mirror of https://github.com/restic/restic.git
Add code for content defined chunking (CDC)
This commit is contained in:
Alexander Neumann
parent
40aea3d612
commit
18131f88bf
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@ -0,0 +1,255 @@
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package chunker
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import (
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"io"
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"sync"
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)
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const (
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KiB = 1024
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MiB = 1024 * KiB
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// randomly generated irreducible polynomial of degree 53 in Z_2[X]
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Polynomial = 0x3DA3358B4DC173
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// use a sliding window of 64 byte.
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WindowSize = 64
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// aim to create chunks of 20 bits or about 1MiB on average.
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AverageBits = 20
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// Chunks should be in the range of 512KiB to 8MiB.
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MinSize = 512 * KiB
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MaxSize = 8 * MiB
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splitmask = (1 << AverageBits) - 1
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)
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var (
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pol_shift = deg(Polynomial) - 8
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once sync.Once
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mod_table [256]uint64
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out_table [256]uint64
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)
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// A chunk is one content-dependent chunk of bytes whose end was cut when the
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// Rabin Fingerprint had the value stored in Cut.
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type Chunk struct {
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Start int
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Length int
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Cut uint64
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Data []byte
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}
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// A chunker takes a stream of bytes and emits average size chunks.
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type Chunker interface {
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Next() (*Chunk, error)
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}
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// A chunker internally holds everything needed to split content.
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type chunker struct {
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rd io.Reader
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window []byte
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wpos int
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buf []byte
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bpos int
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bmax int
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data []byte
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start int
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count int
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pos int
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digest uint64
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}
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// New returns a new Chunker that reads from data from rd.
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func New(rd io.Reader) Chunker {
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c := &chunker{
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rd: rd,
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window: make([]byte, WindowSize),
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buf: make([]byte, MaxSize),
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data: make([]byte, 0, MaxSize),
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}
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once.Do(c.fill_tables)
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c.reset()
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return c
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}
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func (c *chunker) reset() {
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for i := 0; i < WindowSize; i++ {
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c.window[i] = 0
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}
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c.digest = 0
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c.wpos = 0
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c.pos = 0
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c.count = 0
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c.slide(1)
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c.data = make([]byte, 0, MaxSize)
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}
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// Calculate out_table and mod_table for optimization. Must be called only once.
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func (c *chunker) fill_tables() {
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// calculate table for sliding out bytes. The byte to slide out is used as
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// the index for the table, the value contains the following:
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// out_table[b] = Hash(b || 0 || ... || 0)
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// \ windowsize-1 zero bytes /
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// To slide out byte b_0 for window size w with known hash
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// H := H(b_0 || ... || b_w), it is sufficient to add out_table[b_0]:
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// H(b_0 || ... || b_w) + H(b_0 || 0 || ... || 0)
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// = H(b_0 + b_0 || b_1 + 0 || ... || b_w + 0)
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// = H( 0 || b_1 || ... || b_w)
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//
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// Afterwards a new byte can be shifted in.
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for b := 0; b < 256; b++ {
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var hash uint64
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hash = append_byte(hash, byte(b), Polynomial)
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for i := 0; i < WindowSize-1; i++ {
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hash = append_byte(hash, 0, Polynomial)
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}
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out_table[b] = hash
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}
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// calculate table for reduction mod Polynomial
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k := deg(Polynomial)
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for b := 0; b < 256; b++ {
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// mod_table[b] = A | B, where A = (b(x) * x^k mod pol) and B = b(x) * x^k
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//
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// The 8 bits above deg(Polynomial) determine what happens next and so
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// these bits are used as a lookup to this table. The value is split in
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// two parts: Part A contains the result of the modulus operation, part
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// B is used to cancel out the 8 top bits so that one XOR operation is
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// enough to reduce modulo Polynomial
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mod_table[b] = mod(uint64(b)<<uint(k), Polynomial) | (uint64(b) << uint(k))
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}
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}
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func (c *chunker) Next() (*Chunk, error) {
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for {
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if c.bpos >= c.bmax {
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n, err := io.ReadFull(c.rd, c.buf)
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if err == io.ErrUnexpectedEOF {
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err = nil
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}
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if err != nil {
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return &Chunk{
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Start: c.start,
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Length: c.count,
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Cut: c.digest,
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Data: c.data,
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}, err
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}
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c.bpos = 0
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c.bmax = n
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}
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for i, b := range c.buf[c.bpos:c.bmax] {
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// inline c.slide(b) and append(b) to increase performance
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out := c.window[c.wpos]
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c.window[c.wpos] = b
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c.digest ^= out_table[out]
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c.wpos = (c.wpos + 1) % WindowSize
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// c.append(b)
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index := c.digest >> uint(pol_shift)
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c.digest <<= 8
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c.digest |= uint64(b)
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c.digest ^= mod_table[index]
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if (c.count+i+1 >= MinSize && (c.digest&splitmask) == 0) || c.count+i+1 >= MaxSize {
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c.data = append(c.data, c.buf[c.bpos:c.bpos+i]...)
