SSD Fears Remedied By SAS, Flash

The performance of solid-state drives (SSDs) has been limited by SAS interface capacity, but 12G SAS and enhanced flash promise to address those issues.

Jim O'Reilly

May 13, 2013

3 Min Read
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The SSD and flash space has suffered from a lack of user trust when it comes to wearout and exceeding interface bandwidth needs. Two product announcements incorporating SAS and enhanced flash address these issues and indicate that vendors are responding to enterprise expectations

The performance of solid-state drives (SSDs) is phenomenal, but the flip side of this story is that the relatively new 6G SAS interface is already inadequate to meet the bandwidth possible from current drives, never mind products just months away.

One solution to the problem is to extend PCIe out to the drive, and so cut out a layer of hardware and logic. This effort is coupled with a new protocol that uses a new software stack to replace the SCSI/SAS driver architecture we've used for years.

Now the SAS teams are fighting back. With limited availability of 12G SAS gear from LSI, HGST, a subsidiary of Western Digital, has announced the first 12G SAS SSD, and provided some specs and benchmarks.

This family is a real performer. Ranging up to a terabyte in capacity, they can move 1.2 Gbit/s reading sequentially, which is twice the performance of their 6G SAS predecessors. Clearly, the interface was a major bottleneck, and we can expect all of the enterprise-class vendors and wannabees to follow suit as quickly as they can.

HGST enjoys much higher capacities than the Micron products, but Micron's PCIe SSD family still has an edge in performance, with 40 percent faster random and sequential operation, though the gap has closed a lot. I suspect that protocol differences and interrupt handling are now the big issues, and HGST must be thinking of how to merge ideas from NVMe into the SAS stack.

HGST addressed the other issue, wearout, by having three grades of unit, all using MLC flash. The most reliable has 100k total write cycles, which is a good step up from the competition, which are matched by HGST's lowest grade of the three at 20k writes. 100k writes is roughly 50 full writes per day for five years, making it plenty adequate for most applications.

This is where the Micron announcement falls behind. Write durability is quoted at 20k write cycles. This is only as good as HGST's lowest grade but is enterprise-class, at 10 full writes per day for five years. However, the Micron drive is still 6G SAS, so it turns in only half the throughput of the HGST unit.

With this durability, we can expect some of the fear of using SSD in the enterprise to vanish. In fact, worldwide SSD shipments are expected to rise to 83 million units this year, up from 39 million in 2012, according to a recent report from IHS iSuppli Research. Shipments are expected to rise 239 million units in 2016, amounting to about 40 percent of the size of the hard disk drive (HDD) market. It's clear that both drives, and especially HGST's, will contribute to a substantially increased uptake of SSD.

There are still some signs of vendor denial in the market. The HDD vendors are still in shock at the SSD assault on their markets, and they don't fully accept that the playing field is tilted against them. What is surprising is that HGST predicts price parity between enterprise SSD and enterprise HDD in 2016, while EMC claims that the ratio will be 8:1.

This may be EMC conditioning the market for its traditional high markup on drives, or it could be a sign that the company that achieved greatness by shipping lots of slow drives is threatened by a mix of fewer SSD and cheap SATA HDD. I suspect HGST is nearer correct, since Micron just shipped a terabyte notebook SSD using new 128 Gbit/s flash chips at a price around $1 per gigabyte.

Any way you look at it, the SSD is going to make much better progress in the enterprise in 2013 and 2014, with a ripple effect throughout the infrastructure.

About the Author(s)

Jim O'Reilly


Jim O'Reilly was Vice President of Engineering at Germane Systems, where he created ruggedized servers and storage for the US submarine fleet. He has also held senior management positions at SGI/Rackable and Verari; was CEO at startups Scalant and CDS; headed operations at PC Brand and Metalithic; and led major divisions of Memorex-Telex and NCR, where his team developed the first SCSI ASIC, now in the Smithsonian. Jim is currently a consultant focused on storage and cloud computing.

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