fs-v1-multipart: Remove backgroundAppend operation

The backgroundAppend was meant as a performance optimization where
uploaded chunks would be aggregated to a complete file in the background
while more parts were uploaded at the same time.

This might be an optimization on actual filesystem paths, but on Gluster
it made the operation a lot slower because many redundant filesystem
calls were executed. The CompleteMultipartUpload request which requires
the uploaded chunks to be put together has to

1. wait for any ongoing backgroundAppend operations to complete,
2. enumerate all chunks and check if they were put together in the right
   order (otherwise start from scratch),
3. move the aggregated file.

Removing this piece of code started out as an experiment, because I
expected the chunks to not be aggregated at all. It turned out that
there is a fallback which is also necessary in case the final object
should have a different order or not contain some of the uploaded parts.
This also ensures that a final aggregate is created. At least on
GlusterFS this makes any CMU request run almost twice as fast as when
using backgroundAppend.

(cherry picked from commit e61a333)

cmd/fs-v1-multipart.go

@@ -62,63 +62,6 @@ func (fs *FSObjects) decodePartFile(name string) (partNumber int, etag string, a
 	return partNumber, result[1], actualSize, nil
 }
 
-// Appends parts to an appendFile sequentially.
-func (fs *FSObjects) backgroundAppend(ctx context.Context, bucket, object, uploadID string) {
-	fs.appendFileMapMu.Lock()
-	logger.GetReqInfo(ctx).AppendTags("uploadID", uploadID)
-	file := fs.appendFileMap[uploadID]
-	if file == nil {
-		file = &fsAppendFile{
-			filePath: fmt.Sprintf("%s.%s", uploadID, mustGetUUID()),
-		}
-		fs.appendFileMap[uploadID] = file
-	}
-	fs.appendFileMapMu.Unlock()
-
-	file.Lock()
-	defer file.Unlock()
-
-	// Since we append sequentially nextPartNumber will always be len(file.parts)+1
-	nextPartNumber := len(file.parts) + 1
-	uploadIDPath := fs.getUploadIDDir(bucket, object, uploadID)
-	parts, err := fs.getRemainingUploadedChunks(ctx, bucket, object, uploadID, nextPartNumber)
-	if err != nil {
-		logger.LogIf(ctx, err)
-		return
-	}
-
-	for _, part := range parts {
-		partNumber, etag, actualSize := part.Number, part.ETag, part.ActualSize
-		if partNumber < nextPartNumber {
-			// Part already appended.
-			continue
-		}
-		if partNumber > nextPartNumber {
-			// Required part number is not yet uploaded.
-			return
-		}
-
-		partPath := pathJoin(uploadIDPath, fs.encodePartFile(partNumber, etag, actualSize))
-		entryBuf, err := fs.disk.ReadAll(ctx, minioMetaMultipartBucket, partPath)
-		if err != nil {
-			reqInfo := logger.GetReqInfo(ctx).AppendTags("partPath", partPath)
-			reqInfo.AppendTags("filepath", file.filePath)
-			logger.LogIf(ctx, err)
-			return
-		}
-		err = fs.disk.AppendFile(ctx, fs.disk.MetaTmpBucket(), file.filePath, entryBuf)
-		if err != nil {
-			reqInfo := logger.GetReqInfo(ctx).AppendTags("partPath", partPath)
-			reqInfo.AppendTags("filepath", file.filePath)
-			logger.LogIf(ctx, err)
-			return
-		}
-
-		file.parts = append(file.parts, partNumber)
-		nextPartNumber++
-	}
-}
-
 // Get uploaded chunks from multipart folder in metabucket
 func (fs *FSObjects) getRemainingUploadedChunks(ctx context.Context, bucket, object, uploadID string, nextPartNumber int) ([]ObjectPartInfo, error) {
 	// Setting count to -1 will read everything.
@@ -442,8 +385,6 @@ func (fs *FSObjects) PutObjectPart(rctx context.Context, bucket, object, uploadI
 		return pi, toObjectErr(err, minioMetaMultipartBucket, partPath)
 	}
 
-	go fs.backgroundAppend(wctx, bucket, object, uploadID)
-
 	return PartInfo{
 		PartNumber:   partID,
 		LastModified: UTCNow(),
@@ -701,7 +642,6 @@ func (fs *FSObjects) CompleteMultipartUpload(ctx context.Context, bucket string,
 	// 1. The last PutObjectPart triggers go-routine fs.backgroundAppend, this go-routine has not started yet.
 	// 2. Now CompleteMultipartUpload gets called which sees that lastPart is not appended and starts appending
 	//    from the beginning
-	fs.backgroundAppend(ctx, bucket, object, uploadID)
 
 	fs.appendFileMapMu.Lock()
 	file := fs.appendFileMap[uploadID]