XL/erasure-read: Add Comments and enable bitrot detection.

erasure-readfile.go

@@ -17,7 +17,6 @@
 package main
 
 import (
-	"bytes"
 	"encoding/hex"
 	"errors"
 	"io"
@@ -28,51 +27,72 @@ import (
 
 // erasureReadFile - read bytes from erasure coded files and writes to given writer.
 // Erasure coded files are read block by block as per given erasureInfo and data chunks
-// are decoded into a data block.  Data block is trimmed for given offset and length,
-// then written to given writer.  This function also supports bit-rot detection by
+// are decoded into a data block. Data block is trimmed for given offset and length,
+// then written to given writer. This function also supports bit-rot detection by
 // verifying checksum of individual block's checksum.
 func erasureReadFile(writer io.Writer, disks []StorageAPI, volume string, path string, partName string, eInfos []erasureInfo, offset int64, length int64, totalLength int64) (int64, error) {
-	min := func(a int64, b int64) int {
-		if a < b {
-			return int(a)
-		}
-		return int(b)
-	}
-	// Total bytes written to writer
-	bytesWritten := int64(0)
-
-	// Gather previously calculated block checksums.
-	// blockCheckSums := metaPartBlockChecksums(disks, eInfos, partName)
-
 	// Pick one erasure info.
 	eInfo := pickValidErasureInfo(eInfos)
 
-	// Data chunk size on each block.
-	chunkSize := getEncodedBlockLen(eInfo.BlockSize, eInfo.DataBlocks)
-
-	// Get block info for given offset, length and block size.
-	startBlock, bytesToSkip := getBlockInfo(offset, eInfo.BlockSize)
+	// Gather previously calculated block checksums.
+	blockCheckSums := metaPartBlockChecksums(disks, eInfos, partName)
+	orderedBlockCheckSums := make([]checkSumInfo, len(disks))
 
+	// []orderedDisks will have first eInfo.DataBlocks disks as data disks and rest will be parity.
 	orderedDisks := make([]StorageAPI, len(disks))
 	for index := range disks {
 		blockIndex := eInfo.Distribution[index]
 		orderedDisks[blockIndex-1] = disks[index]
+		orderedBlockCheckSums[blockIndex-1] = blockCheckSums[index]
 	}
 
+	// bitrotVerify verifies if the file on a particular disk does not have bitrot by verifying the hash of
+	// the contents of the file.
+	bitrotVerify := func() func(diskIndex int) bool {
+		verified := make([]bool, len(orderedDisks))
+		// Return closure so that we have reference to []verified and not recalculate the hash on it
+		// everytime the function is called for the same disk.
+		return func(diskIndex int) bool {
+			if verified[diskIndex] {
+				return true
+			}
+			isValid := isValidBlock(orderedDisks[diskIndex], volume, path, orderedBlockCheckSums[diskIndex])
+			verified[diskIndex] = isValid
+			return isValid
+		}
+	}()
+
+	// Total bytes written to writer
+	bytesWritten := int64(0)
+
+	// chunkSize is roughly BlockSize/DataBlocks.
+	// chunkSize is calculated such that chunkSize*DataBlocks accommodates BlockSize bytes.
+	// So chunkSize*DataBlocks can be slightly larger than BlockSize if BlockSize is not divisible by
+	// DataBlocks. The extra space will have 0-padding.
+	chunkSize := getEncodedBlockLen(eInfo.BlockSize, eInfo.DataBlocks)
+
+	startBlock, endBlock, bytesToSkip := getBlockInfo(offset, totalLength, eInfo.BlockSize)
+
+	// For each block, read chunk from each disk. If we are able to read all the data disks then we don't
+	// need to read parity disks. If one of the data disk is missing we need to read DataBlocks+1 number
+	// of disks. Once read, we Reconstruct() missing data if needed and write it to the given writer.
 	for block := startBlock; bytesWritten < length; block++ {
+		// curChunkSize will be chunkSize except for the last block because the size of the last block
+		// can be less than BlockSize.
 		curChunkSize := chunkSize
-		if totalLength-offset+bytesWritten < curChunkSize {
-			curChunkSize = getEncodedBlockLen(totalLength-offset+bytesWritten, eInfo.DataBlocks)
+		if block == endBlock && (totalLength%eInfo.BlockSize != 0) {
+			// If this is the last block and size of the block is < BlockSize.
+			curChunkSize = getEncodedBlockLen(totalLength%eInfo.BlockSize, eInfo.DataBlocks)
 		}
 
