path: root/trie/database.go

// Copyright 2018 The go-ethereum Authors
// This file is part of the go-ethereum library.
//
// The go-ethereum library is free software: you can redistribute it and/or modify
// it under the terms of the GNU Lesser General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// The go-ethereum library 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 Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public License
// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.

package trie

import (
    "fmt"
    "io"
    "sync"
    "time"

    "github.com/ethereum/go-ethereum/common"
    "github.com/ethereum/go-ethereum/ethdb"
    "github.com/ethereum/go-ethereum/log"
    "github.com/ethereum/go-ethereum/metrics"
    "github.com/ethereum/go-ethereum/rlp"
)

var (
    memcacheFlushTimeTimer  = metrics.NewRegisteredResettingTimer("trie/memcache/flush/time", nil)
    memcacheFlushNodesMeter = metrics.NewRegisteredMeter("trie/memcache/flush/nodes", nil)
    memcacheFlushSizeMeter  = metrics.NewRegisteredMeter("trie/memcache/flush/size", nil)

    memcacheGCTimeTimer  = metrics.NewRegisteredResettingTimer("trie/memcache/gc/time", nil)
    memcacheGCNodesMeter = metrics.NewRegisteredMeter("trie/memcache/gc/nodes", nil)
    memcacheGCSizeMeter  = metrics.NewRegisteredMeter("trie/memcache/gc/size", nil)

    memcacheCommitTimeTimer  = metrics.NewRegisteredResettingTimer("trie/memcache/commit/time", nil)
    memcacheCommitNodesMeter = metrics.NewRegisteredMeter("trie/memcache/commit/nodes", nil)
    memcacheCommitSizeMeter  = metrics.NewRegisteredMeter("trie/memcache/commit/size", nil)
)

// secureKeyPrefix is the database key prefix used to store trie node preimages.
var secureKeyPrefix = []byte("secure-key-")

// secureKeyLength is the length of the above prefix + 32byte hash.
const secureKeyLength = 11 + 32

// DatabaseReader wraps the Get and Has method of a backing store for the trie.
type DatabaseReader interface {
    // Get retrieves the value associated with key form the database.
    Get(key []byte) (value []byte, err error)

    // Has retrieves whether a key is present in the database.
    Has(key []byte) (bool, error)
}

// Database is an intermediate write layer between the trie data structures and
// the disk database. The aim is to accumulate trie writes in-memory and only
// periodically flush a couple tries to disk, garbage collecting the remainder.
type Database struct {
    diskdb ethdb.Database // Persistent storage for matured trie nodes

    nodes  map[common.Hash]*cachedNode // Data and references relationships of a node
    oldest common.Hash                 // Oldest tracked node, flush-list head
    newest common.Hash                 // Newest tracked node, flush-list tail

    preimages map[common.Hash][]byte // Preimages of nodes from the secure trie
    seckeybuf [secureKeyLength]byte  // Ephemeral buffer for calculating preimage keys

    gctime  time.Duration      // Time spent on garbage collection since last commit
    gcnodes uint64             // Nodes garbage collected since last commit
    gcsize  common.StorageSize // Data storage garbage collected since last commit

    flushtime  time.Duration      // Time spent on data flushing since last commit
    flushnodes uint64             // Nodes flushed since last commit
    flushsize  common.StorageSize // Data storage flushed since last commit

    nodesSize     common.StorageSize // Storage size of the nodes cache (exc. flushlist)
    preimagesSize common.StorageSize // Storage size of the preimages cache

    lock sync.RWMutex
}

// rawNode is a simple binary blob used to differentiate between collapsed trie
// nodes and already encoded RLP binary blobs (while at the same time store them
// in the same cache fields).
type rawNode []byte

func (n rawNode) canUnload(uint16, uint16) bool { panic("this should never end up in a live trie") }
func (n rawNode) cache() (hashNode, bool)       { panic("this should never end up in a live trie") }
func (n rawNode) fstring(ind string) string     { panic("this should never end up in a live trie") }

