GOMODULES
/
bbolt
mirror of https://github.com/etcd-io/bbolt.git
1
0
Fork 0
Code Issues Packages Projects Releases Wiki Activity

Intermediate.

pull/34/head
Ben Johnson 2014-01-27 10:11:54 -05:00
parent 1baa6d576a
commit 192649f453
12 changed files with 246 additions and 327 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

18
bnode.go
View File

@ -4,20 +4,16 @@ import (
"unsafe"
)
const bnodeSize = int(unsafe.Sizeof(lnode{}))
// bnode represents a node on a branch page.
type bnode struct {
flags uint16
keySize uint16
pgid pgid
data uintptr // Pointer to the beginning of the data.
pos uint32
ksize uint32
pgid pgid
}
// key returns a byte slice that of the key data.
// key returns a byte slice of the node key.
func (n *bnode) key() []byte {
return (*[MaxKeySize]byte)(unsafe.Pointer(&n.data))[:n.keySize]
}
// bnodeSize returns the number of bytes required to store a key as a branch node.
func bnodeSize(key []byte) int {
return int(unsafe.Offsetof((*bnode)(nil)).data) + len(key)
return (*[MaxKeySize]byte)(unsafe.Pointer(&n))[n.pos : n.pos+n.ksize]
}

4
bnodes.go Normal file
View File

@ -0,0 +1,4 @@
package bolt
type bnodes []bnode

43
bucket.go
View File

@ -1,37 +1,40 @@
package bolt
var (
InvalidBucketError = &Error{"invalid bucket", nil}
)
type bucketid uint32
type Bucket struct {
*bucket
name string
transaction Transaction,
cursors []*Cursor,
name string
transaction *Transaction
cursors []*Cursor
}
type bucket struct {
id bucketid
flags uint32
root pgid
branches pgid
leafs pgid
entries uint64
id bucketid
flags uint32
root pgid
}
func (b *Bucket) Close() error {
// TODO: Close cursors.
return nil
// Get retrieves the value for a key in the bucket.
func (b *Bucket) Get(key []byte) []byte {
return b.cursor().Get(key)
}
func (b *Bucket) Cursor() (*Cursor, error) {
if b.transaction == nil {
return nil, InvalidBucketError
}
// Cursor creates a new cursor for this bucket.
func (b *Bucket) Cursor() *Cursor {
c := b.cursor()
b.cursors = append(b.cursors, c)
return c
}
c := &Cursor{
// cursor creates a new untracked cursor for this bucket.
func (b *Bucket) cursor() *Cursor {
return &Cursor{
bucket: b,
stack: make([]elem, 0),
stack: make([]elem, 0),
}
return nil
}

37
cursor.go
View File

@ -1,8 +1,8 @@
package bolt
type Cursor struct {
bucket *Bucket
stack []elem
bucket *Bucket
stack []elem
}
// elem represents a node on a page that's on the cursor's stack.
@ -16,22 +16,43 @@ func (c *Cursor) Bucket() *Bucket {
}
// First moves the cursor to the first item in the bucket and returns its key and data.
func (c *Cursor) First() ([]byte, []byte, error) {
func (c *Cursor) First() ([]byte, []byte) {
// TODO: Traverse to the first key.
return nil, nil, nil
return nil, nil
}
// Move the cursor to the next key/value.
func (c *Cursor) Next() ([]byte, []byte, error) {
return nil, nil, nil
func (c *Cursor) Next() ([]byte, []byte) {
return nil, nil
}
// Get positions the cursor at a specific key and returns the its value.
func (c *Cursor) Get(key []byte) []byte {
if c.Goto(key) {
return c.node().value()
}
return nil
}
// Goto positions the cursor at a specific key.
func (c *Cursor) Goto(key []byte) ([]byte, error) {
// Returns true if an exact match or false if positioned after the closest match.
func (c *Cursor) Goto(key []byte) bool {
// TODO(benbjohnson): Optimize for specific use cases.
// TODO: Check if len(key) > 0.
// TODO: Start from root page and traverse to correct page.
return nil, nil
return false
}
// current the page and leaf node that the cursor is currently pointing at.
func (c *Cursor) current() (*page, *lnode) {
elem := c.stack[len(c.stack)-1]
return elem.page, elem.page.lnode(elem.index)
}
// node returns the leaf node that the cursor is currently positioned on.
func (c *Cursor) node() *lnode {
elem := c.stack[len(c.stack)-1]
return elem.page.lnode(elem.index)
}

