The advent of cheap solid
state storage has been one of the most significant performance
enhancements to hit the PC market in years, but the SATA standard has
had a difficult time keeping pace with new product introductions. The
new SATA Express standard promises to change that by combining the
bandwidth of PCI-Express with a cable-based SATA-like connector that
maintains backwards compatibility with the old SATA standard.
A better SATA standard
SATA was originally designed to handle the needs of next-generation spinning
disks. The earliest SATA controllers were essentially Parallel ATA
(PATA) products with different cabling. Later SATA hard drives
introduced the Advanced Host Controller Interface (AHCI), which offers
support for features like Native Command Queuing as well as limited
multithreading support and hot-swapping.
AHCI, however, was explicitly designed to meet the needs of magnetic
disks. As solid-state storage has proliferated, it’s become clear that
the old standard was deficient in a number of key areas. Unlike hard
drives, SSDs have sophisticated processors on board that handle
everything from security to local memory management.
The new standard, NVMe, is something we’ve discussed before
— this is the protocol that will replace AHCI and offer superior
performance at vastly reduced latencies compared to the old protocol.
Along with that improvement in latency comes a massive kick in the pants
for performance. SATA 6 tops out at 550MB/s, while PCIe 2.0-based SATA
Express can deliver up to 800MB/s of bandwidth. SATA Express implemented
using PCIe 3.0 would deliver up to 1600GB/s of storage bandwidth. This
isn’t necessarily desirable at this point, as it would divert valued
PCIe 3.0 lanes from the graphics card, but it shows the long-term
scaling potential. This standard could allow for drives four to five
times faster than the current top-end storage solutions available today,
and it would scale seamlessly with future PCI-Express versions.
Will the new standard be backwards-compatible?
Yes,
with appropriate plug interfaces. Exactly how this will play out is a
bit muddled and will vary from manufacturer to manufacturer. The SATA
Express standard supports both PCIe-based and SATA-based signaling in
the same protocol and manufacturers are expected to create adapters that
allow customers to plug current-generation SATA SSDs into SATA Express
connectors. Current SSDs that plug into SATA Express ports will still
use conventional SATA — you won’t be able to accelerate a modern SSD by
hooking it up to a SATA Express port.
We’ll start to see
controllers that support SATA Express this year and into 2015, but
motherboard and shipping drive support remains questionable. As with
SATA 6G, we’ll see edge support from third-party manufacturers first,
with integration into AMD and Intel chipsets following a year or two
later. With USB 3.0 and SATA 6G, the third-party controllers did a
fairly good job, but Intel’s controllers still managed to wind up in the
top spot, using the least amount of CPU time and delivering the highest
level of performance.
Performance tests performed by Anandtech
show that SATA Express can deliver full PCIe performance over cables —
an impressive feat considering its early days for hardware availability.
Do we need SATA Express in the first place?
SATA
Express is widely expected to find use in the desktop client
environment; enterprise will be much slower to move (PCI-Express based
storage is already entrenched ) and mobile already has the M.2 standard.
As Kristian Vättö says, the value of SATA-Express when desktops already
have plenty of PCIe slots seems limited. This is a fair point, so long
as we’re only considering mainstream desktop form factors.
The
problem is, the future of the desktop market — to the extent that one
exists — isn’t based in mainstream form factors. Intel continues to
refine and polish its Next Unit of Computing. Gigabyte has new SFFs based on AMD and Intel platforms, as does Zotac. AMD pushed the A8-7600
out for its Kaveri launch precisely because it believes that 45-65W
performance is the future of computing. It’s own next-generation Carrizo
reportedly targets a 65W form factor.
In other words, the market
is moving towards system configurations with fewer and fewer PCI-Express
slots. That doesn’t mean desktop workstations are going to cease
existing — there will always be a market for users who want a huge
number of cores or maximum frequency — but it means that the concept of
“desktop” is getting smaller for an awful lot of people.
This, in turn, suggests that SATA Express actually offers a way to use
the PCI-E lanes on modern chipsets. It may never make sense for
high-end desktops, but it could offer a way to squeeze PCIe performance
into a mini-ITX form factor. That’s where the new standard is likely to
shine — particularly if SATA Express SSDs sell at a lower price point
than PCI-Express-based storage.