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Add code for content defined chunking (CDC)

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This commit is contained in:
Alexander Neumann 2014-09-11 20:43:12 +02:00
parent 40aea3d612
commit 18131f88bf
4 changed files with 473 additions and 0 deletions
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255
chunker/chunker.go Normal file
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package chunker
import (
"io"
"sync"
)
const (
KiB = 1024
MiB = 1024 * KiB
// randomly generated irreducible polynomial of degree 53 in Z_2[X]
Polynomial = 0x3DA3358B4DC173
// use a sliding window of 64 byte.
WindowSize = 64
// aim to create chunks of 20 bits or about 1MiB on average.
AverageBits = 20
// Chunks should be in the range of 512KiB to 8MiB.
MinSize = 512 * KiB
MaxSize = 8 * MiB
splitmask = (1 << AverageBits) - 1
)
var (
pol_shift = deg(Polynomial) - 8
once sync.Once
mod_table [256]uint64
out_table [256]uint64
)
// A chunk is one content-dependent chunk of bytes whose end was cut when the
// Rabin Fingerprint had the value stored in Cut.
type Chunk struct {
Start int
Length int
Cut uint64
Data []byte
}
// A chunker takes a stream of bytes and emits average size chunks.
type Chunker interface {
Next() (*Chunk, error)
}
// A chunker internally holds everything needed to split content.
type chunker struct {
rd io.Reader
window []byte
wpos int
buf []byte
bpos int
bmax int
data []byte
start int
count int
pos int
digest uint64
}
// New returns a new Chunker that reads from data from rd.
func New(rd io.Reader) Chunker {
c := &chunker{
rd: rd,
window: make([]byte, WindowSize),
buf: make([]byte, MaxSize),
data: make([]byte, 0, MaxSize),
}
once.Do(c.fill_tables)
c.reset()
return c
}
func (c *chunker) reset() {
for i := 0; i < WindowSize; i++ {
c.window[i] = 0
}
c.digest = 0
c.wpos = 0
c.pos = 0
c.count = 0
c.slide(1)
c.data = make([]byte, 0, MaxSize)
}
// Calculate out_table and mod_table for optimization. Must be called only once.
func (c *chunker) fill_tables() {
// calculate table for sliding out bytes. The byte to slide out is used as
// the index for the table, the value contains the following:
// out_table[b] = Hash(b || 0 || ... || 0)
// \ windowsize-1 zero bytes /
// To slide out byte b_0 for window size w with known hash
// H := H(b_0 || ... || b_w), it is sufficient to add out_table[b_0]:
// H(b_0 || ... || b_w) + H(b_0 || 0 || ... || 0)
// = H(b_0 + b_0 || b_1 + 0 || ... || b_w + 0)
// = H( 0 || b_1 || ... || b_w)
//
// Afterwards a new byte can be shifted in.
for b := 0; b < 256; b++ {
var hash uint64
hash = append_byte(hash, byte(b), Polynomial)
for i := 0; i < WindowSize-1; i++ {
hash = append_byte(hash, 0, Polynomial)
}
out_table[b] = hash
}
// calculate table for reduction mod Polynomial
k := deg(Polynomial)
for b := 0; b < 256; b++ {
// mod_table[b] = A | B, where A = (b(x) * x^k mod pol) and B = b(x) * x^k
//
// The 8 bits above deg(Polynomial) determine what happens next and so
// these bits are used as a lookup to this table. The value is split in
// two parts: Part A contains the result of the modulus operation, part
// B is used to cancel out the 8 top bits so that one XOR operation is
// enough to reduce modulo Polynomial
mod_table[b] = mod(uint64(b)<<uint(k), Polynomial) | (uint64(b) << uint(k))
}
}
func (c *chunker) Next() (*Chunk, error) {
for {
if c.bpos >= c.bmax {
n, err := io.ReadFull(c.rd, c.buf)
if err == io.ErrUnexpectedEOF {
err = nil
}
if err != nil {
return &Chunk{
Start: c.start,
Length: c.count,
Cut: c.digest,
Data: c.data,
}, err
}
c.bpos = 0
c.bmax = n
}
for i, b := range c.buf[c.bpos:c.bmax] {
// inline c.slide(b) and append(b) to increase performance
out := c.window[c.wpos]
c.window[c.wpos] = b
c.digest ^= out_table[out]
c.wpos = (c.wpos + 1) % WindowSize
// c.append(b)
index := c.digest >> uint(pol_shift)
c.digest <<= 8
c.digest |= uint64(b)
c.digest ^= mod_table[index]
if (c.count+i+1 >= MinSize && (c.digest&splitmask) == 0) || c.count+i+1 >= MaxSize {
c.data = append(c.data, c.buf[c.bpos:c.bpos+i]...)