48
db.go
View File

@ -12,6 +12,8 @@ const (
db_nometasync
)
const minPageSize = 0x1000
var (
DatabaseNotOpenError = &Error{"db is not open", nil}
DatabaseAlreadyOpenedError = &Error{"db already open", nil}
@ -28,23 +30,11 @@ type DB struct {
file file
metafile file
data []byte
buf []byte
meta0 *meta
meta1 *meta
pageSize int
rwtransaction *RWTransaction
transactions []*Transaction
maxPageNumber int /**< me_mapsize / me_psize */
freePages []int /** IDL of pages that became unused in a write txn */
dirtyPages []int /** ID2L of pages written during a write txn. Length MDB_IDL_UM_SIZE. */
// TODO: scratch []*page // list of temp pages for writing.
readers []*reader
maxFreeOnePage int /** Max number of freelist items that can fit in a single overflow page */
maxPageDataSize int
maxNodeSize int /** Max size of a node on a page */
maxKeySize int /**< max size of a key */
}
// NewDB creates a new DB instance.
@ -91,10 +81,10 @@ func (db *DB) Open(path string, mode os.FileMode) error {
// Read enough data to get both meta pages.
var m, m0, m1 *meta
var buf [pageHeaderSize + int(unsafe.Sizeof(meta{}))]byte
var buf [minPageSize]byte
if _, err := db.file.ReadAt(buf[:], 0); err == nil {
if m0, _ = db.pageInBuffer(buf[:], 0).meta(); m0 != nil {
db.pageSize = int(m0.free.pad)
db.pageSize = int(m0.pageSize)
}
}
if _, err := db.file.ReadAt(buf[:], int64(db.pageSize)); err == nil {
@ -115,12 +105,6 @@ func (db *DB) Open(path string, mode os.FileMode) error {
}
}
// Initialize db fields.
db.buf = make([]byte, db.pageSize)
db.maxPageDataSize = ((db.pageSize - pageHeaderSize) / int(unsafe.Sizeof(pgno(0)))) - 1
db.maxNodeSize = (((db.pageSize - pageHeaderSize) / minKeyCount) & -2) - int(unsafe.Sizeof(indx(0)))
// TODO?: env->me_maxpg = env->me_mapsize / env->me_psize;
// Memory map the data file.
if err := db.mmap(); err != nil {
db.close()
@ -181,8 +165,8 @@ func (db *DB) init() error {
// Create two meta pages on a buffer.
buf := make([]byte, db.pageSize*2)
for i := 0; i < 2; i++ {
p := db.pageInBuffer(buf[:], i)
p.id = pgno(i)
p := db.pageInBuffer(buf[:], pgid(i))
p.id = pgid(i)
p.init(db.pageSize)
}
@ -198,7 +182,7 @@ func (db *DB) init() error {
func (db *DB) Close() {
db.Lock()
defer db.Unlock()
s.close()
db.close()
}
func (db *DB) close() {
@ -245,13 +229,13 @@ func (db *DB) RWTransaction() (*RWTransaction, error) {
}
// page retrieves a page reference from the mmap based on the current page size.
func (db *DB) page(id int) *page {
return (*page)(unsafe.Pointer(&db.data[id*db.pageSize]))
func (db *DB) page(id pgid) *page {
return (*page)(unsafe.Pointer(&db.data[id*pgid(db.pageSize)]))
}
// pageInBuffer retrieves a page reference from a given byte array based on the current page size.
func (db *DB) pageInBuffer(b []byte, id int) *page {
return (*page)(unsafe.Pointer(&b[id*db.pageSize]))
func (db *DB) pageInBuffer(b []byte, id pgid) *page {
return (*page)(unsafe.Pointer(&b[id*pgid(db.pageSize)]))
}
// meta retrieves the current meta page reference.
@ -262,17 +246,15 @@ func (db *DB) meta() *meta {
return db.meta1
}
// sync flushes the file descriptor to disk unless "no sync" is enabled.
// sync flushes the file descriptor to disk.
func (db *DB) sync(force bool) error {
if !db.noSync {
if err := syscall.Fsync(int(db.file.Fd())); err != nil {
return err
}
if err := syscall.Fsync(int(db.file.Fd())); err != nil {
return err
}
return nil
}
func (db *DB) Stat() *stat {
func (db *DB) Stat() *Stat {
// TODO: Calculate size, depth, page count (by type), entry count, readers, etc.
return nil
}