// rawFullNode represents only the useful data content of a full node, with the
// caches and flags stripped out to minimize its data storage. This type honors
// the same RLP encoding as the original parent.
type rawFullNode [17]node

func (n rawFullNode) canUnload(uint16, uint16) bool { panic("this should never end up in a live trie") }
func (n rawFullNode) cache() (hashNode, bool)       { panic("this should never end up in a live trie") }
func (n rawFullNode) fstring(ind string) string     { panic("this should never end up in a live trie") }

func (n rawFullNode) EncodeRLP(w io.Writer) error {
    var nodes [17]node

    for i, child := range n {
        if child != nil {
            nodes[i] = child
        } else {
            nodes[i] = nilValueNode
        }
    }
    return rlp.Encode(w, nodes)
}

// rawShortNode represents only the useful data content of a short node, with the
// caches and flags stripped out to minimize its data storage. This type honors
// the same RLP encoding as the original parent.
type rawShortNode struct {
    Key []byte
    Val node
}

func (n rawShortNode) canUnload(uint16, uint16) bool { panic("this should never end up in a live trie") }
func (n rawShortNode) cache() (hashNode, bool)       { panic("this should never end up in a live trie") }
func (n rawShortNode) fstring(ind string) string     { panic("this should never end up in a live trie") }

// cachedNode is all the information we know about a single cached node in the
// memory database write layer.
type cachedNode struct {
    node node   // Cached collapsed trie node, or raw rlp data
    size uint16 // Byte size of the useful cached data

    parents  uint16                 // Number of live nodes referencing this one
    children map[common.Hash]uint16 // External children referenced by this node

    flushPrev common.Hash // Previous node in the flush-list
    flushNext common.Hash // Next node in the flush-list
}

// rlp returns the raw rlp encoded blob of the cached node, either directly from
// the cache, or by regenerating it from the collapsed node.
func (n *cachedNode) rlp() []byte {
    if node, ok := n.node.(rawNode); ok {
        return node
    }
    blob, err := rlp.EncodeToBytes(n.node)
    if err != nil {
        panic(err)
    }
    return blob
}

// obj returns the decoded and expanded trie node, either directly from the cache,
// or by regenerating it from the rlp encoded blob.
func (n *cachedNode) obj(hash common.Hash, cachegen uint16) node {
    if node, ok := n.node.(rawNode); ok {
        return mustDecodeNode(hash[:], node, cachegen)
    }
    return expandNode(hash[:], n.node, cachegen)
}

// childs returns all the tracked children of this node, both the implicit ones
// from inside the node as well as the explicit ones from outside the node.
func (n *cachedNode) childs() []common.Hash {
    children := make([]common.Hash, 0, 16)
    for child := range n.children {
        children = append(children, child)
    }
    if _, ok := n.node.(rawNode); !ok {
        gatherChildren(n.node, &children)
    }
    return children
}

// gatherChildren traverses the node hierarchy of a collapsed storage node and
// retrieves all the hashnode children.
func gatherChildren(n node, children *[]common.Hash) {
    switch n := n.(type) {
    case *rawShortNode:
        gatherChildren(n.Val, children)

    case rawFullNode:
        for i := 0; i < 16; i++ {
            gatherChildren(n[i], children)
        }
    case hashNode:
        *children = append(*children, common.BytesToHash(n))

    case valueNode, nil:

    default:
        panic(fmt.Sprintf("unknown node type: %T", n))
    }
}

// simplifyNode traverses the hierarchy of an expanded memory node and discards
// all the internal caches, returning a node that only contains the raw data.
func simplifyNode(n node) node {
    switch n := n.(type) {
    case *shortNode:
        // Short nodes discard the flags and cascade
        return &rawShortNode{Key: n.Key, Val: simplifyNode(n.Val)}

    case *fullNode:
        // Full nodes discard the flags and cascade
        node := rawFullNode(n.Children)
        for i := 0; i < len(node); i++ {
            if node[i] != nil {
                node[i] = simplifyNode(node[i])
            }
        }
        return node

    case valueNode, hashNode, rawNode:
        return n

    default:
        panic(fmt.Sprintf("unknown node type: %T", n))
    }
}

// expandNode traverses the node hierarchy of a collapsed storage node and converts
// all fields and keys into expanded memory form.
func expandNode(hash hashNode, n node, cachegen uint16) node {
    switch n := n.(type) {
    case *rawShortNode:
        // Short nodes need key and child expansion
        return &shortNode{
            Key: compactToHex(n.Key),
            Val: expandNode(nil, n.Val, cachegen),
            flags: nodeFlag{
                hash: hash,
                gen:  cachegen,
            },
        }