c.count += i + 1
c.pos += i + 1
c.bpos += i + 1
chunk := &Chunk{
Start: c.start,
Length: c.count,
Cut: c.digest,
Data: c.data,
}
// keep position
pos := c.pos
c.reset()
c.pos = pos
c.start = pos
return chunk, nil
}
}
steps := c.bmax - c.bpos
if steps > 0 {
c.data = append(c.data, c.buf[c.bpos:c.bpos+steps]...)
}
c.count += steps
c.pos += steps
c.bpos = c.bmax
}
return nil, nil
}
func (c *chunker) append(b byte) {
index := c.digest >> uint(pol_shift)
c.digest <<= 8
c.digest |= uint64(b)
c.digest ^= mod_table[index]
}
func (c *chunker) slide(b byte) {
out := c.window[c.wpos]
c.window[c.wpos] = b
c.digest ^= out_table[out]
c.wpos = (c.wpos + 1) % WindowSize
c.append(b)
}
func append_byte(hash uint64, b byte, pol uint64) uint64 {
hash <<= 8
hash |= uint64(b)
return mod(hash, pol)
}
// Mod calculates the remainder of x divided by p.
func mod(x, p uint64) uint64 {
for deg(x) >= deg(p) {
shift := uint(deg(x) - deg(p))
x = x ^ (p << shift)
}
return x
}
// Deg returns the degree of the polynomial p, this is equivalent to the number
// of the highest bit set in p.
func deg(p uint64) int {
var mask uint64 = 0x8000000000000000
for i := 0; i < 64; i++ {
if mask&p > 0 {
return 63 - i
}
mask >>= 1
}
return -1
}

133
chunker/chunker_test.go Normal file
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package chunker_test
import (
"bytes"
"io"
"math/rand"
"testing"
"github.com/fd0/khepri/chunker"
)
type chunk struct {
Length int
CutFP uint64
}
// created for 32MB of random data out of math/rand's Uint32() seeded by
// constant 23
//
// chunking configuration:
// window size 64, avg chunksize 1<<20, min chunksize 1<<19, max chunksize 1<<23
// polynom 0x3DA3358B4DC173
var chunks1 = []chunk{
chunk{2163460, 0x000b98d4cdf00000},
chunk{643703, 0x000d4e8364d00000},
chunk{1528956, 0x0015a25c2ef00000},
chunk{1955808, 0x00102a8242e00000},
chunk{2222372, 0x00045da878000000},
chunk{2538687, 0x00198a8179900000},
chunk{609606, 0x001d4e8d17100000},
chunk{1205738, 0x000a7204dd600000},
chunk{959742, 0x00183e71e1400000},
chunk{4036109, 0x001fec043c700000},
chunk{1525894, 0x000b1574b1500000},
chunk{1352720, 0x00018965f2e00000},
chunk{811884, 0x00155628aa100000},
chunk{1282314, 0x001909a0a1400000},
chunk{1318021, 0x001cceb980000000},
chunk{948640, 0x0011f7a470a00000},
chunk{645464, 0x00030ce2d9400000},
chunk{533758, 0x0004435c53c00000},
chunk{1128303, 0x0000c48517800000},
chunk{800374, 0x000968473f900000},
chunk{2453512, 0x001e197c92600000},
chunk{2651975, 0x000ae6c868000000},
chunk{237392, 0x00184c5825e18636},
}
func test_with_data(t *testing.T, chunker chunker.Chunker, chunks []chunk) {
for i, chunk := range chunks {
c, err := chunker.Next()
if i < len(chunks)-1 {
if err != nil {
t.Fatalf("Error returned with chunk %d: %v", i, err)
}
} else {
if err != io.EOF {
t.Fatalf("EOF not returned with chunk %d", i)
}
}
if c == nil {
t.Fatalf("Nil chunk returned")
}
if c != nil {
if c.Length != chunk.Length {
