diff --git a/accounts-db/src/accounts_hash.rs b/accounts-db/src/accounts_hash.rs
index ac4134cf8..124e5b069 100644
--- a/accounts-db/src/accounts_hash.rs
+++ b/accounts-db/src/accounts_hash.rs
@@ -19,8 +19,7 @@ use {
     std::{
         borrow::Borrow,
         convert::TryInto,
-        fs::File,
-        io::{BufWriter, Write},
+        io::{Seek, SeekFrom, Write},
         path::PathBuf,
         sync::{
             atomic::{AtomicU64, AtomicUsize, Ordering},
@@ -33,81 +32,96 @@ pub const MERKLE_FANOUT: usize = 16;
 
 /// 1 file containing account hashes sorted by pubkey, mapped into memory
 struct MmapAccountHashesFile {
+    /// raw slice of `Hash` values. Can be a larger slice than `count`
     mmap: MmapMut,
+    /// # of valid Hash entries in `mmap`
+    count: usize,
 }
 
 impl MmapAccountHashesFile {
     /// return a slice of account hashes starting at 'index'
     fn read(&self, index: usize) -> &[Hash] {
         let start = std::mem::size_of::<Hash>() * index;
-        let item_slice: &[u8] = &self.mmap[start..];
+        let item_slice: &[u8] = &self.mmap[start..self.count * std::mem::size_of::<Hash>()];
         let remaining_elements = item_slice.len() / std::mem::size_of::<Hash>();
         unsafe {
             let item = item_slice.as_ptr() as *const Hash;
             std::slice::from_raw_parts(item, remaining_elements)
         }
     }
+
+    /// write a hash to the end of mmap file.
+    fn write(&mut self, hash: &Hash) {
+        let start = self.count * std::mem::size_of::<Hash>();
+        let end = start + std::mem::size_of::<Hash>();
+        self.mmap[start..end].copy_from_slice(hash.as_ref());
+        self.count += 1;
+    }
 }
 
 /// 1 file containing account hashes sorted by pubkey
 pub struct AccountHashesFile {
     /// # hashes and an open file that will be deleted on drop. None if there are zero hashes to represent, and thus, no file.
-    count_and_writer: Option<(usize, BufWriter<File>)>,
+    writer: Option<MmapAccountHashesFile>,
     /// The directory where temporary cache files are put
     dir_for_temp_cache_files: PathBuf,
+    /// # bytes allocated
+    capacity: usize,
 }
 
 impl AccountHashesFile {
-    /// map the file into memory and return a reader that can access it by slice
-    fn get_reader(&mut self) -> Option<(usize, MmapAccountHashesFile)> {
-        std::mem::take(&mut self.count_and_writer).map(|(count, writer)| {
-            let file = Some(writer.into_inner().unwrap());
-            (
-                count,
-                MmapAccountHashesFile {
-                    mmap: unsafe { MmapMut::map_mut(file.as_ref().unwrap()).unwrap() },
-                },
-            )
-        })
+    /// return a mmap reader that can be accessed  by slice
+    fn get_reader(&mut self) -> Option<MmapAccountHashesFile> {
+        std::mem::take(&mut self.writer)
     }
 
     /// # hashes stored in this file
     pub fn count(&self) -> usize {
-        self.count_and_writer
+        self.writer
             .as_ref()
-            .map(|(count, _)| *count)
+            .map(|writer| writer.count)
             .unwrap_or_default()
     }
 
     /// write 'hash' to the file
     /// If the file isn't open, create it first.
     pub fn write(&mut self, hash: &Hash) {
-        if self.count_and_writer.is_none() {
+        if self.writer.is_none() {
             // we have hashes to write but no file yet, so create a file that will auto-delete on drop
-            self.count_and_writer = Some((
-                0,
-                BufWriter::new(
-                    tempfile_in(&self.dir_for_temp_cache_files).unwrap_or_else(|err| {
-                        panic!(
-                            "Unable to create file within {}: {err}",
-                            self.dir_for_temp_cache_files.display()
-                        )
-                    }),
-                ),
-            ));
-        }
-        let count_and_writer = self.count_and_writer.as_mut().unwrap();
-        count_and_writer
-            .1
-            .write_all(hash.as_ref())
-            .unwrap_or_else(|err| {
+
+            let mut data = tempfile_in(&self.dir_for_temp_cache_files).unwrap_or_else(|err| {
                 panic!(
-                    "Unable to write file within {}: {err}",
+                    "Unable to create file within {}: {err}",
                     self.dir_for_temp_cache_files.display()
                 )
             });
 
-        count_and_writer.0 += 1;
+            // Theoretical performance optimization: write a zero to the end of
+            // the file so that we won't have to resize it later, which may be
+            // expensive.
+            data.seek(SeekFrom::Start((self.capacity - 1) as u64))
+                .unwrap();
+            data.write_all(&[0]).unwrap();
+            data.rewind().unwrap();
+            data.flush().unwrap();
+
+            //UNSAFE: Required to create a Mmap
+            let map = unsafe { MmapMut::map_mut(&data) };
+            let map = map.unwrap_or_else(|e| {
+                error!(
+                    "Failed to map the data file (size: {}): {}.\n
+                        Please increase sysctl vm.max_map_count or equivalent for your platform.",
+                    self.capacity, e
+                );
+                std::process::exit(1);
+            });
+
+            self.writer = Some(MmapAccountHashesFile {
+                mmap: map,
+                count: 0,
+            });
+        }
+        self.writer.as_mut().unwrap().write(hash);
     }
 }
 
