diff --git a/store/src/pmmr.rs b/store/src/pmmr.rs
index b4f569464d..ded9679b19 100644
--- a/store/src/pmmr.rs
+++ b/store/src/pmmr.rs
@@ -21,7 +21,7 @@ use crate::core::core::BlockHeader;
 use crate::core::ser::{FixedLength, PMMRable};
 use crate::leaf_set::LeafSet;
 use crate::prune_list::PruneList;
-use crate::types::DataFile;
+use crate::types::{AppendOnlyFile, DataFile, SizeEntry, SizeInfo};
 use croaring::Bitmap;
 use std::path::{Path, PathBuf};
 
@@ -201,21 +201,22 @@ impl<T: PMMRable> PMMRBackend<T> {
 	) -> io::Result<PMMRBackend<T>> {
 		let data_dir = data_dir.as_ref();
 
-		// We either have a fixed size *or* a path to a file for tracking sizes.
-		let (elmt_size, size_path) = if fixed_size {
-			(Some(T::E::LEN as u16), None)
+		// Are we dealing with "fixed size" data elements or "variable size" data elements
+		// maintained in an associated size file?
+		let size_info = if fixed_size {
+			SizeInfo::FixedSize(T::E::LEN as u16)
 		} else {
-			(None, Some(data_dir.join(PMMR_SIZE_FILE)))
+			SizeInfo::VariableSize(Box::new(AppendOnlyFile::open(
+				data_dir.join(PMMR_SIZE_FILE),
+				SizeInfo::FixedSize(SizeEntry::LEN as u16),
+			)?))
 		};
 
 		// Hash file is always "fixed size" and we use 32 bytes per hash.
-		let hash_file =
-			DataFile::open(&data_dir.join(PMMR_HASH_FILE), None, Some(Hash::LEN as u16))?;
-		let data_file = DataFile::open(
-			&data_dir.join(PMMR_DATA_FILE),
-			size_path.as_ref(),
-			elmt_size,
-		)?;
+		let hash_size_info = SizeInfo::FixedSize(Hash::LEN as u16);
+
+		let hash_file = DataFile::open(&data_dir.join(PMMR_HASH_FILE), hash_size_info)?;
+		let data_file = DataFile::open(&data_dir.join(PMMR_DATA_FILE), size_info)?;
 
 		let leaf_set_path = data_dir.join(PMMR_LEAF_FILE);
 
diff --git a/store/src/types.rs b/store/src/types.rs
index 1192ee2fd0..5f681d8a6b 100644
--- a/store/src/types.rs
+++ b/store/src/types.rs
@@ -59,7 +59,15 @@ impl Writeable for SizeEntry {
 	}
 }
 
-/// Data file (MMR) wrapper around an append only file.
+/// Are we dealing with "fixed size" data or "variable size" data in a data file?
+pub enum SizeInfo {
+	/// Fixed size data.
+	FixedSize(u16),
+	/// Variable size data.
+	VariableSize(Box<AppendOnlyFile<SizeEntry>>),
+}
+
+/// Data file (MMR) wrapper around an append-only file.
 pub struct DataFile<T> {
 	file: AppendOnlyFile<T>,
 }
@@ -69,21 +77,13 @@ where
 	T: Readable + Writeable + Debug,
 {
 	/// Open (or create) a file at the provided path on disk.
-	pub fn open<P>(path: P, size_path: Option<P>, elmt_size: Option<u16>) -> io::Result<DataFile<T>>
+	pub fn open<P>(path: P, size_info: SizeInfo) -> io::Result<DataFile<T>>
 	where
 		P: AsRef<Path> + Debug,
 	{
-		let size_file = if let Some(size_path) = size_path {
-			Some(AppendOnlyFile::open(
-				size_path,
-				None,
-				Some(SizeEntry::LEN as u16),
-			)?)
-		} else {
-			None
-		};
-		let file = AppendOnlyFile::open(path, size_file, elmt_size)?;
-		Ok(DataFile { file })
+		Ok(DataFile {
+			file: AppendOnlyFile::open(path, size_info)?,
+		})
 	}
 