    case rawFullNode:
        // Full nodes need child expansion
        node := &fullNode{
            flags: nodeFlag{
                hash: hash,
                gen:  cachegen,
            },
        }
        for i := 0; i < len(node.Children); i++ {
            if n[i] != nil {
                node.Children[i] = expandNode(nil, n[i], cachegen)
            }
        }
        return node

    case valueNode, hashNode:
        return n

    default:
        panic(fmt.Sprintf("unknown node type: %T", n))
    }
}

// NewDatabase creates a new trie database to store ephemeral trie content before
// its written out to disk or garbage collected.
func NewDatabase(diskdb ethdb.Database) *Database {
    return &Database{
        diskdb:    diskdb,
        nodes:     map[common.Hash]*cachedNode{{}: {}},
        preimages: make(map[common.Hash][]byte),
    }
}

// DiskDB retrieves the persistent storage backing the trie database.
func (db *Database) DiskDB() DatabaseReader {
    return db.diskdb
}

// InsertBlob writes a new reference tracked blob to the memory database if it's
// yet unknown. This method should only be used for non-trie nodes that require
// reference counting, since trie nodes are garbage collected directly through
// their embedded children.
func (db *Database) InsertBlob(hash common.Hash, blob []byte) {
    db.lock.Lock()
    defer db.lock.Unlock()

    db.insert(hash, blob, rawNode(blob))
}

// insert inserts a collapsed trie node into the memory database. This method is
// a more generic version of InsertBlob, supporting both raw blob insertions as
// well ex trie node insertions. The blob must always be specified to allow proper
// size tracking.
func (db *Database) insert(hash common.Hash, blob []byte, node node) {
    // If the node's already cached, skip
    if _, ok := db.nodes[hash]; ok {
        return
    }
    // Create the cached entry for this node
    entry := &cachedNode{
        node:      simplifyNode(node),
        size:      uint16(len(blob)),
        flushPrev: db.newest,
    }
    for _, child := range entry.childs() {
        if c := db.nodes[child]; c != nil {
            c.parents++
        }
    }
    db.nodes[hash] = entry

    // Update the flush-list endpoints
    if db.oldest == (common.Hash{}) {
        db.oldest, db.newest = hash, hash
    } else {
        db.nodes[db.newest].flushNext, db.newest = hash, hash
    }
    db.nodesSize += common.StorageSize(common.HashLength + entry.size)
}

// insertPreimage writes a new trie node pre-image to the memory database if it's
// yet unknown. The method will make a copy of the slice.
//
// Note, this method assumes that the database's lock is held!
func (db *Database) insertPreimage(hash common.Hash, preimage []byte) {
    if _, ok := db.preimages[hash]; ok {
        return
    }
    db.preimages[hash] = common.CopyBytes(preimage)
    db.preimagesSize += common.StorageSize(common.HashLength + len(preimage))
}

// node retrieves a cached trie node from memory, or returns nil if none can be
// found in the memory cache.
func (db *Database) node(hash common.Hash, cachegen uint16) node {
    // Retrieve the node from cache if available
    db.lock.RLock()
    node := db.nodes[hash]
    db.lock.RUnlock()

    if node != nil {
        return node.obj(hash, cachegen)
    }
    // Content unavailable in memory, attempt to retrieve from disk
    enc, err := db.diskdb.Get(hash[:])
    if err != nil || enc == nil {
        return nil
    }
    return mustDecodeNode(hash[:], enc, cachegen)
}

// Node retrieves an encoded cached trie node from memory. If it cannot be found
// cached, the method queries the persistent database for the content.
func (db *Database) Node(hash common.Hash) ([]byte, error) {
    // Retrieve the node from cache if available
    db.lock.RLock()
    node := db.nodes[hash]
    db.lock.RUnlock()

    if node != nil {
        return node.rlp(), nil
    }
    // Content unavailable in memory, attempt to retrieve from disk
    return db.diskdb.Get(hash[:])
}

