[minor] note to use vdev-dir in setup if it makes sense (#8735)

Added to how-to-run doc. We've only used this in deploy scripts.

Author: zeeshanlakhani

docs/how-to-run.adoc

@@ -227,6 +227,27 @@ If you're using an existing external network, you likely don't need to specify a
     --physical-link igb0           # The physical link for your external network.
 ----
 
+When running `cargo xtask virtual-hardware create`, consider using the
+`--vdev-dir` option to specify where the tool will create files that will back
+the virtual U2/M2 devices:
+
+```bash
+cargo xtask virtual-hardware create \
+   --vdev-dir /scratch \
+   --gateway-ip 192.168.1.199 \
+   --pxa-start 192.168.1.20 \
+   --pxa-end 192.168.1.40
+```
+
+If the directory is changed, the `vdevs` field in
+`smf/sled-agent/non-gimlet/config.toml` needs to be updated to point to the
+new location.
+
+Why is this useful? Separating vdev storage allows you to granularly control how
+you want to manage storage resources. For example, you can use a separate disk
+or partition for vdevs, which can be useful for performance and alleviating hard
+disk stress.
+
 If you're running on a bench Gimlet, you may not need (or want) most of what `cargo xtask virtual-hardware create` does, but you do still need SoftNPU. You can tweak what resources are created with the `--scope` flag.
 
 Later, you can clean up the resources created by `cargo xtask virtual-hardware create` with: