Page
使用 BoltDB 的时候我们会初始化一个 DB 对象，一个 DB 对应的就是一个磁盘文件。而对应的磁盘文件，BoltDB 是以 Page 为单位进行数据读写的，换句话说数据在磁盘上是以 Page 为单位存储的。这里的 Page 和操作系统的 Page cache 大小一致，即 4K。
Page 结构
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type page struct {
// id 页id
id pgid
// flags 页类型，分为：分支，叶子节点，元信息，空闲列表
flags uint16
// count 统计叶子节点、非叶子节点、空闲列表页的个数
count uint16
// overflow 数据是否有溢出，主要在空闲列表上有用
overflow uint32
// ptr 无类线指针，指向真实数据
ptr uintptr
}
由以上定义可知，页由页头和页数据两部分组成。
在 boltDB 中页分为四种类型，如以下定义：
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const (
branchPageFlag = 0x01 // 存储索引信息(页号、元素key值)
leafPageFlag = 0x02 // 存储数据信息(页号、插入的key值、插入的value值)
metaPageFlag = 0x04 // 存储数据库的元信息，例如空闲列表页id、放置桶的根页等
freelistPageFlag = 0x10 // 存储哪些页是空闲页，可以用来后续分配空间时，优先考虑分配
)
每个数据页有一个 meta 方法，这个方法主要用于获取元数据信息，前提是改页类型是元数据页类型
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// meta returns a pointer to the metadata section of the page.
func (p *page) meta() *meta {
return (*meta)(unsafe.Pointer(&p.ptr))
}
Meta 结构
meta 结构如下：
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type meta struct {
// magic 魔数
magic uint32
// version 版本
version uint32
// pageSize page页的大小，该值和操作系统默认的页大小保持一致
pageSize uint32
// flags 保留值
flags uint32
// root 所有 bucket 的根
root bucket
// freelist 空闲列表页的id
freelist pgid
// pgid 元数据页的id
pgid pgid
// txid 最大的事务id
txid txid
// checksum 用作校验的校验和
checksum uint64
}
meta 写入到 page 中：
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// write writes the meta onto a page.
func (m *meta) write(p *page) {
if m.root.root >= m.pgid {
panic(fmt.Sprintf("root bucket pgid (%d) above high water mark (%d)", m.root.root, m.pgid))
} else if m.freelist >= m.pgid {
panic(fmt.Sprintf("freelist pgid (%d) above high water mark (%d)", m.freelist, m.pgid))
}
// Page id is either going to be 0 or 1 which we can determine by the transaction ID.
p.id = pgid(m.txid % 2)
p.flags |= metaPageFlag
// Calculate the checksum.
m.checksum = m.sum64()
m.copy(p.meta())
}
// meta returns a pointer to the metadata section of the page.
func (p *page) meta() *meta {
return (*meta)(unsafe.Pointer(&p.ptr))
}
// copy copies one meta object to another.
func (m *meta) copy(dest *meta) {
*dest = *m
}
Freelist 结构
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// freelist represents a list of all pages that are available for allocation.
// It also tracks pages that have been freed but are still in use by open transactions.
type freelist struct {
// ids 可用的空闲页 id
ids []pgid // all free and available free page ids.
// pending 不久将会释放的空闲页
pending map[txid][]pgid // mapping of soon-to-be free page ids by tx.
// cache 所有可用的空闲页及不久将释放的页 ID，便于快速查找
cache map[pgid]bool // fast lookup of all free and pending page ids.
}
freelist 写入到 page 中：
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// write writes the page ids onto a freelist page. All free and pending ids are
// saved to disk since in the event of a program crash, all pending ids will
// become free.
func (f *freelist) write(p *page) error {
// Combine the old free pgids and pgids waiting on an open transaction.
// Update the header flag.
// 设置页标识
p.flags |= freelistPageFlag
// The page.count can only hold up to 64k elements so if we overflow that
// number then we handle it by putting the size in the first element.
// 获取空闲页 ID 数，总 ID 数 = 空闲页ID + pending ID
lenids := f.count()
// 如果空闲页为0，记录空闲页数目到 p.count
if lenids == 0 {
p.count = uint16(lenids)
} else if lenids < 0xFFFF {
// 如果空闲页数目小于 0xFFFF，记录空闲页数目到 p.count
p.count = uint16(lenids)
f.copyall(((*[maxAllocSize]pgid)(unsafe.Pointer(&p.ptr)))[:])
} else {
// 如果空闲页数目大于 0xFFFF，p.count = 0xFFFF 因为p.count是uint64，最多只能记录 65535 大小
// 然后使用 p.ptr 的第一个字节记录空闲页大小
p.count = 0xFFFF
// 使用 p.ptr 的第一个字节记录空闲页大小
((*[maxAllocSize]pgid)(unsafe.Pointer(&p.ptr)))[0] = pgid(lenids)
// 从第一个元素位置拷贝，把空闲页 id 拷贝到 p.ptr
f.copyall(((*[maxAllocSize]pgid)(unsafe.Pointer(&p.ptr)))[1:])
}
return nil
}
// copyall copies into dst a list of all free ids and all pending ids in one sorted list.