// preimage retrieves a cached trie node pre-image from memory. If it cannot be
// found cached, the method queries the persistent database for the content.
func (db *Database) preimage(hash common.Hash) ([]byte, error) {
    // Retrieve the node from cache if available
    db.lock.RLock()
    preimage := db.preimages[hash]
    db.lock.RUnlock()

    if preimage != nil {
        return preimage, nil
    }
    // Content unavailable in memory, attempt to retrieve from disk
    return db.diskdb.Get(db.secureKey(hash[:]))
}

// secureKey returns the database key for the preimage of key, as an ephemeral
// buffer. The caller must not hold onto the return value because it will become
// invalid on the next call.
func (db *Database) secureKey(key []byte) []byte {
    buf := append(db.seckeybuf[:0], secureKeyPrefix...)
    buf = append(buf, key...)
    return buf
}

// Nodes retrieves the hashes of all the nodes cached within the memory database.
// This method is extremely expensive and should only be used to validate internal
// states in test code.
func (db *Database) Nodes() []common.Hash {
    db.lock.RLock()
    defer db.lock.RUnlock()

    var hashes = make([]common.Hash, 0, len(db.nodes))
    for hash := range db.nodes {
        if hash != (common.Hash{}) { // Special case for "root" references/nodes
            hashes = append(hashes, hash)
        }
    }
    return hashes
}

// Reference adds a new reference from a parent node to a child node.
func (db *Database) Reference(child common.Hash, parent common.Hash) {
    db.lock.RLock()
    defer db.lock.RUnlock()

    db.reference(child, parent)
}

// reference is the private locked version of Reference.
func (db *Database) reference(child common.Hash, parent common.Hash) {
    // If the node does not exist, it's a node pulled from disk, skip
    node, ok := db.nodes[child]
    if !ok {
        return
    }
    // If the reference already exists, only duplicate for roots
    if db.nodes[parent].children == nil {
        db.nodes[parent].children = make(map[common.Hash]uint16)
    } else if _, ok = db.nodes[parent].children[child]; ok && parent != (common.Hash{}) {
        return
    }
    node.parents++
    db.nodes[parent].children[child]++
}

// Dereference removes an existing reference from a root node.
func (db *Database) Dereference(root common.Hash) {
    db.lock.Lock()
    defer db.lock.Unlock()

    nodes, storage, start := len(db.nodes), db.nodesSize, time.Now()
    db.dereference(root, common.Hash{})

    db.gcnodes += uint64(nodes - len(db.nodes))
    db.gcsize += storage - db.nodesSize
    db.gctime += time.Since(start)

    memcacheGCTimeTimer.Update(time.Since(start))
    memcacheGCSizeMeter.Mark(int64(storage - db.nodesSize))
    memcacheGCNodesMeter.Mark(int64(nodes - len(db.nodes)))

    log.Debug("Dereferenced trie from memory database", "nodes", nodes-len(db.nodes), "size", storage-db.nodesSize, "time", time.Since(start),
        "gcnodes", db.gcnodes, "gcsize", db.gcsize, "gctime", db.gctime, "livenodes", len(db.nodes), "livesize", db.nodesSize)
}

// dereference is the private locked version of Dereference.
func (db *Database) dereference(child common.Hash, parent common.Hash) {
    // Dereference the parent-child
    node := db.nodes[parent]

    if node.children != nil && node.children[child] > 0 {
        node.children[child]--
        if node.children[child] == 0 {
            delete(node.children, child)
        }
    }
    // If the child does not exist, it's a previously committed node.
    node, ok := db.nodes[child]
    if !ok {
        return
    }
    // If there are no more references to the child, delete it and cascade
    node.parents--
    if node.parents == 0 {
        // Remove the node from the flush-list
        if child == db.oldest {
            db.oldest = node.flushNext
        } else {
            db.nodes[node.flushPrev].flushNext = node.flushNext
            db.nodes[node.flushNext].flushPrev = node.flushPrev
        }
        // Dereference all children and delete the node
        for _, hash := range node.childs() {
            db.dereference(hash, child)
        }
        delete(db.nodes, child)
        db.nodesSize -= common.StorageSize(common.HashLength + int(node.size))
    }
}

// Cap iteratively flushes old but still referenced trie nodes until the total
// memory usage goes below the given threshold.
func (db *Database) Cap(limit common.StorageSize) error {
    // Create a database batch to flush persistent data out. It is important that
    // outside code doesn't see an inconsistent state (referenced data removed from
    // memory cache during commit but not yet in persistent storage). This is ensured
    // by only uncaching existing data when the database write finalizes.
    db.lock.RLock()

    nodes, storage, start := len(db.nodes), db.nodesSize, time.Now()
    batch := db.diskdb.NewBatch()

