SSD performance -- is a slowdown inevitable?

Solid state drives could suffer performance issues after prolonged use

When you compare the size of NAND blocks with the typical write request used by Windows, there's a mismatch because most writes are small. (Mas OS X is less affected by this issue because its write requests are smaller.)

The amount of space required for each new write can vary, but according to Knut Grimsrud, a director of storage architecture in Intel's research and development laboratory, write amplification on many consumer SSDs is anywhere from 15 to 20. That means for every 1MB of data written to the drive, 15MB to 20MBs of space is actually needed.

Read-write algorithms matter

For example, a read-modify-write algorithm in an SSD controller will take a block about to be written to, retrieve any data already in it, mark the block for deletion, redistribute the old data, then lay down the new data in the old block.

"So you had to write that old data back again," said Grimsrud, whose group developed some of the core technology for Intel's SSDs. "None of that is progress in terms of what the user was trying to do with the new data. It was all just overhead. That's the crux of the problem with NAND [memory] management -- all the granularity involved in managing it.

"It's a general issue of all NAND-based SSDs that these are issues that have to be grappled with and it's just a matter of how well manufacturers grapple with it," Grimsrud added.

Because of the limited number of writes and erases an SSD can sustain, manufacturers try to reduce write amplification and reduce overhead. Some use algorithms that combine writes to more efficiently use NAND flash memory space; others use cache to store writes in order to lay them down more efficiently. But details about the techniques used are hard to come by, as each manufacturer considers that technology proprietary.

Intel has addressed write amplification through controller firmware that combines writes to reduce the amount of capacity needed to store data. Intel states that its write amplification is a low 1.1, meaning for every 1MB of data written to the SSD, 1.1MB of capacity is actually used. Another manufacturer, Samsung, pegs the "Wear Acceleration Index" for its SSDs at 1.03, a 3% average overhead for writes.

Many SSD manufacturers also use mean time between (or before) failure (MBTF) on their marketing material, a metric given to hard disk drives that may or may not be accurate. All things being equal, a drive's MTBF all depends on how the drive is used. Intel's X25-M's MTBF is 1.2 million hours, about the same as the average consumer hard-disk drive. To put it another way, Intel predicts its X25-M will last for five years -- assuming 100GB or more of write-erase operations per day.

Much depends on whether an SSD uses MLC or SLC technology. The SLC version of Intel's X25-E 64GB SSD can handle up to 2 petabytes of random writes. By comparison, the MLC-based X25-M can handle only 15TB of random writes over its lifetime. Intel said users should think about it as analogous to a car.

"If you have one car that can go 10,000 miles and another that can go 100,000 miles, how much longer will it last?" said an Intel spokeswoman. "[It] really depends on how much [it's] being used. So that is why [lifespan is based] on the number of sustained random writes. In general..., SLC lasts longer, but the duration of the life expectancy depends on the user's usage model and amount of wear and tear."

Bugs can cause slowdowns, too

Though it's highly regarded, Intel's X25-M SSD had a firmware bug that adjusted the priorities of random and sequential writes, leading to a major fragmentation problem that dropped throughput dramatically. The issue was originally uncovered by PC Perspective after two months of testing. Those tests showed that write speeds dropped from 80MB/sec. to 30MB/sec. over time, and read speeds dropped from 250MB/sec. to 60MB/sec. for some large block writes.

"I supposed if you ran the same tests across many SSDs, most of them have a similar problem...," said Pat Wilkinson, vice president of marketing and business development at SSD vendor STEC Inc.

Algorithms used for wear-leveling are complex and still in their infancy, so while they are likely to improve over time, drive makers cannot eliminate fragmentation all together, McGregor said.

Although Intel acknowledged that all of its SSDs will suffer from reduced performance because of significant fragmentation, the type of write levels needed to reproduce PC Perspective's results aren't likely for everyday users, whether they're running Windows and Apple's Mac OS X. Even so, it still released the firmware upgrade to slow fragmentation.

"The 8820 firmware now services both random and sequential write to ensure that fragmentation does not put the drive in a lower-than-expected performance state," Intel said.

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