25
lnode.go
View File

@ -4,27 +4,22 @@ import (
"unsafe"
)
type nodeid uint16
const lnodeSize = int(unsafe.Sizeof(lnode{}))
// lnode represents a node on a leaf page.
type lnode struct {
flags uint16
keySize uint16
dataSize uint32
data uintptr // Pointer to the beginning of the data.
flags uint32
pos uint32
ksize uint32
vsize uint32
}
// key returns a byte slice that of the node key.
// key returns a byte slice of the node key.
func (n *lnode) key() []byte {
return (*[MaxKeySize]byte)(unsafe.Pointer(&n.data))[:n.keySize]
return (*[MaxKeySize]byte)(unsafe.Pointer(&n))[n.pos : n.pos+n.ksize]
}
// data returns a byte slice that of the node data.
func (n *lnode) data() []byte {
return (*[MaxKeySize]byte)(unsafe.Pointer(&n.data))[n.keySize : n.keySize+n.dataSize]
}
// lnodeSize returns the number of bytes required to store a key+data as a leaf node.
func lnodeSize(key []byte, data []byte) int {
return int(unsafe.Offsetof((*lnode)(nil)).data) + len(key) + len(data)
// value returns a byte slice of the node value.
func (n *lnode) value() []byte {
return (*[MaxKeySize]byte)(unsafe.Pointer(&n))[n.pos+n.ksize : n.pos+n.ksize+n.vsize]
}

17
lnodes.go Normal file
View File

@ -0,0 +1,17 @@
package bolt
type lnodes []lnode
// replace replaces the node at the given index with a new key/value size.
func (s lnodes) replace(key, value []byte, index int) lnodes {
n := &s[index]
n.pos = 0
n.ksize = len(key)
n.vsize = len(value)
return s
}
// insert places a new node at the given index with a key/value size.
func (s lnodes) insert(key, value []byte, index int) lnodes {
return append(s[0:index], lnode{ksize: len(key), vsize: len(value)}, s[index:len(s)])
}

4
meta.go
View File

@ -1,7 +1,9 @@
package bolt
var (
InvalidMetaPageError = &Error{"Invalid meta page", nil}
InvalidError = &Error{"Invalid database", nil}
VersionMismatchError = &Error{"version mismatch", nil}
InvalidMetaPageError = &Error{"invalid meta page", nil}
)
const magic uint32 = 0xC0DEC0DE

1
os_test.go
View File

@ -2,6 +2,7 @@ package bolt
import (
"os"
"time"
"github.com/stretchr/testify/mock"
)

27
page.go
View File

@ -4,12 +4,11 @@ import (
"unsafe"
)
const pageHeaderSize = int(unsafe.Offsetof(((*page)(nil)).ptr))
const maxPageSize = 0x8000
const minKeyCount = 2
const pageHeaderSize = int(unsafe.Offsetof(((*page)(nil)).data))
const minPageKeys = 2
const minKeysPerPage = 2
const maxNodesPerPage = 65535
const fillThreshold = 250 // 25%
const (
@ -21,12 +20,11 @@ const (
type pgid uint64
type page struct {
id pgid
flags uint32
lower uint16
upper uint16
count uint32
data uintptr
id pgid
flags uint16
count uint16
overflow uint32
ptr uintptr
}
// meta returns a pointer to the metadata section of the page.
@ -52,5 +50,10 @@ func (p *page) init(pageSize int) {
m.version = version
m.pageSize = uint32(pageSize)
m.pgid = 1
m.buckets.root = 0
m.sys.root = 0
}
// lnode retrieves the leaf node by index
func (p *page) lnode(index int) *lnode {
return &((*[maxNodesPerPage]lnode)(unsafe.Pointer(&p.ptr)))[index]
}

