PCIe is a downward-compatible interface technology with standardized slots, but the chaos with interface cards has not gone away. If you want to use fast mass storage to upgrade older systems without UEFI BIOS and NVMe support, you will certainly benefit from the advantages of the Peripheral Component Interconnect Express (PCIe) bus. However, before you buy expensive components, it makes sense to investigate the specifications; otherwise, you may end up spending several hundred dollars on equipment you can't use.

Understanding PCIe

PCIe [1] is a standard for extensions in personal computers that has been in use for around 15 years, and by now, it has completely replaced its predecessors PCI, PCI-X, and AGP.

The PCIe bus thoroughly eliminates many of the problems of its predecessors. Unlike the PCI bus and its variations, PCIe works with serial connections. Point-to-point connections are created between the components, which leads to significantly higher data transfer rates with significantly less overhead, because slow devices on the bus cannot slow down the faster devices (Table 1).

With the legacy bus topology used by the PCI standard, all the connected components have to share the available bandwidth; with the PCIe specification, a switch integrated in the chipset enables the individual connections. The devices can use several lanes for data transmission; unlike the PCI bus, lanes do not have a specified and fixed clock signal. Depending on the specification, up to almost 2000MB/s can theoretically be transferred per lane. The PCIe bus operates in duplex mode, which enables simultaneous data transmission in both directions.

Since the PCIe bus is hot-pluggable, the system is also suitable for high-performance mass storage devices that can be swapped on-the-fly if defects occur. Especially in high-performance workstations and servers, the PCIe standard replaced the PCI standard at a very early stage.

However, even today, you occasionally find a mixture of one or two conventional PCI slots and (usually) four to six PCIe slots in such computer systems. Mixed operation of the different standards is also possible. Thanks to upward and downward compatibility, you can operate plugin cards of different specifications without the cards influencing each other in terms of performance.

Lanes

Data transfer within a PCIe infrastructure takes place along one or more lanes. The maximum transfer speed has increased with each new specification, and the connectors have changed accordingly. In addition to PCIe x1 connectors with only one lane, there are also standardized x4, x8, and x16 slots.

Short slots are sufficient for relatively slow plugin cards (such as USB 3.0 interfaces), but graphic cards, PCIe SSDs, and high-performance network cards in particular depend on many lanes and thus long slots. In the case of graphics cards, the problem of power supply also arises: Modern graphics adapters consume 300W or more of power.

Because a PCIe slot typically does not deliver more than 75W, such graphics cards require external power connections. The standard provides for standardized connectors for this purpose. In high-performance workstations, for simultaneous use with multiple graphics cards and multi-monitor operation, you'll need multiple PCIe x16 slots, along with a correspondingly dimensioned power supply (Figure 1).

Figure 1: Different PCIe slots in an HP Z600 workstation.

Disadvantages of Modern SSDs

Especially with high-performance components such as PCIe SSDs, the use of the devices must always be compared with the technical capabilities of the host computer before you decide to purchase. Although the slots match mechanically, even across different specifications, and could therefore accommodate components of all standards, not every PCIe SSD is suitable for every computer.

Older computers with the PCIe 1.x and 2.x standards do not support all modern NVMe SSDs, but mostly only older ones that follow the AHCI standard. Since such computer systems still rely on a legacy BIOS, the SSDs must also provide a Boot ROM that allows the computer system to boot from the SSD despite having an older BIOS. Without the Boot ROM, an SSD in a computer with a legacy BIOS is not recognized as a bootable device.

The Boot ROMs of modern PCIe SSDs, however, usually only have routines for devices with a UEFI BIOS, so such mass storage devices do not work with older computer hardware despite the manufacturer claiming Linux compatibility. However, problems can also arise with seemingly suitable specifications. For example, certain older high-performance SSDs with a PCIe 2.x interface must be integrated with proprietary driver modules on Linux for them to be recognized as fast mass storage devices (Figure 2).

Figure 2: A PCIe SSD that can only be addressed with its own kernel driver on Linux.

Kernel modules for these SSDs are usually available directly from the manufacturer, but only for a few distributions and often only for certain kernel versions. Although the support and thus also the driver development for such very expensive high-end components is generally much better (and also designed for longer periods of time) than for consumer products, the manufacturer's support expires after a few years at the latest.

In these cases, there is also the drawback that such SSDs promise an enormous speed boost in servers and workstations but are not bootable due to the lack of operating system integration. In such cases, another mass storage device is needed to boot the computer.

A further drawback when using high-performance SSDs lies in the high power draw and what can be a massive heat generation associated with it. Very fast PCIe SSDs therefore come with active cooling out of the box, as do high-performance graphics cards. Since these SSDs generally require longer slots with a higher number of lanes, it is important to ensure during installation that there is sufficient clearance for air to circulate between the components.

High-performance workstations, often equipped with eight to twelve internal fans, provide good air circulation out the box. For smaller systems, you need to make sure that there is enough space between an actively cooled graphics card and an SSD with its own fan, otherwise heat build-up may occur.

Since the individual memory cells of the SSDs usually also passively dissipate heat through a large heat sink, it may not be possible to use two adjacent PCIe slots simultaneously for such high-energy components.