Make 30 workers for local storage disks (#9639)

In testing on Cosmo We determined that 30 worker threads for each local
storage disk seemed about right, so let's make Omicron tell propolis that
when it sends over the information about disks attached to an instance.

Co-authored-by: Alan Hanson <alan@oxide.computer>

Author: leftwo

nexus/src/app/instance_platform/mod.rs

@@ -77,6 +77,7 @@ use std::collections::HashMap;
 use crate::app::instance::InstanceRegisterReason;
 use crate::cidata::InstanceCiData;
 
+use super::LOCAL_STORAGE_WORKERS;
 use crate::db::datastore::Disk;
 use nexus_db_queries::db;
 use omicron_common::api::external::Error;
@@ -279,7 +280,7 @@ impl DisksByIdBuilder {
             path,
             readonly: false,
             block_size: disk.block_size().to_bytes(),
-            workers: None,
+            workers: Some(LOCAL_STORAGE_WORKERS),
         });
 
         self.add_generic_disk(disk, backend)

nexus/src/app/mod.rs

@@ -45,6 +45,7 @@ use slog_error_chain::InlineErrorChain;
 use std::collections::HashMap;
 use std::net::SocketAddrV6;
 use std::net::{IpAddr, Ipv6Addr};
+use std::num::NonZeroU32;
 use std::sync::Arc;
 use std::sync::OnceLock;
 use tokio::sync::mpsc;
@@ -156,6 +157,12 @@ pub const MAX_MEMORY_BYTES_PER_INSTANCE: u64 = 1536 * (1 << 30); // 1.5 TiB
 pub const MIN_DISK_SIZE_BYTES: u32 = 1 << 30; // 1 GiB
 pub const MAX_DISK_SIZE_BYTES: u64 = 1023 * (1 << 30); // 1023 GiB
 
+/// This was number was chosen as the best-ish measured on a Cosmo when more or
+/// less fully dedicating an SN861 as a disk for a single VM. This is certainly
+/// higher than necessary for Gimlet, and is chosen far higher than may be
+/// appropriate if the disk is shared across several or more instances.
+pub const LOCAL_STORAGE_WORKERS: NonZeroU32 = NonZeroU32::new(30).unwrap();
+
 /// This value is aribtrary
 pub const MAX_SSH_KEYS_PER_INSTANCE: u32 = 100;