Hard disk vs. solid-state drive

By , Computerworld |  Storage, HDD, SSD

Solid-state disk (SSD) drives are all the rage among techies. The drives use non-volatile NAND flash memory, meaning there are no moving parts. Because there is no actuator arm and read/write head that must seek out data on a platter like on a hard disk drive (HDD), they are faster in reading and, in most cases, writing data.

But SSDs are also much more expensive than their hard-disk drive (HDD) counterparts, which offer 300GB of capacity or more for less than $100.

Most consumer-grade SSDs from leading vendors now cost around $3 per gigabyte, while traditional hard drives cost about 20 to 30 cents per gigabyte for 2.5-in. laptop drives and 10 to 20 cents per gigabyte for 3.5-in. desktop drives, according storage market research firm Coughlin Associates Inc. In other words, even the cheapest 120GB SSDs are going to be around $300, though some are available on sale for less. So should you buy a high-capacity HDD for little cash or plunk down hundreds of dollars more for a fast, but lower-capacity, SSD? Or, should you wait?

Coughlin Associates founder Tom Coughlin said per-gigabyte prices for HDDs and SSDs are dropping at the same pace -- about 50% per year -- so the sizeble price gap between the two will remain for years to come.

Gregory Wong, a solid-state drive analyst with market research firm Forward Insights agreed.

"I think the issue with SSD adoption is that prices have not been favorable," he said. "And there's still going to be a gap between HDD and SSD prices, even five years from now."

The average consumer in the market for a desktop or laptop doesn't pay a lot of attention to drive I/O speeds -- he tends to focus more on capacity. So when a drive offers less capacity, even when it's an SSD, it's apt to get passed over, according to Wong.

A lot depends on how you expect to use your computer. If you're a college student writing papers and surfing the Internet for information, the advantages of an SSD are negligible, but if you're downloading video and using multiple applications at the same time, an SSD will give you a very noticeable performance boost, Wong said.

Head to head: SSD vs. HDD

I decided to test two popular 2.5-in. laptop drives -- a Seagate Momentus 7200.4 500GB HDD and an OCZ Vertex Series SATA II SSD -- to determine the advantages each offers. I tested their impact on battery life, the read and write speeds, cold boot-ups and restarts, and CPU utilization.

I briefly considered pitting the SSD against a higher-end, 10,000-rpm Western Digital VelociRaptor as the test unit for the hard disk drive or using the 2.5-in Intel X25-E for the solid-state drive unit. The VelociRaptor yielded a 105MB/sec random read rate in Computerworld's testing and the X25-E blew by everything with a 250MB/sec peak read rate. But I wanted to evaluate something more accessible price-wise for the average consumer, someone who generally wouldn't normally wouldn't consider a top-end drive for her laptop or desktop.

Besides, even the VelociRaptor is no match for a decent SSD.

As you'd expect, the two drives I tested have vastly different prices. The Seagate Momentus HDD (model - ST9500420AS) will run you between $127 and $140, while the OCZ 120GB Vertex SSD goes for between $376 and $400. Both drives use a SATA 3GB/sec interface and both use cache to increase write performance. The OCZ has 64MB of cache, the Seagate, 16MB of cache.

SSDs are naturally more rugged than HDDs because they have no moving parts. OCZ claims its Vertex drive can sustain up to 1,500 Gs of shock before sustaining damage or a drop in performance. Seagate claims its Momentus drive can withstand up to 350 Gs while operating and 800 Gs when turned off.

The Vertex is OCZ's second iteration of an SSD, and it uses 64MB of cache to artificially enhance the write performance and a more advanced Indillinx controller than its slower predecessor, the OCZ Apex Series SSD, which uses a controller from JMicron and has no cache memory. The Vertex drive's packaging lists a maximum read rate of 250MB/sec and a sustained write rate of 100MB/sec. It also claims a 1.5 million-hour mean time between failure (MTBF) rate, if MTBF can actually be applied accurately to an SSD. Most experts don't believe it can.