t.Fatalf("Length for chunk %d does not match: expected %d, got %d",
i, chunk.Length, c.Length)
}
if c.Cut != chunk.CutFP {
t.Fatalf("Cut fingerprint for chunk %d does not match: expected %016x, got %016x",
i, chunk.CutFP, c.Cut)
}
}
}
}
func get_random(seed, count int) []byte {
buf := make([]byte, count)
rnd := rand.New(rand.NewSource(23))
for i := 0; i < count; i += 4 {
r := rnd.Uint32()
buf[i] = byte(r)
buf[i+1] = byte(r >> 8)
buf[i+2] = byte(r >> 16)
buf[i+3] = byte(r >> 24)
}
return buf
}
func TestChunker(t *testing.T) {
// setup data source
buf := get_random(23, 32*1024*1024)
ch := chunker.New(bytes.NewReader(buf))
test_with_data(t, ch, chunks1)
}
func BenchmarkChunker(b *testing.B) {
size := 10 * 1024 * 1024
buf := get_random(23, size)
b.ResetTimer()
b.SetBytes(int64(size))
var chunks int
for i := 0; i < b.N; i++ {
chunks = 0
ch := chunker.New(bytes.NewReader(buf))
for {
_, err := ch.Next()
chunks++
if err != nil && err != io.EOF {
b.Fatalf("Unexpected error occurred: %v", err)
}
if err == io.EOF {
break
}
}
}
b.Logf("%d chunks, average chunk size: %d bytes", chunks, size/chunks)
}

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chunker/doc.go Normal file
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// Copyright 2014 Alexander Neumann. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
/*
Package chunker implements Content Defined Chunking (CDC) based on a rolling
Rabin Checksum.
Background Literature
An introduction to Rabin Fingerprints/Checksums can be found in the following articles:
Michael O. Rabin (1981): "Fingerprinting by Random Polynomials"
http://www.xmailserver.org/rabin.pdf
Ross N. Williams (1993): "A Painless Guide to CRC Error Detection Algorithms"
http://www.zlib.net/crc_v3.txt
Andrei Z. Broder (1993): "Some Applications of Rabin's Fingerprinting Method"
http://www.xmailserver.org/rabin_apps.pdf
Andrew Kadatch, Bob Jenkins (2007): "Everything we know about CRC but afraid to forget"
http://crcutil.googlecode.com/files/crc-doc.1.0.pdf
*/
package chunker

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cmd/splits/main.go Normal file
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package main
import (
"crypto/sha256"
"fmt"
"io"
"os"
"github.com/fd0/khepri/chunker"
)
func main() {
count, bytes := 0, 0
min := 0
max := 0
var (
err error
file *os.File = os.Stdin
)
if len(os.Args) > 1 {
file, err = os.Open(os.Args[1])
if err != nil {
panic(err)
}
}
ch := chunker.New(file)
for {
chunk, err := ch.Next()
if chunk != nil {
fmt.Printf("%d %016x %02x\n", chunk.Length, chunk.Cut, sha256.Sum256(chunk.Data))
count++
bytes += chunk.Length
if chunk.Length == chunker.MaxSize {
max++
} else if chunk.Length == chunker.MinSize {
min++
}
}
if err == io.EOF {
break
}
}
var avg int
if count > 0 {
avg = bytes / count
}
fmt.Fprintf(os.Stderr, "%d chunks, average size %d (%d min size, %d max size chunks)\n",
count, avg, min, max)
}