guide: document StorVSP channels and subchannels

Guide/src/SUMMARY.md

@@ -94,6 +94,7 @@
       - [virtio-pmem]()
   - [VMBus]()
       - [storvsp](./reference/devices/vmbus/storvsp.md)
+        - [Channels & Subchannels](./reference/devices/vmbus/storvsp_channels.md)
       - [netvsp]()
       - [vpci]()
       - [serial]()

Guide/src/reference/architecture/devices/storage.md

@@ -81,7 +81,16 @@ but they all produce `DiskIo` calls on the backend side.
 
 **NVMe** is the simplest path. The NVMe controller's namespace directly holds a `Disk`. NVM opcodes (READ, WRITE, FLUSH, DSM) map nearly 1:1 to `DiskIo` methods. The FUA bit from the NVMe write command is forwarded directly.
 
-**StorVSP / SCSI** has a two-layer design. StorVSP handles the VMBus transport — negotiation, ring buffer management, sub-channel allocation. It dispatches each SCSI request to an [`AsyncScsiDisk`](https://openvmm.dev/rustdoc/linux/scsi_core/trait.AsyncScsiDisk.html) implementation. For hard drives, that's [`SimpleScsiDisk`](https://openvmm.dev/rustdoc/linux/scsidisk/struct.SimpleScsiDisk.html), which parses the SCSI CDB and translates it to `DiskIo` calls. For optical drives, it's [`SimpleScsiDvd`](https://openvmm.dev/rustdoc/linux/scsidisk/scsidvd/struct.SimpleScsiDvd.html).
+**StorVSP / SCSI** has a two-layer design. StorVSP handles the VMBus
+transport — negotiation, ring buffer management,
+[sub-channel allocation](../../devices/vmbus/storvsp_channels.md). It
+dispatches each SCSI request to an
+[`AsyncScsiDisk`](https://openvmm.dev/rustdoc/linux/scsi_core/trait.AsyncScsiDisk.html)
+implementation. For hard drives, that's
+[`SimpleScsiDisk`](https://openvmm.dev/rustdoc/linux/scsidisk/struct.SimpleScsiDisk.html),
+which parses the SCSI CDB and translates it to `DiskIo` calls. For
+optical drives, it's
+[`SimpleScsiDvd`](https://openvmm.dev/rustdoc/linux/scsidisk/scsidvd/struct.SimpleScsiDvd.html).
 
 **IDE** is the legacy path. ATA commands for hard drives call `DiskIo` directly. ATAPI commands for optical drives delegate to `SimpleScsiDvd` through an ATAPI-to-SCSI translation layer — the same DVD implementation that StorVSP uses. IDE also supports [enlightened INT13 commands](../../emulated/legacy_x86/ide.md#enlightened-io), a Microsoft-specific optimization that collapses the multi-exit register-programming sequence into a single VM exit.
 

Guide/src/reference/architecture/openhcl/storage_configuration.md

@@ -23,6 +23,21 @@ The same tree appears in three places:
 
 This page lines those views up so you do not have to infer the schema from the JSON alone.
 
+## Fixed settings
+
+The `fixed` section of VTL2 settings contains values that apply
+globally, not per-controller. The key storage-related field is
+`scsi_sub_channels`:
+
+| Field | Type | Default | Meaning |
+|-------|------|---------|---------|
+| `scsi_sub_channels` | `u16` | `0` | Maximum sub-channel count for all SCSI controllers. Clamped to 256 at runtime. |
+| `io_ring_size` | `u32` | `256` | Size of each per-CPU io_uring submission queue in the OpenHCL threadpool. Not StorVSP-specific — affects all async I/O. |
+
+See [StorVSP Channels & Subchannels](../../devices/vmbus/storvsp_channels.md)
+for the full subchannel model, performance characteristics, and
+configuration guidance.
+
 ## One controller tree
 
 ```text
@@ -175,6 +190,7 @@ The runtime model and schema enforce several important constraints:
 | `Striped` must carry at least two backing devices | `schema::v1` rejects too few devices |
 | NVMe namespace IDs cannot be `0` or `!0` | `schema::v1` validates `location` for NVMe |
 | NVMe children cannot be DVDs | `schema::v1` rejects `is_dvd` on NVMe |
+| SCSI subchannel count is clamped to 256 | `underhill_core::dispatch::vtl2_settings_worker`, `scsi_sub_channels.min(256)` |
 | IDE can be a guest-visible target but not a VTL2 backing source | `petri::vtl2_settings` and `openvmm_entry::storage_builder` reject IDE backing |
 
 These rules are useful when debugging configuration failures because they tell you whether the problem is in controller selection, child addressing, or backing-device declaration.

Guide/src/reference/devices/vmbus/storvsp.md

@@ -32,7 +32,9 @@ calls (read, write, flush, unmap).
 - **Protocol.** Hyper-V SCSI (SRB-based), with version negotiation
   (Win6 through Blue).
 - **Sub-channels.** StorVSP supports multiple VMBus sub-channels
-  for parallel I/O, one worker per channel.
+  for parallel I/O, one worker per channel. See
+  [Channels & Subchannels](storvsp_channels.md) for the full
+  model, configuration, and performance characteristics.
 - **Hot-add / hot-remove.** SCSI devices can be attached and
   detached at runtime via `ScsiControllerRequest`.
 - **Performance.** Poll-mode optimization — when pending I/O count