// f.count returns the minimum length required for dst.
func (f *freelist) copyall(dst []pgid) {
m := make(pgids, 0, f.pending_count())
for _, list := range f.pending {
m = append(m, list...)
}
sort.Sort(m)
// 对 f.ids 和 f.pending 进行合并，并且拷贝到 dst
mergepgids(dst, f.ids, m)
}
// mergepgids copies the sorted union of a and b into dst.
// If dst is too small, it panics.
func mergepgids(dst, a, b pgids) {
if len(dst) < len(a)+len(b) {
panic(fmt.Errorf("mergepgids bad len %d < %d + %d", len(dst), len(a), len(b)))
}
// Copy in the opposite slice if one is nil.
if len(a) == 0 {
copy(dst, b)
return
}
if len(b) == 0 {
copy(dst, a)
return
}
// Merged will hold all elements from both lists.
merged := dst[:0]
// Assign lead to the slice with a lower starting value, follow to the higher value.
lead, follow := a, b
if b[0] < a[0] {
lead, follow = b, a
}
// Continue while there are elements in the lead.
for len(lead) > 0 {
// Merge largest prefix of lead that is ahead of follow[0].
n := sort.Search(len(lead), func(i int) bool { return lead[i] > follow[0] })
merged = append(merged, lead[:n]...)
if n >= len(lead) {
break
}
// Swap lead and follow.
lead, follow = follow, lead[n:]
}
// Append what's left in follow.
_ = append(merged, follow...)
}
从磁盘中加载 freelist 页，并转为 freelist 结构：
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// read initializes the freelist from a freelist page.
func (f *freelist) read(p *page) {
// If the page.count is at the max uint16 value (64k) then it's considered
// an overflow and the size of the freelist is stored as the first element.
idx, count := 0, int(p.count)
// 如果 count == 0xFFFF，从 ptr 第一个字节读取实际空闲页个数
if count == 0xFFFF {
idx = 1
count = int(((*[maxAllocSize]pgid)(unsafe.Pointer(&p.ptr)))[0])
}
// Copy the list of page ids from the freelist.
if count == 0 {
f.ids = nil
} else {
ids := ((*[maxAllocSize]pgid)(unsafe.Pointer(&p.ptr)))[idx:count]
f.ids = make([]pgid, len(ids))
copy(f.ids, ids)
// Make sure they're sorted.
sort.Sort(pgids(f.ids))
}
// Rebuild the page cache.
f.reindex()
}
// reindex rebuilds the free cache based on available and pending free lists.
func (f *freelist) reindex() {
f.cache = make(map[pgid]bool, len(f.ids))
for _, id := range f.ids {
f.cache[id] = true
}
for _, pendingIDs := range f.pending {
for _, pendingID := range pendingIDs {
f.cache[pendingID] = true
}
}
}
allocate 分配指定大小空闲页
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// allocate returns the starting page id of a contiguous list of pages of a given size.
// If a contiguous block cannot be found then 0 is returned.
// 分配n个连续的页，如果没有连续的页则返回0
func (f *freelist) allocate(n int) pgid {
if len(f.ids) == 0 {
return 0
}
var initial, previd pgid
for i, id := range f.ids {
if id <= 1 {
panic(fmt.Sprintf("invalid page allocation: %d", id))
}
// Reset initial page if this is not contiguous.
if previd == 0 || id-previd != 1 {
initial = id
}
// If we found a contiguous block then remove it and return it.
// 如果找到连续的块，则返回并且从f.ids，f.cache 中移除空闲页
if (id-initial)+1 == pgid(n) {
// If we're allocating off the beginning then take the fast path
// and just adjust the existing slice. This will use extra memory
// temporarily but the append() in free() will realloc the slice
// as is necessary.
if (i + 1) == n {
f.ids = f.ids[i+1:]
} else {
copy(f.ids[i-n+1:], f.ids[i+1:])
f.ids = f.ids[:len(f.ids)-n]
}
// Remove from the free cache.
for i := pgid(0); i < pgid(n); i++ {
delete(f.cache, initial+i)
}
return initial
}
previd = id
}
return 0
}
branchPage 结构
分支节点主要用来构建索引，提升查询效率。
分支节点在存储时，一个分支节点页上会存储多个分支页元素即branchPageElement。这个信息可以记做为分支页元素元信息。
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// branchPageElement represents a node on a branch page.
type branchPageElement struct {
// pos 最小key的值存储的位置距离当前的元信息的偏移量pos
pos uint32
// ksize 该元素所指向的页中存储的最小key的值大小
ksize uint32
// pgid 元素页 id
pgid pgid
}
分支节点页 page 获取下标为 index 的某一个element 的信息
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// leafPageElement retrieves the leaf node by index
func (p *page) leafPageElement(index uint16) *leafPageElement {
n := &((*[0x7FFFFFF]leafPageElement)(unsafe.Pointer(&p.ptr)))[index]
return n
}
非叶子结点图示：
在内存中，分支节点和叶子节点都是通过 node 来表示，它们是通过 node 里面的 isleaf 字段进行类型区分
node 结构
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// node represents an in-memory, deserialized page.
type node struct {
bucket *Bucket
isLeaf bool
unbalanced bool
spilled bool
key []byte
pgid pgid
parent *node
children nodes
inodes inodes
}
type inodes []inode
// inode represents an internal node inside of a node.