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c.count += i + 1
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c.pos += i + 1
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c.bpos += i + 1
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chunk := &Chunk{
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Start: c.start,
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Length: c.count,
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Cut: c.digest,
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Data: c.data,
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}
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// keep position
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pos := c.pos
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c.reset()
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c.pos = pos
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c.start = pos
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return chunk, nil
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}
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}
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steps := c.bmax - c.bpos
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if steps > 0 {
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c.data = append(c.data, c.buf[c.bpos:c.bpos+steps]...)
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}
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c.count += steps
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c.pos += steps
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c.bpos = c.bmax
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}
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return nil, nil
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}
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func (c *chunker) append(b byte) {
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index := c.digest >> uint(pol_shift)
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c.digest <<= 8
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c.digest |= uint64(b)
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c.digest ^= mod_table[index]
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}
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func (c *chunker) slide(b byte) {
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out := c.window[c.wpos]
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c.window[c.wpos] = b
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c.digest ^= out_table[out]
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c.wpos = (c.wpos + 1) % WindowSize
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c.append(b)
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}
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func append_byte(hash uint64, b byte, pol uint64) uint64 {
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hash <<= 8
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hash |= uint64(b)
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return mod(hash, pol)
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}
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// Mod calculates the remainder of x divided by p.
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func mod(x, p uint64) uint64 {
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for deg(x) >= deg(p) {
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shift := uint(deg(x) - deg(p))
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x = x ^ (p << shift)
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}
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return x
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}
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// Deg returns the degree of the polynomial p, this is equivalent to the number
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// of the highest bit set in p.
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func deg(p uint64) int {
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var mask uint64 = 0x8000000000000000
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for i := 0; i < 64; i++ {
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if mask&p > 0 {
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return 63 - i
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}
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mask >>= 1
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}
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return -1
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}
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@ -0,0 +1,133 @@
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package chunker_test
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import (
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"bytes"
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"io"
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"math/rand"
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"testing"
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"github.com/fd0/khepri/chunker"
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)
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type chunk struct {
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Length int
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CutFP uint64
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}
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// created for 32MB of random data out of math/rand's Uint32() seeded by
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// constant 23
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//
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// chunking configuration:
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// window size 64, avg chunksize 1<<20, min chunksize 1<<19, max chunksize 1<<23
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// polynom 0x3DA3358B4DC173
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var chunks1 = []chunk{
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chunk{2163460, 0x000b98d4cdf00000},
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chunk{643703, 0x000d4e8364d00000},
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chunk{1528956, 0x0015a25c2ef00000},
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chunk{1955808, 0x00102a8242e00000},
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chunk{2222372, 0x00045da878000000},
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chunk{2538687, 0x00198a8179900000},
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chunk{609606, 0x001d4e8d17100000},
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chunk{1205738, 0x000a7204dd600000},
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chunk{959742, 0x00183e71e1400000},
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chunk{4036109, 0x001fec043c700000},
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chunk{1525894, 0x000b1574b1500000},
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chunk{1352720, 0x00018965f2e00000},
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chunk{811884, 0x00155628aa100000},
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chunk{1282314, 0x001909a0a1400000},
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chunk{1318021, 0x001cceb980000000},
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chunk{948640, 0x0011f7a470a00000},
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chunk{645464, 0x00030ce2d9400000},
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chunk{533758, 0x0004435c53c00000},
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chunk{1128303, 0x0000c48517800000},
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chunk{800374, 0x000968473f900000},
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chunk{2453512, 0x001e197c92600000},
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chunk{2651975, 0x000ae6c868000000},
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chunk{237392, 0x00184c5825e18636},
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}
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func test_with_data(t *testing.T, chunker chunker.Chunker, chunks []chunk) {
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for i, chunk := range chunks {
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c, err := chunker.Next()
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if i < len(chunks)-1 {
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if err != nil {
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t.Fatalf("Error returned with chunk %d: %v", i, err)
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}
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} else {
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if err != io.EOF {
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t.Fatalf("EOF not returned with chunk %d", i)
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}
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}