-		// Allocate encoded blocks up to storage disks.
+		// Each element of enBlocks holds curChunkSize'd amount of data read from its corresponding disk.
 		enBlocks := make([][]byte, len(disks))
 
 		// Figure out the number of disks that are needed for the read.
-		// If all the data disks are available then dataDiskCount = eInfo.DataBlocks
-		// Else dataDiskCount = eInfo.DataBlocks + 1
-
+		// We will need DataBlocks number of disks if all the data disks are up.
+		// We will need DataBlocks+1 number of disks even if one of the data disks is down.
 		diskCount := 0
+		// Count the number of data disks that are up.
 		for _, disk := range orderedDisks[:eInfo.DataBlocks] {
 			if disk == nil {
 				continue
@@ -81,11 +101,15 @@ func erasureReadFile(writer io.Writer, disks []StorageAPI, volume string, path s
 		}
 
 		if diskCount < eInfo.DataBlocks {
+			// Not enough data disks up, so we need DataBlocks+1 number of disks for reed-solomon Reconstruct()
 			diskCount = eInfo.DataBlocks + 1
 		}
 
 		wg := &sync.WaitGroup{}
+
+		// current disk index from which to read, this will be used later in case one of the parallel reads fails.
 		index := 0
+		// Read from the disks in parallel.
 		for _, disk := range orderedDisks {
 			if disk == nil {
 				index++
@@ -94,9 +118,19 @@ func erasureReadFile(writer io.Writer, disks []StorageAPI, volume string, path s
 			wg.Add(1)
 			go func(index int, disk StorageAPI) {
 				defer wg.Done()
+				ok := bitrotVerify(index)
+				if !ok {
+					// So that we don't read from this disk for the next block.
+					orderedDisks[index] = nil
+					return
+				}
 				buf := make([]byte, curChunkSize)
-				n, err := disk.ReadFile(volume, path, block*curChunkSize, buf)
+				// Note that for the offset calculation we have to use chunkSize and not
+				// curChunkSize. If we use curChunkSize for offset calculation then it
+				// can result in wrong offset for the last block.
+				n, err := disk.ReadFile(volume, path, block*chunkSize, buf)
 				if err != nil {
+					// So that we don't read from this disk for the next block.
 					orderedDisks[index] = nil
 					return
 				}
@@ -110,7 +144,7 @@ func erasureReadFile(writer io.Writer, disks []StorageAPI, volume string, path s
 		}
 		wg.Wait()
 
-		// Counter to keep success data blocks.
+		// Count number of data and parity blocks that were read.
 		var successDataBlocksCount = 0
 		var successParityBlocksCount = 0
 		for bufidx, buf := range enBlocks {
@@ -125,50 +159,63 @@ func erasureReadFile(writer io.Writer, disks []StorageAPI, volume string, path s
 		}
 
 		if successDataBlocksCount < eInfo.DataBlocks {
+			// If we don't have DataBlocks number of data blocks we will have to read enough
+			// parity blocks such that we have DataBlocks+1 number for blocks for reedsolomon.Reconstruct()
 			for ; index < len(orderedDisks); index++ {
 				if (successDataBlocksCount + successParityBlocksCount) == (eInfo.DataBlocks + 1) {
+					// We have DataBlocks+1 blocks, enough for reedsolomon.Reconstruct()
 					break
 				}
+				ok := bitrotVerify(index)
+				if !ok {
+					// Mark nil so that we don't read from this disk for the next block.
+					orderedDisks[index] = nil
+					continue
+				}
 				buf := make([]byte, curChunkSize)
-				n, err := orderedDisks[index].ReadFile(volume, path, block*curChunkSize, buf)
+				n, err := orderedDisks[index].ReadFile(volume, path, block*chunkSize, buf)
 				if err != nil {
+					// Mark nil so that we don't read from this disk for the next block.
 					orderedDisks[index] = nil
 					continue
 				}
 				successParityBlocksCount++
 				enBlocks[index] = buf[:n]
 			}
+			// Reconstruct the missing data blocks.
 			err := decodeData(enBlocks, eInfo.DataBlocks, eInfo.ParityBlocks)
 			if err != nil {
 				return bytesWritten, err
 			}
 		}
 
