avoid floating point math

reservation right now is 25%, which means that the requested size of a
region can be divided by 4. avoid floating point math where possible.

change the reservation percentage stored with the region to an enum,
where the only value is 25%. this limits what can be done with manual
database edits, and restricts what the Region::reserved_size function
has to guard against.

it'd be nice if Region::new was a test-only function but the crate
doesn't have the same idea of a integration test feature.

Author: jmpesp

nexus/db-model/src/region.rs

@@ -4,6 +4,7 @@
 
 use super::ByteCount;
 use crate::SqlU16;
+use crate::impl_enum_type;
 use crate::schema::region;
 use crate::typed_uuid::DbTypedUuid;
 use db_macros::Asset;
@@ -15,6 +16,19 @@ use omicron_uuid_kinds::VolumeUuid;
 use serde::{Deserialize, Serialize};
 use uuid::Uuid;
 
+impl_enum_type!(
+    #[derive(SqlType, Debug, QueryId)]
+    #[diesel(postgres_type(name = "region_reservation_percent", schema = "public"))]
+    pub struct RegionReservationPercentEnum;
+
+    #[derive(Copy, Clone, Debug, AsExpression, FromSqlRow, Serialize, Deserialize, PartialEq)]
+    #[diesel(sql_type = RegionReservationPercentEnum)]
+    pub enum RegionReservationPercent;
+
+    // Enum values
+    TwentyFive => b"25"
+);
+
 /// Database representation of a Region.
 ///
 /// A region represents a portion of a Crucible Downstairs dataset
@@ -58,13 +72,13 @@ pub struct Region {
 
     // The Agent will reserve space for Downstairs overhead when creating the
     // corresponding ZFS dataset. Nexus has to account for that: store that
-    // reservation factor here as it may change in the future, and it can be
-    // used during Crucible related accounting.
-    reservation_factor: f64,
+    // reservation percent here as it may change in the future, and it can be
+    // used during Crucible related accounting. This is stored as an enum to
+    // restrict the values to what the Crucible Agent uses.
+    reservation_percent: RegionReservationPercent,
 }
 
 impl Region {
-    #[allow(clippy::too_many_arguments)]
     pub fn new(
         dataset_id: DatasetUuid,
         volume_id: VolumeUuid,
@@ -73,7 +87,6 @@ impl Region {
         extent_count: u64,
         port: u16,
         read_only: bool,
-        reservation_factor: f64,
     ) -> Self {
         Self {
             identity: RegionIdentity::new(Uuid::new_v4()),
@@ -85,7 +98,10 @@ impl Region {
             port: Some(port.into()),
             read_only,
             deleting: false,
-            reservation_factor,
+            // When the Crucible agent's reservation percentage changes, this
+            // function should accept that as argument. Until then, it can only
+            // ever be 25%.
+            reservation_percent: RegionReservationPercent::TwentyFive,
         }
     }
 
@@ -122,7 +138,9 @@ impl Region {
         self.deleting
     }
 
-    /// The size of the Region without accounting for any overhead
+    /// The size of the Region without accounting for any overhead. The
+    /// `allocation_query` function should have validated that this won't
+    /// overflow.
     pub fn requested_size(&self) -> u64 {
         self.block_size().to_bytes()
             * self.blocks_per_extent()
@@ -133,6 +151,10 @@ impl Region {
     /// which is some factor higher than the requested region size to account
     /// for on-disk overhead.
     pub fn reserved_size(&self) -> u64 {
-        (self.requested_size() as f64 * self.reservation_factor).round() as u64
+        let overhead = match &self.reservation_percent {
+            RegionReservationPercent::TwentyFive => self.requested_size() / 4,
+        };
+
+        self.requested_size() + overhead
     }
 }

nexus/db-model/src/schema.rs

@@ -1126,7 +1126,7 @@ table! {
 
         deleting -> Bool,
 
-        reservation_factor -> Float8,
+        reservation_percent -> crate::RegionReservationPercentEnum,
     }
 }
 

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

@@ -269,7 +269,7 @@ impl DataStore {
             },
             allocation_strategy,
             num_regions_required,
-        );
+        )?;
 
         let conn = self.pool_connection_authorized(&opctx).await?;
 
@@ -317,7 +317,7 @@ impl DataStore {
                     // Update datasets to which the regions belonged.
                     // XXX put this file somewhere else
                     sql_query(include_str!(
-                        "../../../../../schema/crdb/crucible-agent-reservation-overhead/up03.sql"
+                        "../../../../../schema/crdb/crucible-agent-reservation-overhead/up04.sql"
                     ))
                     .execute_async(&conn)
                     .await?;

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

@@ -4715,7 +4715,6 @@ mod tests {
                 region.extent_count(),
                 111,
                 false, // read-write
-                1.0,
             );
 
             use nexus_db_model::schema::region::dsl;
@@ -4953,7 +4952,6 @@ mod tests {
                 region.extent_count(),
                 111,
                 true, // read-only
-                1.0,
             );
 
             use nexus_db_model::schema::region::dsl;