    // db.nodesSize only contains the useful data in the cache, but when reporting
    // the total memory consumption, the maintenance metadata is also needed to be
    // counted. For every useful node, we track 2 extra hashes as the flushlist.
    size := db.nodesSize + common.StorageSize((len(db.nodes)-1)*2*common.HashLength)

    // If the preimage cache got large enough, push to disk. If it's still small
    // leave for later to deduplicate writes.
    flushPreimages := db.preimagesSize > 4*1024*1024
    if flushPreimages {
        for hash, preimage := range db.preimages {
            if err := batch.Put(db.secureKey(hash[:]), preimage); err != nil {
                log.Error("Failed to commit preimage from trie database", "err", err)
                db.lock.RUnlock()
                return err
            }
            if batch.ValueSize() > ethdb.IdealBatchSize {
                if err := batch.Write(); err != nil {
                    db.lock.RUnlock()
                    return err
                }
                batch.Reset()
            }
        }
    }
    // Keep committing nodes from the flush-list until we're below allowance
    oldest := db.oldest
    for size > limit && oldest != (common.Hash{}) {
        // Fetch the oldest referenced node and push into the batch
        node := db.nodes[oldest]
        if err := batch.Put(oldest[:], node.rlp()); err != nil {
            db.lock.RUnlock()
            return err
        }
        // If we exceeded the ideal batch size, commit and reset
        if batch.ValueSize() >= ethdb.IdealBatchSize {
            if err := batch.Write(); err != nil {
                log.Error("Failed to write flush list to disk", "err", err)
                db.lock.RUnlock()
                return err
            }
            batch.Reset()
        }
        // Iterate to the next flush item, or abort if the size cap was achieved. Size
        // is the total size, including both the useful cached data (hash -> blob), as
        // well as the flushlist metadata (2*hash). When flushing items from the cache,
        // we need to reduce both.
        size -= common.StorageSize(3*common.HashLength + int(node.size))
        oldest = node.flushNext
    }
    // Flush out any remainder data from the last batch
    if err := batch.Write(); err != nil {
        log.Error("Failed to write flush list to disk", "err", err)
        db.lock.RUnlock()
        return err
    }
    db.lock.RUnlock()

    // Write successful, clear out the flushed data
    db.lock.Lock()
    defer db.lock.Unlock()

    if flushPreimages {
        db.preimages = make(map[common.Hash][]byte)
        db.preimagesSize = 0
    }
    for db.oldest != oldest {
        node := db.nodes[db.oldest]
        delete(db.nodes, db.oldest)
        db.oldest = node.flushNext

        db.nodesSize -= common.StorageSize(common.HashLength + int(node.size))
    }
    if db.oldest != (common.Hash{}) {
        db.nodes[db.oldest].flushPrev = common.Hash{}
    }
    db.flushnodes += uint64(nodes - len(db.nodes))
    db.flushsize += storage - db.nodesSize
    db.flushtime += time.Since(start)

    memcacheFlushTimeTimer.Update(time.Since(start))
    memcacheFlushSizeMeter.Mark(int64(storage - db.nodesSize))
    memcacheFlushNodesMeter.Mark(int64(nodes - len(db.nodes)))

    log.Debug("Persisted nodes from memory database", "nodes", nodes-len(db.nodes), "size", storage-db.nodesSize, "time", time.Since(start),
        "flushnodes", db.flushnodes, "flushsize", db.flushsize, "flushtime", db.flushtime, "livenodes", len(db.nodes), "livesize", db.nodesSize)

    return nil
}

// Commit iterates over all the children of a particular node, writes them out
// to disk, forcefully tearing down all references in both directions.
//
// As a side effect, all pre-images accumulated up to this point are also written.
func (db *Database) Commit(node common.Hash, report bool) error {
    // Create a database batch to flush persistent data out. It is important that
    // outside code doesn't see an inconsistent state (referenced data removed from
    // memory cache during commit but not yet in persistent storage). This is ensured
    // by only uncaching existing data when the database write finalizes.
    db.lock.RLock()

    start := time.Now()
    batch := db.diskdb.NewBatch()