230
rwtransaction.go
View File

@ -1,19 +1,13 @@
package bolt
import (
"unsafe"
)
// RWTransaction represents a transaction that can read and write data.
// Only one read/write transaction can be active for a DB at a time.
type RWTransaction struct {
Transaction
dirtyPages map[pgid]*page
freelist []pgid
}
// init initializes the transaction and associates it with a database.
func (t *RWTransaction) init(db *DB, meta *meta) {
t.dirtyPages = make(map[pgid]*page)
t.freelist = make([]pgid)
t.Transaction.init(db, meta)
}
// TODO: Allocate scratch meta page.
@ -50,150 +44,86 @@ func (t *RWTransaction) close() error {
}
// CreateBucket creates a new bucket.
func (t *RWTransaction) CreateBucket(name string, dupsort bool) (*Bucket, error) {
func (t *RWTransaction) CreateBucket(name string) error {
if t.db == nil {
return nil, InvalidTransactionError
return InvalidTransactionError
}
// Check if bucket already exists.
if b := t.buckets[name]; b != nil {
return nil, &Error{"bucket already exists", nil}
if b, err := t.Bucket(name); err != nil {
return err
} else if b != nil {
return &Error{"bucket already exists", nil}
}
// Create a new bucket entry.
var buf [unsafe.Sizeof(bucket{})]byte
var raw = (*bucket)(unsafe.Pointer(&buf[0]))
raw.root = p_invalid
// TODO: Set dupsort flag.
raw.root = 0
// Open cursor to system bucket.
c, err := t.Cursor(&t.sysbuckets)
if err != nil {
return nil, err
c := t.sys.cursor()
if c.Goto([]byte(name)) {
// TODO: Delete node first.
}
// Put new entry into system bucket.
if err := c.Put([]byte(name), buf[:]); err != nil {
return nil, err
// Insert new node.
if err := t.insert([]byte(name), buf[:]); err != nil {
return err
}
// Save reference to bucket.
b := &Bucket{name: name, bucket: raw, isNew: true}
t.buckets[name] = b
// TODO: dbflag |= DB_DIRTY;
return b, nil
return nil
}
// DropBucket deletes a bucket.
func (t *RWTransaction) DeleteBucket(b *Bucket) error {
// TODO: Find bucket.
// TODO: Remove entry from system bucket.
// TODO: Remove from main DB.
// TODO: Delete entry from system bucket.
// TODO: Free all pages.
// TODO: Remove cursor.
return nil
}
// Put sets the value for a key in a given bucket.
func (t *Transaction) Put(name string, key []byte, value []byte) error {
c, err := t.Cursor(name)
if err != nil {
return nil, err
}
return c.Put(key, value)
}
// page returns a reference to the page with a given id.
// If page has been written to then a temporary bufferred page is returned.
func (t *Transaction) page(id int) *page {
// Check the dirty pages first.
if p, ok := t.pages[id]; ok {
return p
}
// Otherwise return directly from the mmap.
return t.Transaction.page(id)
}
// Flush (some) dirty pages to the map, after clearing their dirty flag.
// @param[in] txn the transaction that's being committed
// @param[in] keep number of initial pages in dirty_list to keep dirty.
// @return 0 on success, non-zero on failure.
func (t *Transaction) flush(keep bool) error {
func (t *RWTransaction) flush(keep bool) error {
// TODO(benbjohnson): Use vectorized I/O to write out dirty pages.
// TODO: Loop over each dirty page and write it to disk.
return nil
}
func (t *RWTransaction) DeleteBucket(name string) error {
// TODO: Remove from main DB.
// TODO: Delete entry from system bucket.
// TODO: Free all pages.
// TODO: Remove cursor.
return nil
}
func (c *RWCursor) Put(key []byte, value []byte) error {
// Make sure this cursor was created by a transaction.
if c.transaction == nil {
return &Error{"invalid cursor", nil}
func (t *RWTransaction) Put(name string, key []byte, value []byte) error {
b := t.Bucket(name)
if b == nil {
return BucketNotFoundError
}
db := c.transaction.db
// Validate the key we're using.
if key == nil {
// Validate the key and data size.
if len(key) == 0 {
