Preallocate m-map file only for Windows (#7306)

Signed-off-by: Ganesh Vernekar <cs15btech11018@iith.ac.in>

tsdb/chunks/head_chunks.go

@@ -188,7 +188,7 @@ func (cdm *ChunkDiskMapper) openMMapFiles() (returnErr error) {
 	for seq, fn := range files {
 		f, err := fileutil.OpenMmapFile(fn)
 		if err != nil {
-			return errors.Wrap(err, "mmap files")
+			return errors.Wrapf(err, "mmap files, file: %s", fn)
 		}
 		cdm.closers[seq] = f
 		cdm.mmappedChunkFiles[seq] = &mmappedChunkFile{byteSlice: realByteSlice(f.Bytes())}
@@ -337,7 +337,7 @@ func (cdm *ChunkDiskMapper) cut(mint int64) (returnErr error) {
 		return err
 	}
 
-	n, newFile, seq, err := cutSegmentFile(cdm.dir, MagicHeadChunks, headChunksFormatV1, int64(MaxHeadChunkFileSize))
+	n, newFile, seq, err := cutSegmentFile(cdm.dir, MagicHeadChunks, headChunksFormatV1, HeadChunkFilePreallocationSize)
 	if err != nil {
 		return err
 	}

tsdb/chunks/head_chunks_other.go

@@ -0,0 +1,22 @@
+// Copyright 2020 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// +build !windows
+
+package chunks
+
+var (
+	// HeadChunkFilePreallocationSize is the size to which the m-map file should be preallocated when a new file is cut.
+	// Windows needs pre-allocations while the other OS does not.
+	HeadChunkFilePreallocationSize int64
+)

tsdb/chunks/head_chunks_windows.go

@@ -0,0 +1,20 @@
+// Copyright 2020 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package chunks
+
+var (
+	// HeadChunkFilePreallocationSize is the size to which the m-map file should be preallocated when a new file is cut.
+	// Windows needs pre-allocation to m-map the file.
+	HeadChunkFilePreallocationSize int64 = MaxHeadChunkFileSize
+)