diff --git a/io/blob_store.cc b/io/blob_store.cc
index 9e1bda1..6b33186 100644
--- a/io/blob_store.cc
+++ b/io/blob_store.cc
@@ -411,17 +411,19 @@ void BlobWriter::WriteAll(hwy::ThreadPool& pool, const Path& filename) {
   std::unique_ptr<File> file = OpenFileOrNull(filename, "w+");
   if (!file) HWY_ABORT("Failed to open for writing %s", filename.path.c_str());
 
-  pool.Run(0, writes.size(),
-           [this, &file, &writes](uint64_t i, size_t /*thread*/) {
-             const BlobRange& range = writes[i].range;
+  hwy::ThreadPool null_pool(0);
+  hwy::ThreadPool& pool_or_serial = file->IsAppendOnly() ? null_pool : pool;
+  pool_or_serial.Run(
+      0, writes.size(), [this, &file, &writes](uint64_t i, size_t /*thread*/) {
+        const BlobRange& range = writes[i].range;
 
-             if (!file->Write(writes[i].data, range.bytes, range.offset)) {
-               const std::string& key = StringFromKey(keys_[range.key_idx]);
-               HWY_ABORT("Write failed for %s from %zu, %zu bytes to %p.",
-                         key.c_str(), static_cast<size_t>(range.offset),
-                         range.bytes, writes[i].data);
-             }
-           });
+        if (!file->Write(writes[i].data, range.bytes, range.offset)) {
+          const std::string& key = StringFromKey(keys_[range.key_idx]);
+          HWY_ABORT("Write failed for %s from %zu, %zu bytes to %p.",
+                    key.c_str(), static_cast<size_t>(range.offset), range.bytes,
+                    writes[i].data);
+        }
+      });
 }
 
 }  // namespace gcpp
diff --git a/io/io.cc b/io/io.cc
index 1f7eda6..df39f7b 100644
--- a/io/io.cc
+++ b/io/io.cc
@@ -88,6 +88,9 @@ class FilePosix : public File {
     }
   }
 
+  // pwrite is thread-safe and allows arbitrary offsets.
+  bool IsAppendOnly() const override { return false; }
+
   uint64_t FileSize() const override {
     static_assert(sizeof(off_t) == 8, "64-bit off_t required");
     const off_t size = lseek(fd_, 0, SEEK_END);
diff --git a/io/io.h b/io/io.h
index a14889b..d9481bc 100644
--- a/io/io.h
+++ b/io/io.h
@@ -46,6 +46,10 @@ class File {
   File(const File& other) = delete;
   const File& operator=(const File& other) = delete;
 
+  // If true, Write() should only be called with `offset` equal to the number
+  // of bytes already written to the file, which rules out parallel writes.
+  virtual bool IsAppendOnly() const = 0;
+
   // Returns size in bytes or 0.
   virtual uint64_t FileSize() const = 0;
 
diff --git a/io/io_win.cc b/io/io_win.cc
index 1f35a96..34773d3 100644
--- a/io/io_win.cc
+++ b/io/io_win.cc
@@ -47,6 +47,9 @@ class FileWin : public File {
     }
   }
 
+  // WriteFile is thread-safe and allows arbitrary offsets.
+  bool IsAppendOnly() const override { return false; }
+
   uint64_t FileSize() const override {
     DWORD hi;
     const DWORD lo = GetFileSize(hFile_, &hi);