return &Error{"key required", nil}
} else if len(key) > db.maxKeySize {
} else if len(key) > MaxKeySize {
return &Error{"key too large", nil}
}
// TODO: Validate data size based on MaxKeySize if DUPSORT.
// Validate the size of our data.
if len(data) > MaxDataSize {
} else if len(value) > MaxDataSize {
return &Error{"data too large", nil}
}
// If we don't have a root page then add one.
if c.bucket.root == p_invalid {
p, err := c.newLeafPage()
if err != nil {
return err
}
c.push(p)
c.bucket.root = p.id
c.bucket.root++
// TODO: *mc->mc_dbflag |= DB_DIRTY;
// TODO? mc->mc_flags |= C_INITIALIZED;
}
// Move cursor to insertion position.
c := b.cursor()
replace := c.Goto()
p, index := c.current()
// TODO: Move to key.
exists, err := c.moveTo(key)
if err != nil {
// Insert a new node.
if err := t.insert(p, index, key, value, replace); err != nil {
return err
}
// TODO: spill?
if err := c.spill(key, data); err != nil {
return err
}
// Make sure all cursor pages are writable
if err := c.touch(); err != nil {
return err
}
// If key does not exist the
if exists {
node := c.currentNode()
}
return nil
}
func (c *Cursor) Delete(key []byte) error {
func (t *RWTransaction) Delete(key []byte) error {
// TODO: Traverse to the correct node.
// TODO: If missing, exit.
// TODO: Remove node from page.
@ -201,38 +131,6 @@ func (c *Cursor) Delete(key []byte) error {
return nil
}
// newLeafPage allocates and initialize new a new leaf page.
func (c *RWCursor) newLeafPage() (*page, error) {
// Allocate page.
p, err := c.allocatePage(1)
if err != nil {
return nil, err
}
// Set flags and bounds.
p.flags = p_leaf | p_dirty
p.lower = pageHeaderSize
p.upper = c.transaction.db.pageSize
return p, nil
}
// newBranchPage allocates and initialize new a new branch page.
func (b *RWCursor) newBranchPage() (*page, error) {
// Allocate page.
p, err := c.allocatePage(1)
if err != nil {
return nil, err
}
// Set flags and bounds.
p.flags = p_branch | p_dirty
p.lower = pageHeaderSize
p.upper = c.transaction.db.pageSize
return p, nil
}
// allocate returns a contiguous block of memory starting at a given page.
func (t *RWTransaction) allocate(count int) (*page, error) {
// TODO: Find a continuous block of free pages.
@ -240,11 +138,59 @@ func (t *RWTransaction) allocate(count int) (*page, error) {
return nil, nil
}
func (t *RWTransaction) insert(p *page, index int, key []byte, data []byte, replace bool) error {
nodes := copy(p.lnodes())
if replace {
nodes = nodes.replace(index, key, data)
} else {
nodes = nodes.insert(index, key, data)
}
// If our page fits in the same size page then just write it.
if pageHeaderSize + nodes.size() < p.size() {
// TODO: Write new page.
// TODO: Update parent branches.
}
// Calculate total page size.
size := pageHeaderSize
for _, n := range nodes {
size += lnodeSize + n.ksize + n.vsize
}
// If our new page fits in our current page size then just write it.
if size < t.db.pageSize {
return t.writeLeafPage(p.id, nodes)
}
var nodesets [][]lnodes
if size < t.db.pageSize {
nodesets = [][]lnodes{nodes}
}
nodesets := t.split(nodes)
func (t *RWTransaction) insert(key []byte, data []byte) error {
// TODO: If there is not enough space on page for key+data then split.
// TODO: Move remaining data on page forward.
// TODO: Write leaf node to current location.
// TODO: Adjust available page size.
return nil
}
// split takes a list of nodes and returns multiple sets of nodes if a
// page split is required.
func (t *RWTransaction) split(nodes []lnodes) [][]lnodes {
// If the size is less than the page size then just return the current set.
if size < t.db.pageSize {
return [][]lnodes{nodes}
}
// Otherwise loop over nodes and split up into multiple pages.
var nodeset []lnodes
var nodesets [][]lnodes
for _, n := range nodes {
}
}

