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When considering to buy a SSD, how should I interpret the different specifications of the SSD?

Here are some specific things that need to be deciphered:

  • Controller (this can affect performance and endurance more than all other factors combined)
  • Bus Technology
  • Form Factor (Physical Size)
  • Capacity
  • NAND or NOR technology
  • Power Consumption during Read, during Write, when Idle
  • Read/Write Burst and Sustained Throughput

All of these things I would like to be explained in more detail and their actual importance in selecting an SSD.

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I wonder how long a SSD can last with intensive R/W operations. I remember USB and Compact Flash could go wrong at some point, due to the Flash technology. For information here some aspects of SSD, including lifetime (2 years ago): imation.com/PageFiles/83/… with this excerpt: "Recall for the SLC NAND flash, which is the present technology used today, P/E cycle endurance is approximately 100,000 cycles, as opposed to MLC which is.." –  bitlocked Jan 31 '11 at 13:49
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3 Answers

I have a couple of laptops with SSDs - a recent MacBook Air and a 64GB Kingston V100 placed into a Lenovo Thinkpad T60p. Both are fast - the Thinkpad is much faster than previously, with boot times below 30 seconds, and much improved battery life.

Your points:

Controller (this can affect performance and endurance more than all other factors combined)

I don't fully agree - if both the drive and the hard drive controller follows SATA 1.5, SATA 3.0 (aka SATA II) or SATA 6.0 (aka SATA III) standard, they'll probably work together: the performance limits in the standard are UPPER LIMITS: many things can make your performance worse. I have not yet found a source of reliable test results allowing SSD performance comparison across brands, devices and OSes.

Bus Technology

Some SATA II devices claim SATA III compatibility but really just take advantage of SATA III's downward compatibility with SATA II. It's easy to see that only few of the spinning SATA III drives actually push more than 3Gb/sec of data. SSDs have an easier time of it, but I've not seen benchmarks showing any pair of SATA III SSDs + SATA III controllers reliably pushing anywhere near 6 Gb/sec.

Form Factor (Physical Size)

Some devices expect a 9.5mm 2.5in drive; the 7mm 2.5in drives don't fit as well. The Kingston drive fit perfectly into the T60p's 9.5mm slot.

Capacity

This seems linearly correlated to price, as you'd expect. Increasingly, SSDs are over-provisioned, where a 64GB drive actually has 72GB addressable. The excess allows for longer drive life in the face of SSD "bits" write cycles being limited to about 100,000. To compensate, SSD controllers move rather than overwrite changing file sectors, in order to balance the limited lifespan. Some OCZ devices destined for servers are reputedly 25% over-provisioned.

NAND or NOR technology

NAND technology appears to have won: NOR technology either isn't as cost-effective, or it is hidden under more meaningful wrappers, such as wear-levelling, mean-time-between-failure (MTBF), and ever higher rates of over-provisioning

Power Consumption during Read, during Write, when Idle

It is difficult to measure this: as with many parameters for electronic devices, you're trusting the reliability of manufacturer vs. the difficulty of measuring these by reviewers or consumers. Having said that, common SSDs are said to consume ~2W during operation, and 0.5W while idle.

Read/Write Burst and Sustained Throughput

Complex: Some SATA II SSDs can't saturate SATA I's 1.5Mb/sec pipe: others can. And you're taking manufacturer's word for it, despite the fact it's possible to measure, with low precision, at least.

But other issues can obscure the results. For example: TRIM, which must be supported by the OS, the controller and the drive. TRIM is a technique to prevent SSDs from experiencing progressively slower write performance over time. Because SSD sectors are written all at once, if a previously used, partially full sector is to be written to, the drive must read the existing sector, add it to the new data in cache, then write the whole sector, then update the file table, which may require the same read-modify-write cycle. This can take some time.

Recently some reviewers tested whether Macs running Snow Leopard really needed TRIM, which isn't supported by Snow Leopard: the reviewers built some seemingly solid tests, and reported some interesting but reasonable results. But other commenters pointed out the researchers assumed Apple's secure formatting tool worked as promised: overwriting every bit of every sector with zeros, even for un-used file space. It turns out that Apple's tool might not actually do this, and if the zero'd status of the tested SSD isn't reliable, maybe the performance tests of new vs. "dirty" disks wasn't reliable, either.

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When looking to buy a new SSD, the absolute best thing to do is look up hardware reviews. Most major reviewers put new storage devices through a plethora of tests, designed to show you how the drive will perform in various scenarios.

You can't just look at a drive's controller and make a conclusion based off of that. Each has it's advantages and disadvantages. In general, you will be most interested in the drive's capacity, IOPS rating (input/output operations per second), and burst/sustained read/write speeds. Most consumer grade SSDs have similar lifetime (read/write) cycles, power consumption, and form factors.

In general, the specifications are what they are. If a drive has a 200 MB/s sustained write speed, you should expect the same. If a drive can do 50,000 IOPS, then you can expect the same. If a drive only uses 1W under load and 0.1W at idle, again, expect the same. There are no alternative interpretations for specifications, this is why they are called specifications.

The implications of these specifications, however, depend only on your particular computer and your particular usage scenarios. And that is why I recommend you look up reviews on any drives you are considering, as these reviews often include real-life usage scenarios so you can gauge how well the drive will perform for your needs.

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Nice advice. But this doesn't tell anything about how to interpret the specifications... –  Tom Wijsman Jan 31 '11 at 14:34
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There's a very good article here on what to look for in an SSD. It's primarily aimed at buyers, but it also provides good insights into how the different specifications relate to the device.

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