(Keep in mind that most SSD vendors publish sequential read/write rates, which are much faster than random I/O. But most operations on a desktop or laptop are random. For example, file systems and e-mail applications mostly use random operations, while system boot up or copying a large file from a USB drive involves sequential operations. So, in general, don't believe the packaging hype.)

The Seagate Momentus 7200.4 marketing material offers no read/write rates, nor does Seagate offer any information other than a seek time on its Web site: 11 milliseconds for reads and 13 milliseconds for writes. Seagate doesn't use MTBF, preferring its own annualized failure rate (AFR) metric as a method to gauge drive reliability, which is .5%.

The SSD easily beats the HDD in weight. Seagate's Momentus weighs 3.85 ounces; OCZ's drive weighs 2.7 ounces.

Performance tests

Not surprisingly, the Vertex SSD handily beat the Seagate HDD for cold boots: 20 seconds to start up Windows XP for the OCZ and 40 seconds for the Seagate. The SSD also beat the HDD for restarts: 26 seconds versus 37 seconds.

(While it may seem odd that the Seagate drive performed better on a restart than on a cold boot, keep in mind that the drive is still spinning and plenty of OS data is still residing in memory. The drive also has native command queuing (NCQ), which allows its controller to prefetch data in order to access it more quickly on reboots. It works in the same way a grocery list helps you find products as you enter the store. OCZ's Vertex drive with Indillinx controller also has NCQ.)

When it came to I/O speed, there was no match. I used ATTO Technology's ATTO Disk Benchmark v2.3.4 and Simpli Software's HD Tach v3.0.4 benchmarking utilities to perform my read/write performance tests. The ATTO benchmark software showed the OCZ had a read time of 244MB/sec and a write time of 172MB/sec. The Seagate HDD had an average read rate of 98MB/sec and a write time of 87MB/sec.

Using HDTach, the read/write results were quite different. OCZ's drive showed a 196MB/sec read rate, the Seagate, 84.6MB/sec.

The HDTach software also measures CPU utilization and random access times. OCZ's drive had a random access time of .2 milliseconds; Seagate's 16.9 milliseconds. While Seagate's slower random access time wasn't surprising, I was surprised that it actually beat the OCZ drive on CPU utilization: the OCZ SSD used 8%; the Seagate HDD used 5%.

Next I transferred a 1GB folder filled with photos and video files to the drives from a USB drive. Both the SSD and the HDD accomplished the file transfer in about 50 seconds (the Seagate was 2 seconds slower).

For the battery test, I used MobileMark 2007 benchmarking software from Business Applications Performance Corp. (BAPCo). The software simulates more than a dozen programs that people use in everyday life, so it's considered a very accurate measurement of power consumption, and the results from this test were the biggest surprise of all. The battery lasted 132 minutes when powering the Seagate drive and 137 when powering the OCZ -- only a five-minute power difference.

Conclusions

While the SSD outperformed the HDD in most benchmarking tests, as well as handily beating out the competition for boot-ups, whether or not as a consumer you should choose an SSD over a HDD will depend on your needs. HDDs, especially those with 7,200-rpm spindle speeds or higher, offer respectable read/write rates and vastly higher capacity levels.

Typical notebook or desktop users probably won't notice a big difference between an SSD drive and a traditional hard disk drive other than a faster boot-up and quicker application-launch times.

According to Jim Handy, who co-authored a report about digital storage in consumer electronics, SSD will continue to dominate in small handheld devices because the cost to produce flash memory-based drives is significantly cheaper than hard disk drives. So in some cases, SSDs make sense for products such as MP3 players that can store large numbers of compressed music files on drives with relatively small capacities.

"You can buy a $50 HDD with 120GB of storage. A 120GB SSD will set you back around $250-plus. You can buy $30 worth of flash, though -- as long as 16GB is enough for your needs," said Handy, an analyst with Objective Analysis Inc. "That way you can pocket the other $20."

But for laptops and desktops, where consumers will continue to seek as much capacity as money can buy, Handy said SSD adoption will likely suffer for years to come.

The final word: For most users, this a good time to consider buying a higher-end HDD that should deliver more-than-enough performance -- and plenty of room to grow -- while you wait for SSD prices to drop further.

That could be a long wait.

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