Replace local storage allocation search with sort / pair algorithm

nexus/db-queries/Cargo.toml

@@ -116,6 +116,7 @@ pem.workspace = true
 petgraph.workspace = true
 predicates.workspace = true
 pretty_assertions.workspace = true
+proptest.workspace = true
 rcgen.workspace = true
 regex.workspace = true
 rustls.workspace = true

nexus/db-queries/src/db/datastore/disk.rs

@@ -300,10 +300,20 @@ impl LocalStorageDisk {
         self.model().slot()
     }
 
-    pub fn required_dataset_overhead(&self) -> external::ByteCount {
+    fn required_dataset_overhead(&self) -> external::ByteCount {
         self.disk_type_local_storage.required_dataset_overhead()
     }
 
+    /// Size required for the disk's data plus overhead
+    pub fn required_dataset_size(&self) -> i64 {
+        // Both values are ByteCounts, so each fits in i64. The overhead is
+        // roughly 7% of the disk size (see DiskTypeLocalStorage::new), so the
+        // sum only approaches i64::MAX for disks of several EiB, far beyond
+        // what any zpool can hold.
+        self.size().to_bytes() as i64
+            + self.required_dataset_overhead().to_bytes() as i64
+    }
+
     /// Return the full path to the local storage zvol's device
     pub fn zvol_path(&self) -> Result<String, Error> {
         let Some(allocation) = &self.local_storage_dataset_allocation else {