-		// Get data blocks from encoded blocks.
-		dataBlocks, err := getDataBlocks(enBlocks, eInfo.DataBlocks, min(eInfo.BlockSize, totalLength-offset+bytesWritten))
+		// enBlocks data can have 0-padding hence we need to figure the exact number
+		// of bytes we want to read from enBlocks.
+		blockSize := eInfo.BlockSize
+		if block == endBlock && totalLength%eInfo.BlockSize != 0 {
+			// For the last block, the block size can be less than BlockSize.
+			blockSize = totalLength % eInfo.BlockSize
+		}
+		data, err := getDataBlocks(enBlocks, eInfo.DataBlocks, int(blockSize))
 		if err != nil {
 			return bytesWritten, err
 		}
 
-		// Keep required bytes into buf.
-		buf := dataBlocks
-
 		// If this is start block, skip unwanted bytes.
 		if block == startBlock {
-			buf = buf[bytesToSkip:]
+			data = data[bytesToSkip:]
 		}
 
-		if len(buf) > int(length-bytesWritten) {
-			buf = buf[:length-bytesWritten]
+		if len(data) > int(length-bytesWritten) {
+			// We should not send more data than what was requested.
+			data = data[:length-bytesWritten]
 		}
 
-		// Copy data blocks.
-		var n int64
-		n, err = io.Copy(writer, bytes.NewReader(buf))
-		bytesWritten += int64(n)
+		_, err = writer.Write(data)
 		if err != nil {
 			return bytesWritten, err
 		}
+		bytesWritten += int64(len(data))
 	}
 
 	return bytesWritten, nil
@@ -210,23 +257,18 @@ func toDiskIndex(blockIdx int, distribution []int) int {
 
 // isValidBlock - calculates the checksum hash for the block and
 // validates if its correct returns true for valid cases, false otherwise.
-func isValidBlock(disks []StorageAPI, volume, path string, diskIndex int, blockCheckSums []checkSumInfo) (ok bool) {
+func isValidBlock(disk StorageAPI, volume, path string, blockCheckSum checkSumInfo) (ok bool) {
 	ok = false
-	// Unknown block index requested, treat it as error.
-	if diskIndex == -1 {
-		return ok
-	}
-	// Disk is not present, treat entire block to be non existent.
-	if disks[diskIndex] == nil {
-		return ok
+	if disk == nil {
+		return false
 	}
 	// Read everything for a given block and calculate hash.
-	hashWriter := newHash(blockCheckSums[diskIndex].Algorithm)
-	hashBytes, err := hashSum(disks[diskIndex], volume, path, hashWriter)
+	hashWriter := newHash(blockCheckSum.Algorithm)
+	hashBytes, err := hashSum(disk, volume, path, hashWriter)
 	if err != nil {
 		return ok
 	}
-	ok = hex.EncodeToString(hashBytes) == blockCheckSums[diskIndex].Hash
+	ok = hex.EncodeToString(hashBytes) == blockCheckSum.Hash
 	return ok
 }
 

erasure-utils.go

@@ -89,10 +89,11 @@ func getDataBlocks(enBlocks [][]byte, dataBlocks int, curBlockSize int) (data []
 }
 
 // getBlockInfo - find start/end block and bytes to skip for given offset, length and block size.
-func getBlockInfo(offset, blockSize int64) (startBlock, bytesToSkip int64) {
+func getBlockInfo(offset, length, blockSize int64) (startBlock, endBlock, bytesToSkip int64) {
 	// Calculate start block for given offset and how many bytes to skip to get the offset.
 	startBlock = offset / blockSize
 	bytesToSkip = offset % blockSize
+	endBlock = length / blockSize
 	return
 }
 

xl-v1-object.go

@@ -97,7 +97,9 @@ func (xl xlObjects) GetObject(bucket, object string, startOffset int64, length i
 		// Save the current part name and size.
 		partName := xlMeta.Parts[partIndex].Name
 		partSize := xlMeta.Parts[partIndex].Size
+
 		readSize := partSize - partOffset
+		// readSize should be adjusted so that we don't write more data than what was requested.
 		if readSize > (length - totalBytesRead) {
 			readSize = length - totalBytesRead
 		}
@@ -110,7 +112,8 @@ func (xl xlObjects) GetObject(bucket, object string, startOffset int64, length i
 
 		totalBytesRead += n
 
-		// Reset part offset to 0 to read rest of the part from the beginning.
+		// partOffset will be valid only for the first part, hence reset it to 0 for
+		// the remaining parts.
 		partOffset = 0
 	} // End of read all parts loop.