 	/// Append an element to the file.
@@ -168,11 +168,7 @@ where
 pub struct AppendOnlyFile<T> {
 	path: PathBuf,
 	file: Option<File>,
-
-	// We either have a fixed_size or an associated "size" file.
-	elmt_size: Option<u16>,
-	size_file: Option<Box<AppendOnlyFile<SizeEntry>>>,
-
+	size_info: SizeInfo,
 	mmap: Option<memmap::Mmap>,
 
 	// Buffer of unsync'd bytes. These bytes will be appended to the file when flushed.
@@ -187,20 +183,15 @@ where
 	T: Debug + Readable + Writeable,
 {
 	/// Open a file (existing or not) as append-only, backed by a mmap.
-	pub fn open<P>(
-		path: P,
-		size_file: Option<AppendOnlyFile<SizeEntry>>,
-		elmt_size: Option<u16>,
-	) -> io::Result<AppendOnlyFile<T>>
+	pub fn open<P>(path: P, size_info: SizeInfo) -> io::Result<AppendOnlyFile<T>>
 	where
 		P: AsRef<Path> + Debug,
 	{
 		let mut aof = AppendOnlyFile {
 			file: None,
 			path: path.as_ref().to_path_buf(),
-			elmt_size,
+			size_info,
 			mmap: None,
-			size_file: size_file.map(|x| Box::new(x)),
 			buffer: vec![],
 			buffer_start_pos: 0,
 			buffer_start_pos_bak: 0,
@@ -212,7 +203,7 @@ where
 		// This will occur during "fast sync" as we do not sync the size_file
 		// and must build it locally.
 		// And we can *only* do this after init() the data file (so we know sizes).
-		if let Some(ref mut size_file) = &mut aof.size_file {
+		if let SizeInfo::VariableSize(ref mut size_file) = &mut aof.size_info {
 			if size_file.size()? == 0 {
 				aof.rebuild_size_file()?;
 
@@ -228,7 +219,7 @@ where
 	/// (Re)init an underlying file and its associated memmap.
 	/// Taking care to initialize the mmap_offset_cache for each element.
 	pub fn init(&mut self) -> io::Result<()> {
-		if let Some(ref mut size_file) = self.size_file {
+		if let SizeInfo::VariableSize(ref mut size_file) = self.size_info {
 			size_file.init()?;
 		}
 
@@ -252,22 +243,18 @@ where
 	}
 
 	fn size_in_elmts(&self) -> io::Result<u64> {
-		if let Some(elmt_size) = self.elmt_size {
-			Ok(self.size()? / elmt_size as u64)
-		} else if let Some(ref size_file) = &self.size_file {
-			size_file.size_in_elmts()
-		} else {
-			Ok(0)
+		match self.size_info {
+			SizeInfo::FixedSize(elmt_size) => Ok(self.size()? / elmt_size as u64),
+			SizeInfo::VariableSize(ref size_file) => size_file.size_in_elmts(),
 		}
 	}
 
 	fn size_unsync_in_elmts(&self) -> io::Result<u64> {
-		if let Some(elmt_size) = self.elmt_size {
-			Ok(self.buffer_start_pos + (self.buffer.len() as u64 / elmt_size as u64))
-		} else if let Some(ref size_file) = &self.size_file {
-			size_file.size_unsync_in_elmts()
-		} else {
-			Err(io::Error::new(io::ErrorKind::Other, "size file missing"))
+		match self.size_info {
+			SizeInfo::FixedSize(elmt_size) => {
+				Ok(self.buffer_start_pos + (self.buffer.len() as u64 / elmt_size as u64))
+			}
+			SizeInfo::VariableSize(ref size_file) => size_file.size_unsync_in_elmts(),
 		}
 	}
 
