlk;
}

/*
** If page iPg has been redirected according to the redirections in the
** object passed as the second argument, return the destination page to
** which it is redirected. Otherwise, return a copy of iPg.
*/
static Pgno fsRedirectPage(FileSystem *pFS, Redirect *pRedir, Pgno iPg){
  Pgno iReal = iPg;

  if( pRedir ){
    const int nPagePerBlock = (
        pFS->pCompress ? pFS->nBlocksize : (pFS->nBlocksize / pFS->nPagesize)
    );
    int iBlk = fsPageToBlock(pFS, iPg);
................................................................................
    }
  }

  assert( iReal!=0 );
  return iReal;
}

/* Required by the circular fsBlockNext<->fsPageGet dependency. */
static int fsPageGet(FileSystem *, Segment *, Pgno, int, Page **, int *);

/*
** Parameter iBlock is a database file block. This function reads the value 
** stored in the blocks "next block" pointer and stores it in *piNext.
** LSM_OK is returned if everything is successful, or an LSM error code
** otherwise.
*/
static int fsBlockNext(
................................................................................
** Return the page number of the last page on the same block as page iPg.
*/
Pgno fsLastPageOnPagesBlock(FileSystem *pFS, Pgno iPg){
  return fsLastPageOnBlock(pFS, fsPageToBlock(pFS, iPg));
}

/*
** Read nData bytes of data from offset iOff of the database file into
** buffer aData. If this means reading past the end of a block, follow
** the block pointer to the next block and continue reading.
**
** Offset iOff is an absolute offset - not subject to any block redirection.
** However any block pointer followed is. Use pSeg->pRedirect in this case.
**
** This function is only called in compressed database mode.
*/
static int fsReadData(
  FileSystem *pFS,                /* File-system handle */
  Segment *pSeg,                  /* Block redirection */
  i64 iOff,                       /* Read data from this offset */
  u8 *aData,                      /* Buffer to read data into */
................................................................................
  nByte  = (aBuf[0] & 0x7F) << 14;
  nByte += (aBuf[1] & 0x7F) << 7;
  nByte += (aBuf[2] & 0x7F);
  *pbFree = !(aBuf[1] & 0x80);
  return nByte;
}

/*
** Subtract iSub from database file offset iOff and set *piRes to the
** result. If doing so means passing the start of a block, follow the
** block pointer stored in the first 4 bytes of the block.
**
** Offset iOff is an absolute offset - not subject to any block redirection.
** However any block pointer followed is. Use pSeg->pRedirect in this case.
**
** Return LSM_OK if successful or an lsm error code if an error occurs.
*/
static int fsSubtractOffset(
  FileSystem *pFS, 
  Segment *pSeg,
  i64 iOff, 
  int iSub, 
  i64 *piRes
){
................................................................................
  }

  rc = fsBlockPrev(pFS, pSeg, fsPageToBlock(pFS, iOff), &iBlk);
  *piRes = fsLastPageOnBlock(pFS, iBlk) - iSub + (iOff - iStart + 1);
  return rc;
}

/*
** Add iAdd to database file offset iOff and set *piRes to the
** result. If doing so means passing the end of a block, follow the
** block pointer stored in the last 4 bytes of the block.
**
** Offset iOff is an absolute offset - not subject to any block redirection.
** However any block pointer followed is. Use pSeg->pRedirect in this case.
**
** Return LSM_OK if successful or an lsm error code if an error occurs.
*/
static int fsAddOffset(
  FileSystem *pFS, 
  Segment *pSeg,
  i64 iOff, 
  int iAdd, 
  i64 *piRes
){
................................................................................
  }

  rc = fsBlockNext(pFS, pSeg, fsPageToBlock(pFS, iOff), &iBlk);
  *piRes = fsFirstPageOnBlock(pFS, iBlk) + iAdd - (iEob - iOff + 1);
  return rc;
}

/*
** If it is not already allocated, allocate either the FileSystem.aOBuffer (if
** bWrite is true) or the FileSystem.aIBuffer (if bWrite is false). Return
** LSM_OK if successful if the attempt to allocate memory fails.
*/
static int fsAllocateBuffer(FileSystem *pFS, int bWrite){
  u8 **pp;                        /* Pointer to either aIBuffer or aOBuffer */

  assert( pFS->pCompress );

  /* If neither buffer has been allocated, figure out how large they
  ** should be. Store this value in FileSystem.nBuffer.  */
................................................................................
  Page *p;
  int iHash;
  int rc = LSM_OK;

  /* In most cases iReal is the same as iPg. Except, if pSeg->pRedirect is 
  ** not NULL, and the block containing iPg has been redirected, then iReal
  ** is the page number after redirection.  */
  Pgno iReal = fsRedirectPage(pFS, (pSeg ? pSeg->pRedirect : 0), iPg);

  assert_lists_are_ok(pFS);
  assert( iPg>=fsFirstPageOnBlock(pFS, 1) );
  assert( iReal>=fsFirstPageOnBlock(pFS, 1) );
  *ppPg = 0;


................................................................................

#ifndef NDEBUG
/*
** Return true if page iPg, which is a part of segment p, lies on
** a redirected block. 
*/
static int fsPageRedirects(FileSystem *pFS, Segment *p, Pgno iPg){
  return (iPg!=0 && iPg!=fsRedirectPage(pFS, p->pRedirect, iPg));
}

/*
** Return true if the second argument is not NULL and any of the first
** last or root pages lie on a redirected block. 
*/
static int fsSegmentRedirects(FileSystem *pFS, Segment *p){

  }

  return rc;
}

static void dbDeferClose(lsm_db *pDb){
  if( pDb->pFS ){
    LsmFile *pLsmFile = 0;
    Database *p = pDb->pDatabase;
    lsmFsDeferClose(pDb->pFS, &pLsmFile);
    pLsmFile->pNext = p->pLsmFile;
    p->pLsmFile = pLsmFile;
  }
}

LsmFile *lsmDbRecycleFd(lsm_db *db){
  LsmFile *pRet;

  }

  return rc;
}

static void dbDeferClose(lsm_db *pDb){
  if( pDb->pFS ){
    LsmFile *pLsmFile;
    Database *p = pDb->pDatabase;
    pLsmFile = lsmFsDeferClose(pDb->pFS);
    pLsmFile->pNext = p->pLsmFile;
    p->pLsmFile = pLsmFile;
  }
}

LsmFile *lsmDbRecycleFd(lsm_db *db){
  LsmFile *pRet;