    // Move all of the accumulated preimages into a write batch
    for hash, preimage := range db.preimages {
        if err := batch.Put(db.secureKey(hash[:]), preimage); err != nil {
            log.Error("Failed to commit preimage from trie database", "err", err)
            db.lock.RUnlock()
            return err
        }
        if batch.ValueSize() > ethdb.IdealBatchSize {
            if err := batch.Write(); err != nil {
                return err
            }
            batch.Reset()
        }
    }
    // Move the trie itself into the batch, flushing if enough data is accumulated
    nodes, storage := len(db.nodes), db.nodesSize
    if err := db.commit(node, batch); err != nil {
        log.Error("Failed to commit trie from trie database", "err", err)
        db.lock.RUnlock()
        return err
    }
    // Write batch ready, unlock for readers during persistence
    if err := batch.Write(); err != nil {
        log.Error("Failed to write trie to disk", "err", err)
        db.lock.RUnlock()
        return err
    }
    db.lock.RUnlock()

    // Write successful, clear out the flushed data
    db.lock.Lock()
    defer db.lock.Unlock()

    db.preimages = make(map[common.Hash][]byte)
    db.preimagesSize = 0

    db.uncache(node)

    memcacheCommitTimeTimer.Update(time.Since(start))
    memcacheCommitSizeMeter.Mark(int64(storage - db.nodesSize))
    memcacheCommitNodesMeter.Mark(int64(nodes - len(db.nodes)))

    logger := log.Info
    if !report {
        logger = log.Debug
    }
    logger("Persisted trie from memory database", "nodes", nodes-len(db.nodes)+int(db.flushnodes), "size", storage-db.nodesSize+db.flushsize, "time", time.Since(start)+db.flushtime,
        "gcnodes", db.gcnodes, "gcsize", db.gcsize, "gctime", db.gctime, "livenodes", len(db.nodes), "livesize", db.nodesSize)

    // Reset the garbage collection statistics
    db.gcnodes, db.gcsize, db.gctime = 0, 0, 0
    db.flushnodes, db.flushsize, db.flushtime = 0, 0, 0

    return nil
}

// commit is the private locked version of Commit.
func (db *Database) commit(hash common.Hash, batch ethdb.Batch) error {
    // If the node does not exist, it's a previously committed node
    node, ok := db.nodes[hash]
    if !ok {
        return nil
    }
    for _, child := range node.childs() {
        if err := db.commit(child, batch); err != nil {
            return err
        }
    }
    if err := batch.Put(hash[:], node.rlp()); err != nil {
        return err
    }
    // If we've reached an optimal batch size, commit and start over
    if batch.ValueSize() >= ethdb.IdealBatchSize {
        if err := batch.Write(); err != nil {
            return err
        }
        batch.Reset()
    }
    return nil
}

// uncache is the post-processing step of a commit operation where the already
// persisted trie is removed from the cache. The reason behind the two-phase
// commit is to ensure consistent data availability while moving from memory
// to disk.
func (db *Database) uncache(hash common.Hash) {
    // If the node does not exist, we're done on this path
    node, ok := db.nodes[hash]
    if !ok {
        return
    }
    // Node still exists, remove it from the flush-list
    if hash == db.oldest {
        db.oldest = node.flushNext
    } else {
        db.nodes[node.flushPrev].flushNext = node.flushNext
        db.nodes[node.flushNext].flushPrev = node.flushPrev
    }
    // Uncache the node's subtries and remove the node itself too
    for _, child := range node.childs() {
        db.uncache(child)
    }
    delete(db.nodes, hash)
    db.nodesSize -= common.StorageSize(common.HashLength + int(node.size))
}

// Size returns the current storage size of the memory cache in front of the
// persistent database layer.
func (db *Database) Size() (common.StorageSize, common.StorageSize) {
    db.lock.RLock()
    defer db.lock.RUnlock()

    // db.nodesSize only contains the useful data in the cache, but when reporting
    // the total memory consumption, the maintenance metadata is also needed to be
    // counted. For every useful node, we track 2 extra hashes as the flushlist.
    var flushlistSize = common.StorageSize((len(db.nodes) - 1) * 2 * common.HashLength)
    return db.nodesSize + flushlistSize, db.preimagesSize
}