@@ -281,7 +268,7 @@ where
 	/// Append data to the file. Until the append-only file is synced, data is
 	/// only written to memory.
 	pub fn append(&mut self, bytes: &mut [u8]) -> io::Result<()> {
-		if let Some(ref mut size_file) = &mut self.size_file {
+		if let SizeInfo::VariableSize(ref mut size_file) = &mut self.size_info {
 			let next_pos = size_file.size_unsync_in_elmts()?;
 			let offset = if next_pos == 0 {
 				0
@@ -303,18 +290,13 @@ where
 	// If pos is in the buffer then caller needs to remember to account for this
 	// when reading from the buffer.
 	fn offset_and_size(&self, pos: u64) -> io::Result<(u64, u16)> {
-		if let Some(size) = self.elmt_size {
-			// Calculating offset and size is simple if we have fixed size elements.
-			Ok((pos * size as u64, size))
-		} else if let Some(ref size_file) = &self.size_file {
-			// Otherwise we need to calculate offset and size from entries in the size_file.
-			let entry = size_file.read_as_elmt(pos)?;
-			Ok((entry.offset, entry.size))
-		} else {
-			Err(io::Error::new(
-				io::ErrorKind::Other,
-				"variable size, missing size file",
-			))
+		match self.size_info {
+			SizeInfo::FixedSize(elmt_size) => Ok((pos * elmt_size as u64, elmt_size)),
+			SizeInfo::VariableSize(ref size_file) => {
+				// Otherwise we need to calculate offset and size from entries in the size_file.
+				let entry = size_file.read_as_elmt(pos)?;
+				Ok((entry.offset, entry.size))
+			}
 		}
 	}
 
@@ -322,7 +304,7 @@ where
 	/// We simply "rewind" the buffer_start_pos to the specified position.
 	/// Note: We do not currently support rewinding within the buffer itself.
 	pub fn rewind(&mut self, pos: u64) {
-		if let Some(ref mut size_file) = &mut self.size_file {
+		if let SizeInfo::VariableSize(ref mut size_file) = &mut self.size_info {
 			size_file.rewind(pos);
 		}
 
@@ -335,7 +317,7 @@ where
 	/// Syncs all writes (fsync), reallocating the memory map to make the newly
 	/// written data accessible.
 	pub fn flush(&mut self) -> io::Result<()> {
-		if let Some(ref mut size_file) = &mut self.size_file {
+		if let SizeInfo::VariableSize(ref mut size_file) = &mut self.size_info {
 			// Flush the associated size_file if we have one.
 			size_file.flush()?
 		}
@@ -398,7 +380,7 @@ where
 		}
 
 		// Discarding the data file will discard the associated size file if we have one.
-		if let Some(ref mut size_file) = &mut self.size_file {
+		if let SizeInfo::VariableSize(ref mut size_file) = &mut self.size_info {
 			size_file.discard();
 		}
 
@@ -492,7 +474,7 @@ where
 
 		// Now rebuild our size file to reflect the pruned data file.
 		// This will replace the underlying file internally.
-		if let Some(_) = &self.size_file {
+		if let SizeInfo::VariableSize(_) = &self.size_info {
 			self.rebuild_size_file()?;
 		}
 
@@ -503,7 +485,7 @@ where
 	}
 
 	fn rebuild_size_file(&mut self) -> io::Result<()> {
-		if let Some(ref mut size_file) = &mut self.size_file {
+		if let SizeInfo::VariableSize(ref mut size_file) = &mut self.size_info {
 			// Note: Reading from data file and writing sizes to the associated (tmp) size_file.
 			let tmp_path = size_file.path.with_extension("tmp");
 
@@ -563,7 +545,7 @@ where
 		self.file = None;
 
 		// Remember to release the size_file as well if we have one.
-		if let Some(ref mut size_file) = self.size_file {
+		if let SizeInfo::VariableSize(ref mut size_file) = &mut self.size_info {
 			size_file.release();
 		}
 	}