Purge snapshot dirs before loading from an archive (#31283)

* Purge snapshot dirs before loading from an archive

* Refine comment

* Should not remove the bank_snapshots_dir directory

ledger/src/bank_forks_utils.rs

@@ -97,7 +97,6 @@ pub fn load_bank_forks(
             "Initializing bank snapshot path: {}",
             snapshot_config.bank_snapshots_dir.display()
         );
-        let _ = fs::remove_dir_all(&snapshot_config.bank_snapshots_dir);
         fs::create_dir_all(&snapshot_config.bank_snapshots_dir)
             .expect("Couldn't create snapshot directory");
 
@@ -199,6 +198,12 @@ fn bank_forks_from_snapshot(
         process::exit(1);
     }
 
+    // Given that we are going to boot from an archive, the accountvecs held in the snapshot dirs for fast-boot should
+    // be released.  They will be released by the account_background_service anyway.  But in the case of the account_paths
+    // using memory-mounted file system, they are not released early enough to give space for the new append-vecs from
+    // the archives, causing the out-of-memory problem.  So, purge the snapshot dirs upfront before loading from the archive.
+    snapshot_utils::purge_old_bank_snapshots(&snapshot_config.bank_snapshots_dir, 0, None);
+
     let (deserialized_bank, full_snapshot_archive_info, incremental_snapshot_archive_info) =
         snapshot_utils::bank_from_latest_snapshot_archives(
             &snapshot_config.bank_snapshots_dir,