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

@@ -306,17 +306,10 @@ impl LocalStorageDisk {
 
     /// Size required for the disk's data plus overhead
     pub fn required_dataset_size(&self) -> i64 {
-        // The value in ByteCount is capped at i64::MAX, but a disk backed by
-        // local storage cannot be created that does not fit in a zpool at the
-        // time of the disk creation request (note this is different from the
-        // time of instance reservation where Nexus actually assigns the disk an
-        // allocation). This means that the size is well below that MAX, meaning
-        // this won't overflow.
-        //
-        // Additionally, there's a checked_add in DiskTypeLocalStorage::New that
-        // checks this won't happen. Note this won't protect against cases of
-        // database modification, but not much will!
-
+        // 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
     }