Pcie | Damien Dye's Blog

An NVMe drive entering a tri-mode adapter and leaving as a SCSI disk with a queue depth of 128

Tri-Mode Adapters Buy Flexibility With Your NVMe Queues

A tri-mode adapter lets any bay take SAS, SATA or NVMe — real flexibility, and the reason U.3 backplanes exist. What it does not advertise is that your NVMe drives stop being NVMe drives: they arrive in Linux as SCSI disks on mpt3sas, queue depth 128, sharing one tag pool and one x8 uplink. Two Gen4 drives saturate the card. A single Gen5 drive is already past it. Whether that trade still makes sense in 2026, and why the universal bays belong to the backplane rather than the controller.

A small aperture sliding over a large framebuffer, beside the whole framebuffer mapped at once

PCIe Resizable BAR and Modern GPUs — Intel Arc, NVIDIA and AMD

Resizable BAR lets the CPU map a GPU’s whole framebuffer instead of peering at it through a 256MB window. Intel calls it required for Arc, NVIDIA enables it per game, AMD sells it as Smart Access Memory — and for AI work it changes the transfer, not the maths.

A curve rising from 36 percent of bare metal at queue depth 1 to within a few percent by queue depth 32

PCIe Passthrough Performance on Proxmox VE — The IOMMU Tax and How to Minimise It

Why PCIe devices lose throughput when passed through to a VM via VFIO, and the practical tuning steps that claw most of it back.

A latency-percentile curve that stays flat to p90 then climbs steeply, beside one that stays flat

PCIe ASPM and Why You Should Disable It for Passthrough

Active State Power Management saves a few watts on idle PCIe links. Under VFIO passthrough, it adds latency jitter that’s hard to diagnose and easy to fix.

The same 4 KB payload drawn as 32 packets at MPS 128 and 8 packets at MPS 512

PCIe MaxPayloadSize — A Free Performance Win for Passthrough

QEMU’s virtual root complex defaults to 128-byte TLP payloads. Most devices support 256 or 512. One kernel parameter fixes it.

A flat 1996-era PCI bus beside a 2007-era PCIe root complex

Always Use Q35, Not i440fx — Why It Matters on Proxmox VE

The two QEMU virtual chipsets are not interchangeable. Q35 provides a proper PCIe topology that passthrough, modern Windows, and the wider KVM ecosystem all depend on.