Nikkei Electronics Asia -- June 2009
[Feature] SSDs Challenge HDDs, but Quality a Problem [Part 1]

Jun 1, 2009 16:58 Nikkei Electronics Asia

Solid-state drives (SSD) using NAND Flash memory are beginning to show up in a variety of equipment as storage devices. Prices are dropping as semiconductor technology continues to evolve, accelerating adoption of the drives, but manufacturers must also deal with eroding quality.

Internet search service baidu.com is operated by Baidu Inc of China, and according to comScore Inc of the US, achieves the third-largest number of searches worldwide. It serves at least 200 million individual users in China, and is continuing to expand, such as through the launch the baidu.jp search service in Japan in Jan 2008. One of the reasons baidu.com is able to attract so many users is the speed with which it returns search results.

The key to achieving such speed is its use of solid-state drives (SSD). These storage devices make use of NAND Flash memory, and come with the same interfaces as hard disk drives (HDD). Access is extremely fast, however, because they have no mechanical parts - unlike HDDs, which require motors, heads, arms and the like. As of 2009, says Takehito Soeda, marketing director at Baidu Japan Inc of Japan, "We plan to use SSDs for several tenths of all search servers, and 100% of Web search servers."

SSDs Add Value

Baidu's decision is only the tip of the iceberg: SSDs are being adopted for use in all types of equipment that formerly relied on HDDs, from data centers and servers to disk arrays and industrial machinery (Fig 1). In all of these diverse applications, however, SSDs add value to the product by improving performance, reducing power consumption and cutting cost.

Baidu's decision to switch to SSDs is based on its recognition that SSDs are the key to boosting data center performance (Fig 2). The company selected SSDs after analyzing existing data center requirements and reviewing technology trend forecasts for the future. According to Baidu Japan's Soeda, "The volume of data handled by the search engines has grown enormously, and the volume of data to be processed per unit time is still growing at a frightening pace. We don't expect the pace to slacken any, either."

Baidu took a long, hard look at just what type of storage system would be optimal for the needs of the era. Input/output (I/O) performance is low on a per-HDD basis, and the only way to cope with surging data volume would be to install massive numbers of HDDs running in parallel. "We also considered whether this increased scale was the best solution for the future," reveals Soeda. "It became apparent that SSDs, with I/O performance tens of times faster than HDDs, were very promising."

Disk Array Processing

Other companies are also beginning to replace the HDDs in some disk arrays for corporate applications with SSDs to improve performance: Hitachi Ltd of Japan and Hewlett-Packard Co (HP) of the US, for example. HP started using SSDs in its StorageWorks XP24000 and XP20000 in Nov 2008, and in Jan 2009 Hitachi followed suit by mounting SSDs in its Hitachi Universal Storage Platform V and VM.

The use of SSDs provides a significant improvement in disk array processing performance (Fig 3). Akinobu Shimada, department manager, Products Planning Department, Strategic Business Planning of Hitachi Ltd, Disk Array Systems Division explains: "In applications where access tends to concentrate in particular timeframes, such as bank account databases, stock dealing systems, and seat reservation systems for air or rail transportation, for example, even a slight delay can result in customer complaints. Formerly we relied on distributed processing with multiple HDDs, but now we're replacing some of the HDDs with SSDs, and letting the SSDs handle the heavy loads. The total number of drives can be reduced, which means a corresponding reduction in power consumption, floor space and other resources."

SSDs for Cache Memory

Sun Microsystems Inc of the US is putting SSDs in its server systems to achieve significant cost reductions. The Sun Storage 7410 and 7210 server storage systems announced in Nov 2008 were released with SSDs (Fig 4), but they are used a bit differently than the HDD-alternative usage of companies like Baidu, Hitachi or HP. Sun Microsystems has positioned them as a memory layer between dynamic random-access memory (DRAM; main memory) and the HDD (the final storage device), as shown in Fig 4b.

By using SSDs as cache memory, less-expensive HDDs can be used in the product, because a large-capacity SSD cache will improve the cache hit rate (Note 1). A high cache hit rate reduces the frequency of HDD access, allowing the HDDs used to be less expensive than those normally demanded by server applications. To be specific, a serial ATA (SATA) HDD is sufficient for the task.

Note 1: The max capacity of the SSD in the Sun Storage 7410 is 144GB (18GB x 8 units) for write and 600GB (100GB x 6 units) for read. This configuration provides a significant improvement in data read/write performance.

Sun Microsystems' past server systems used expensive, high-speed HDDs turning at about 15,000 rpm, and capable of using high-speed interfaces like Serial Attached Small Computer System Interface (SCSI), generally abbreviated to SAS, or Fibre Channel, for example. Compared to SATA HDDs, SAS-capable HDDs are about double the price per gigabyte.

By using SSDs as cache memory, says Kenichiro Matoba, product manager, x86 System Sales, Systems Practice of Sun Microsystems KK of Japan, "We were able to cut per-gig storage cost in half, and judging from the reaction of our customers, it's a resounding success."

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