// It can be used to point to elements in a page or point
// to an element which hasn't been added to a page yet.
type inode struct {
// flags 表示是否是子桶叶子节点还是普通叶子节点。如果flags值为1表示子桶叶子节点，否则为普通叶子节点
flags uint32
// pgid 当inode为分支元素时，pgid才有值，为叶子元素时，则没值
pgid pgid
key []byte
// 当inode为分支元素时，value为空，为叶子元素时，才有值
value []byte
}
page 转化为 node 结构：
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// read initializes the node from a page.
func (n *node) read(p *page) {
n.pgid = p.id
n.isLeaf = ((p.flags & leafPageFlag) != 0)
n.inodes = make(inodes, int(p.count))
for i := 0; i < int(p.count); i++ {
inode := &n.inodes[i]
if n.isLeaf {
elem := p.leafPageElement(uint16(i))
inode.flags = elem.flags
inode.key = elem.key()
inode.value = elem.value()
} else {
elem := p.branchPageElement(uint16(i))
inode.pgid = elem.pgid
inode.key = elem.key()
}
_assert(len(inode.key) > 0, "read: zero-length inode key")
}
// Save first key so we can find the node in the parent when we spill.
// 存储第一个 key
if len(n.inodes) > 0 {
n.key = n.inodes[0].key
_assert(len(n.key) > 0, "read: zero-length node key")
} else {
n.key = nil
}
}
// key returns a byte slice of the node key.
func (n *branchPageElement) key() []byte {
buf := (*[maxAllocSize]byte)(unsafe.Pointer(n))
return (*[maxAllocSize]byte)(unsafe.Pointer(&buf[n.pos]))[:n.ksize]
}
node 转化为 page：
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// write writes the items onto one or more pages.
func (n *node) write(p *page) {
// Initialize page.
if n.isLeaf {
p.flags |= leafPageFlag
} else {
p.flags |= branchPageFlag
}
if len(n.inodes) >= 0xFFFF {
panic(fmt.Sprintf("inode overflow: %d (pgid=%d)", len(n.inodes), p.id))
}
p.count = uint16(len(n.inodes))
// Stop here if there are no items to write.
// 没有节点可存储，直接返回
if p.count == 0 {
return
}
// Loop over each item and write it to the page.
b := (*[maxAllocSize]byte)(unsafe.Pointer(&p.ptr))[n.pageElementSize()*len(n.inodes):]
for i, item := range n.inodes {
_assert(len(item.key) > 0, "write: zero-length inode key")
// Write the page element.
if n.isLeaf {
elem := p.leafPageElement(uint16(i))
elem.pos = uint32(uintptr(unsafe.Pointer(&b[0])) - uintptr(unsafe.Pointer(elem)))
elem.flags = item.flags
elem.ksize = uint32(len(item.key))
elem.vsize = uint32(len(item.value))
} else {
elem := p.branchPageElement(uint16(i))
// 计算第一个 key 与当前元素的地址偏移量
elem.pos = uint32(uintptr(unsafe.Pointer(&b[0])) - uintptr(unsafe.Pointer(elem)))
elem.ksize = uint32(len(item.key))
elem.pgid = item.pgid
_assert(elem.pgid != p.id, "write: circular dependency occurred")
}
// If the length of key+value is larger than the max allocation size
// then we need to reallocate the byte array pointer.
//
// See: https://github.com/boltdb/bolt/pull/335
klen, vlen := len(item.key), len(item.value)
if len(b) < klen+vlen {
b = (*[maxAllocSize]byte)(unsafe.Pointer(&b[0]))[:]
}
// Write data for the element to the end of the page.
copy(b[0:], item.key)
b = b[klen:]
copy(b[0:], item.value)
b = b[vlen:]
}
// DEBUG ONLY: n.dump()
}
leafPage 结构
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// leafPageElement represents a node on a leaf page.
type leafPageElement struct {
// flags 该值主要用来区分，是子桶叶子节点元素还是普通的key/value叶子节点元素。flags值为1时表示子桶。否则为key/value
flags uint32
// pos 具体存储的值距离元信息的偏移位置pos
pos uint32
// ksize key的长度
ksize uint32
// vsize value的长度
vsize uint32
}
LeafPage 结构图示：
