XL: Implement strided erasure distribution. (#1772)

Strided erasure distribution uses a new randomized
block distribution for each Put operation. This
information is captured inside `xl.json` for subsequent
Get operations.

erasure-createfile.go

@@ -32,8 +32,7 @@ func (e erasure) cleanupCreateFileOps(volume, path string, writers []io.WriteClo
 	}
 }
 
-// WriteErasure reads predefined blocks, encodes them and writes to
-// configured storage disks.
+// WriteErasure reads predefined blocks, encodes them and writes to configured storage disks.
 func (e erasure) writeErasure(volume, path string, reader *io.PipeReader, wcloser *waitCloser) {
 	// Release the block writer upon function return.
 	defer wcloser.release()
@@ -119,7 +118,8 @@ func (e erasure) writeErasure(volume, path string, reader *io.PipeReader, wclose
 				// Write encoded data in routine.
 				go func(index int, writer io.Writer) {
 					defer wg.Done()
-					encodedData := dataBlocks[index]
+					// Pick the block from the distribution.
+					encodedData := dataBlocks[e.distribution[index]-1]
 					_, wErr := writers[index].Write(encodedData)
 					if wErr != nil {
 						wErrs[index] = wErr

erasure-readfile.go

@@ -82,13 +82,14 @@ func (e erasure) ReadFile(volume, path string, startOffset int64, totalSize int6
 			enBlocks := make([][]byte, len(e.storageDisks))
 			// Read all the readers.
 			for index, reader := range readers {
+				blockIndex := e.distribution[index] - 1
 				// Initialize shard slice and fill the data from each parts.
-				enBlocks[index] = make([]byte, curEncBlockSize)
+				enBlocks[blockIndex] = make([]byte, curEncBlockSize)
 				if reader == nil {
 					continue
 				}
 				// Read the necessary blocks.
-				_, rErr := io.ReadFull(reader, enBlocks[index])
+				_, rErr := io.ReadFull(reader, enBlocks[blockIndex])
 				if rErr != nil && rErr != io.ErrUnexpectedEOF {
 					readers[index].Close()
 					readers[index] = nil

erasure.go

@@ -24,10 +24,11 @@ type erasure struct {
 	DataBlocks   int
 	ParityBlocks int
 	storageDisks []StorageAPI
+	distribution []int
 }
 
 // newErasure instantiate a new erasure.
-func newErasure(disks []StorageAPI) *erasure {
+func newErasure(disks []StorageAPI, distribution []int) *erasure {
 	// Initialize E.
 	e := &erasure{}
 
@@ -46,6 +47,9 @@ func newErasure(disks []StorageAPI) *erasure {
 	// Save all the initialized storage disks.
 	e.storageDisks = disks
 
+	// Save the distribution.
+	e.distribution = distribution
+
 	// Return successfully initialized.
 	return e
 }

tree-walk-xl.go

@@ -82,7 +82,7 @@ func (xl xlObjects) listDir(bucket, prefixDir string, filter func(entry string)
 // getRandomDisk - gives a random disk at any point in time from the
 // available pool of disks.
 func (xl xlObjects) getRandomDisk() (disk StorageAPI) {
-	rand.Seed(time.Now().UTC().UnixNano())
+	rand.Seed(time.Now().UTC().UnixNano()) // Seed with current time.
 	randIndex := rand.Intn(len(xl.storageDisks) - 1)
 	disk = xl.storageDisks[randIndex] // Pick a random disk.
 	return disk

xl-v1-metadata.go

@@ -20,6 +20,7 @@ import (
 	"bytes"
 	"encoding/json"
 	"io"
+	"math/rand"
 	"path"
 	"sort"
 	"sync"
@@ -254,16 +255,15 @@ func (xl xlObjects) writeXLMetadata(bucket, prefix string, xlMeta xlMetaV1) erro
 