119
transaction.go
View File

@ -1,7 +1,6 @@
package bolt
import (
"strings"
"unsafe"
)
@ -20,11 +19,12 @@ const (
type txnid uint64
type Transaction struct {
id int
db *DB
meta *meta
sys Bucket
buckets map[string]*Bucket
id int
db *DB
meta *meta
sys Bucket
buckets map[string]*Bucket
pages map[pgid]*page
}
// init initializes the transaction and associates it with a database.
@ -32,6 +32,7 @@ func (t *Transaction) init(db *DB, meta *meta) {
t.db = db
t.meta = meta
t.buckets = make(map[string]*Bucket)
t.pages = nil
t.sys.transaction = t
t.sys.bucket = &t.meta.sys
}
@ -47,27 +48,21 @@ func (t *Transaction) DB() *DB {
// Bucket retrieves a bucket by name.
func (t *Transaction) Bucket(name string) (*Bucket, error) {
return t.bucket(name)
}
func (t *Transaction) bucket(name string) (*Bucket, error) {
// Return bucket if it's already been looked up.
// Return cached reference if it's already been looked up.
if b := t.buckets[name]; b != nil {
return b, nil
}
// Retrieve bucket data from the system bucket.
data, err := c.get(&t.sys, []byte(name))
if err != nil {
return nil, err
} else if data == nil {
return nil, &Error{"bucket not found", nil}
value := t.get(&t.sys, []byte(name))
if value == nil {
return nil
}
// Create a bucket that overlays the data.
b := &Bucket{
bucket: (*bucket)(unsafe.Pointer(&data[0])),
name: name,
bucket: (*bucket)(unsafe.Pointer(&data[0])),
name: name,
transaction: t,
}
t.buckets[name] = b
@ -76,85 +71,39 @@ func (t *Transaction) bucket(name string) (*Bucket, error) {
}
// Cursor creates a cursor associated with a given bucket.
func (t *Transaction) Cursor(b *Bucket) (*Cursor, error) {
if b == nil {
return nil, &Error{"bucket required", nil}
} else
// Create a new cursor and associate it with the transaction and bucket.
c := &Cursor{
transaction: t,
bucket: b,
}
// Set the first page if available.
if b.root != p_invalid {
p := t.db.page(t.db.data, int(b.root))
c.top = 0
c.pages = append(c.pages, p)
}
return nil, nil
}
// Get retrieves the value for a key in a given bucket.
func (t *Transaction) Get(name string, key []byte) ([]byte, error) {
c, err := t.Cursor(name)
func (t *Transaction) Cursor(name string) (*Cursor, error) {
b, err := t.Bucket(name)
if err != nil {
return nil, err
}
return c.Get(key)
return b.Cursor()
}
func (t *Transaction) page(id int) (*page, error) {
return t.db.page(id)
// Get retrieves the value for a key in a named bucket.
func (t *Transaction) Get(name string, key []byte) ([]byte, error) {
b, err := t.Bucket(name)
if err != nil {
return nil, err
}
return b.Get(key)
}
// Stat returns information about a bucket's internal structure.
func (t *Transaction) Stat(name string) *stat {
func (t *Transaction) Stat(name string) *Stat {
// TODO
return nil
}
// //
// //
// //
// //
// //
// ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ CONVERTED ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ //
// //
// //
// //
// //
// //
// Return the data associated with a given node.
// @param[in] txn The transaction for this operation.
// @param[in] leaf The node being read.
// @param[out] data Updated to point to the node's data.
// @return 0 on success, non-zero on failure.
func (t *Transaction) readNode(leaf *node, data []byte) error {
/*
MDB_page *omp; // overflow page
pgno_t pgno;
int rc;
if (!F_ISSET(leaf->mn_flags, F_BIGDATA)) {
data->mv_size = NODEDSZ(leaf);
data->mv_data = NODEDATA(leaf);
return MDB_SUCCESS;
// page returns a reference to the page with a given id.
// If page has been written to then a temporary bufferred page is returned.
func (t *Transaction) page(id pgid) *page {
// Check the dirty pages first.
if t.pages != nil {
if p, ok := t.pages[id]; ok {
return p
}
}
// Read overflow data.
data->mv_size = NODEDSZ(leaf);
memcpy(&pgno, NODEDATA(leaf), sizeof(pgno));
if ((rc = mdb_page_get(txn, pgno, &omp, NULL)) != 0) {
DPRINTF(("read overflow page %"Z"u failed", pgno));
return rc;
}
data->mv_data = METADATA(omp);
return MDB_SUCCESS;
*/
return nil
// Otherwise return directly from the mmap.
return t.db.page(id)
}