@@ -338,7 +352,8 @@ impl CumulativeHashesFromFiles {
         let mut readers = Vec::with_capacity(hashes.len());
         let cumulative = CumulativeOffsets::new(hashes.into_iter().filter_map(|mut hash_file| {
             // ignores all hashfiles that have zero entries
-            hash_file.get_reader().map(|(count, reader)| {
+            hash_file.get_reader().map(|reader| {
+                let count = reader.count;
                 readers.push(reader);
                 count
             })
@@ -985,15 +1000,12 @@ impl<'a> AccountsHasher<'a> {
         // map from index of an item in first_items[] to index of the corresponding item in sorted_data_by_pubkey[]
         // this will change as items in sorted_data_by_pubkey[] are exhausted
         let mut first_item_to_pubkey_division = Vec::with_capacity(len);
-        let mut hashes = AccountHashesFile {
-            count_and_writer: None,
-            dir_for_temp_cache_files: self.dir_for_temp_cache_files.clone(),
-        };
+
         // initialize 'first_items', which holds the current lowest item in each slot group
-        sorted_data_by_pubkey
+        let max_inclusive_num_pubkeys = sorted_data_by_pubkey
             .iter()
             .enumerate()
-            .for_each(|(i, hash_data)| {
+            .map(|(i, hash_data)| {
                 let first_pubkey_in_bin =
                     Self::find_first_pubkey_in_bin(hash_data, pubkey_bin, bins, &binner, stats);
                 if let Some(first_pubkey_in_bin) = first_pubkey_in_bin {
@@ -1001,8 +1013,27 @@ impl<'a> AccountsHasher<'a> {
                     first_items.push(k);
                     first_item_to_pubkey_division.push(i);
                     indexes.push(first_pubkey_in_bin);
+                    let mut first_pubkey_in_next_bin = first_pubkey_in_bin + 1;
+                    while first_pubkey_in_next_bin < hash_data.len() {
+                        if binner.bin_from_pubkey(&hash_data[first_pubkey_in_next_bin].pubkey)
+                            != pubkey_bin
+                        {
+                            break;
+                        }
+                        first_pubkey_in_next_bin += 1;
+                    }
+                    first_pubkey_in_next_bin - first_pubkey_in_bin
+                } else {
+                    0
                 }
-            });
+            })
+            .sum::<usize>();
+        let mut hashes = AccountHashesFile {
+            writer: None,
+            dir_for_temp_cache_files: self.dir_for_temp_cache_files.clone(),
+            capacity: max_inclusive_num_pubkeys * std::mem::size_of::<Hash>(),
+        };
+
         let mut overall_sum = 0;
         let mut duplicate_pubkey_indexes = Vec::with_capacity(len);
         let filler_accounts_enabled = self.filler_accounts_enabled();
@@ -1238,8 +1269,9 @@ pub mod tests {
     impl AccountHashesFile {
         fn new(dir_for_temp_cache_files: PathBuf) -> Self {
             Self {
-                count_and_writer: None,
+                writer: None,
                 dir_for_temp_cache_files,
+                capacity: 1024, /* default 1k for tests */
             }
         }
     }
@@ -1308,16 +1340,16 @@ pub mod tests {
         // 1 hash
         file.write(&hashes[0]);
         let reader = file.get_reader().unwrap();
-        assert_eq!(&[hashes[0]][..], reader.1.read(0));
-        assert!(reader.1.read(1).is_empty());
+        assert_eq!(&[hashes[0]][..], reader.read(0));
+        assert!(reader.read(1).is_empty());
 
         // multiple hashes
         let mut file = AccountHashesFile::new(dir_for_temp_cache_files.path().to_path_buf());
         assert!(file.get_reader().is_none());
         hashes.iter().for_each(|hash| file.write(hash));
         let reader = file.get_reader().unwrap();
-        (0..2).for_each(|i| assert_eq!(&hashes[i..], reader.1.read(i)));
-        assert!(reader.1.read(2).is_empty());
+        (0..2).for_each(|i| assert_eq!(&hashes[i..], reader.read(i)));
+        assert!(reader.read(2).is_empty());
     }
 
     #[test]
@@ -1476,7 +1508,7 @@ pub mod tests {
         let accounts_hasher = AccountsHasher::new(dir_for_temp_cache_files.path().to_path_buf());
         let (mut hashes, lamports) =
             accounts_hasher.de_dup_accounts_in_parallel(&slice, 0, 1, &HashStats::default());
-        assert_eq!(&[Hash::default()], hashes.get_reader().unwrap().1.read(0));
+        assert_eq!(&[Hash::default()], hashes.get_reader().unwrap().read(0));
         assert_eq!(lamports, 1);
     }
 
@@ -1486,7 +1518,7 @@ pub mod tests {
     fn get_vec(mut hashes: AccountHashesFile) -> Vec<Hash> {
         hashes
             .get_reader()
-            .map(|r| r.1.read(0).to_vec())
+            .map(|r| r.read(0).to_vec())
             .unwrap_or_default()
     }