 // randErasureDistribution - uses Knuth Fisher-Yates shuffle algorithm.
 func randErasureDistribution(numBlocks int) []int {
+	rand.Seed(time.Now().UTC().UnixNano()) // Seed with current time.
 	distribution := make([]int, numBlocks)
 	for i := 0; i < numBlocks; i++ {
 		distribution[i] = i + 1
 	}
-	/*
-		for i := 0; i < numBlocks; i++ {
-			// Choose index uniformly in [i, numBlocks-1]
-			r := i + rand.Intn(numBlocks-i)
-			distribution[r], distribution[i] = distribution[i], distribution[r]
-		}
-	*/
+	for i := 0; i < numBlocks; i++ {
+		// Choose index uniformly in [i, numBlocks-1]
+		r := i + rand.Intn(numBlocks-i)
+		distribution[r], distribution[i] = distribution[i], distribution[r]
+	}
 	return distribution
 }

xl-v1-multipart.go

@@ -136,11 +136,14 @@ func (xl xlObjects) putObjectPartCommon(bucket string, object string, uploadID s
 	if err != nil {
 		return "", toObjectErr(err, bucket, object)
 	}
+
 	// Increment version only if we have online disks less than configured storage disks.
 	if diskCount(onlineDisks) < len(xl.storageDisks) {
 		higherVersion++
 	}
-	erasure := newErasure(onlineDisks) // Initialize a new erasure with online disks
+
+	// Initialize a new erasure with online disks and new distribution.
+	erasure := newErasure(onlineDisks, xlMeta.Erasure.Distribution)
 
 	partSuffix := fmt.Sprintf("object%d", partID)
 	tmpPartPath := path.Join(tmpMetaPrefix, bucket, object, uploadID, partSuffix)

xl-v1-object.go

@@ -41,16 +41,20 @@ func (xl xlObjects) GetObject(bucket, object string, startOffset int64) (io.Read
 	if err != nil {
 		return nil, toObjectErr(err, bucket, object)
 	}
+
 	// List all online disks.
 	onlineDisks, _, err := xl.listOnlineDisks(bucket, object)
 	if err != nil {
 		return nil, toObjectErr(err, bucket, object)
 	}
+
 	// For zero byte files, return a null reader.
 	if xlMeta.Stat.Size == 0 {
 		return nullReadCloser{}, nil
 	}
-	erasure := newErasure(onlineDisks) // Initialize a new erasure with online disks
+
+	// Initialize a new erasure with online disks, with previous block distribution.
+	erasure := newErasure(onlineDisks, xlMeta.Erasure.Distribution)
 
 	// Get part index offset.
 	partIndex, partOffset, err := xlMeta.objectToPartOffset(startOffset)
@@ -208,16 +212,22 @@ func (xl xlObjects) PutObject(bucket string, object string, size int64, data io.
 	tempErasureObj := path.Join(tmpMetaPrefix, bucket, object, "object1")
 	tempObj := path.Join(tmpMetaPrefix, bucket, object)
 
+	// Initialize xl meta.
+	xlMeta := newXLMetaV1(xl.dataBlocks, xl.parityBlocks)
+
 	// List all online disks.
 	onlineDisks, higherVersion, err := xl.listOnlineDisks(bucket, object)
 	if err != nil {
 		return "", toObjectErr(err, bucket, object)
 	}
+
 	// Increment version only if we have online disks less than configured storage disks.
 	if diskCount(onlineDisks) < len(xl.storageDisks) {
 		higherVersion++
 	}
-	erasure := newErasure(onlineDisks) // Initialize a new erasure with online disks
+
+	// Initialize a new erasure with online disks and new distribution.
+	erasure := newErasure(onlineDisks, xlMeta.Erasure.Distribution)
 	fileWriter, err := erasure.CreateFile(minioMetaBucket, tempErasureObj)
 	if err != nil {
 		return "", toObjectErr(err, bucket, object)
@@ -301,7 +311,7 @@ func (xl xlObjects) PutObject(bucket string, object string, size int64, data io.
 		return "", toObjectErr(err, bucket, object)
 	}
 
-	xlMeta := newXLMetaV1(xl.dataBlocks, xl.parityBlocks)
+	// Fill all the necessary metadata.
 	xlMeta.Meta = metadata
 	xlMeta.Stat.Size = size
 	xlMeta.Stat.ModTime = modTime