diff --git a/tsdb/chunks/head_chunks_other.go b/tsdb/chunks/head_chunks_other.go
index cd0e258a1a..3d00fddc37 100644
--- a/tsdb/chunks/head_chunks_other.go
+++ b/tsdb/chunks/head_chunks_other.go
@@ -17,5 +17,6 @@
 package chunks
 
 // 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.
-var HeadChunkFilePreallocationSize int64
+// Windows needs pre-allocations while the other OS does not. But we observed that a 0 pre-allocation causes unit tests to flake.
+// This small allocation for non-Windows OSes removes the flake.
+var HeadChunkFilePreallocationSize int64 = MinWriteBufferSize * 2