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if c == nil {
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t.Fatalf("Nil chunk returned")
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}
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if c != nil {
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if c.Length != chunk.Length {
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t.Fatalf("Length for chunk %d does not match: expected %d, got %d",
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i, chunk.Length, c.Length)
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}
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if c.Cut != chunk.CutFP {
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t.Fatalf("Cut fingerprint for chunk %d does not match: expected %016x, got %016x",
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i, chunk.CutFP, c.Cut)
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}
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}
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}
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}
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func get_random(seed, count int) []byte {
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buf := make([]byte, count)
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rnd := rand.New(rand.NewSource(23))
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for i := 0; i < count; i += 4 {
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r := rnd.Uint32()
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buf[i] = byte(r)
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buf[i+1] = byte(r >> 8)
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buf[i+2] = byte(r >> 16)
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buf[i+3] = byte(r >> 24)
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}
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return buf
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}
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func TestChunker(t *testing.T) {
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// setup data source
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buf := get_random(23, 32*1024*1024)
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ch := chunker.New(bytes.NewReader(buf))
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test_with_data(t, ch, chunks1)
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}
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func BenchmarkChunker(b *testing.B) {
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size := 10 * 1024 * 1024
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buf := get_random(23, size)
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b.ResetTimer()
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b.SetBytes(int64(size))
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var chunks int
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for i := 0; i < b.N; i++ {
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chunks = 0
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ch := chunker.New(bytes.NewReader(buf))
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for {
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_, err := ch.Next()
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chunks++
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if err != nil && err != io.EOF {
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b.Fatalf("Unexpected error occurred: %v", err)
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}
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if err == io.EOF {
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break
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}
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}
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}
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b.Logf("%d chunks, average chunk size: %d bytes", chunks, size/chunks)
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}
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@ -0,0 +1,26 @@
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// Copyright 2014 Alexander Neumann. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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/*
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Package chunker implements Content Defined Chunking (CDC) based on a rolling
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Rabin Checksum.
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Background Literature
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An introduction to Rabin Fingerprints/Checksums can be found in the following articles:
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Michael O. Rabin (1981): "Fingerprinting by Random Polynomials"
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http://www.xmailserver.org/rabin.pdf
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Ross N. Williams (1993): "A Painless Guide to CRC Error Detection Algorithms"
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http://www.zlib.net/crc_v3.txt
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Andrei Z. Broder (1993): "Some Applications of Rabin's Fingerprinting Method"
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http://www.xmailserver.org/rabin_apps.pdf
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Andrew Kadatch, Bob Jenkins (2007): "Everything we know about CRC but afraid to forget"
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http://crcutil.googlecode.com/files/crc-doc.1.0.pdf
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*/
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package chunker
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@ -0,0 +1,59 @@
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package main
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import (
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"crypto/sha256"
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"fmt"
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"io"
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"os"
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"github.com/fd0/khepri/chunker"
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)
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func main() {
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count, bytes := 0, 0
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min := 0
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max := 0
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var (
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err error
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file *os.File = os.Stdin
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)
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if len(os.Args) > 1 {
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file, err = os.Open(os.Args[1])
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if err != nil {
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panic(err)
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}
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}
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ch := chunker.New(file)
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for {
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chunk, err := ch.Next()
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if chunk != nil {
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fmt.Printf("%d %016x %02x\n", chunk.Length, chunk.Cut, sha256.Sum256(chunk.Data))
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count++
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bytes += chunk.Length
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if chunk.Length == chunker.MaxSize {
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max++
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} else if chunk.Length == chunker.MinSize {
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min++
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}
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}
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if err == io.EOF {
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break
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}
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}
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var avg int
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if count > 0 {
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avg = bytes / count
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}
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fmt.Fprintf(os.Stderr, "%d chunks, average size %d (%d min size, %d max size chunks)\n",
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count, avg, min, max)
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}
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