add 'oldest_root' stat in hash calc (#26896)

runtime/src/accounts_db.rs

@@ -6886,6 +6886,8 @@ impl AccountsDb {
     ) -> Result<(Hash, u64), BankHashVerificationError> {
         let _guard = self.active_stats.activate(ActiveStatItem::Hash);
 
+        stats.oldest_root = storages.range().start;
+
         self.mark_old_slots_as_dirty(storages, Some(config.epoch_schedule.slots_per_epoch));
 
         let (num_hash_scan_passes, bins_per_pass) = Self::bins_per_pass(self.num_hash_scan_passes);

runtime/src/accounts_hash.rs

@@ -94,6 +94,7 @@ pub struct HashStats {
     pub rehash_required: AtomicUsize,
     /// # rehashes that took place and were UNnecessary
     pub rehash_unnecessary: AtomicUsize,
+    pub oldest_root: Slot,
     pub roots_older_than_epoch: AtomicUsize,
     pub accounts_in_roots_older_than_epoch: AtomicUsize,
     pub append_vec_sizes_older_than_epoch: AtomicUsize,
@@ -218,6 +219,7 @@ impl HashStats {
                 self.roots_older_than_epoch.load(Ordering::Relaxed) as i64,
                 i64
             ),
+            ("oldest_root", self.oldest_root as i64, i64),
             (
                 "ancient_append_vecs",
                 self.ancient_append_vecs.load(Ordering::Relaxed) as i64,