Some Eee PCs have an SSD reliability issue

Tags: Asus Eee PC ssd flash shrinking problem
The first mass produced notebook with flash memory as a primary storage device takes its first steps.

It's been the fear of tech forums everywhere, solid state storage failure. Commercial grade flash memory chips typically have a threshold of 100,000 write/erase cycles per block, after which it could become unusable due to an increased error count while reading. This is a mean value, some fail after, some before. Reads also take a toll on that value, but they are almost negligible when compared to an erase cycle.

This has raised the question as to whether or not NAND is "ready" for high write operations. Is flash memory reliable enough to replace magnetic media?

The ASUS Eee PC is the first multi-purpose computing device available en masse that employs storage based solely on Flash memory, commonly known as a solid state drive (SSD). Being a pioneer, it will be an excellent test subject and a testimonial to the durability of flash based drives for use as an HDD replacement. Other flash based devic es don't have the same kind of usage patterns and therefore aren't good for this purpose.

Since the Eee PC was released in November, one would think it is still quite early to expect some relevant data, but turns out that isn't true. Some users have already reported unusual behavior from their drives; some claim the main volume sizes have shrunk by as much as 500MB. All cases revolve around Eee PCs that feature SLC Hynix HY27UG088G NAND chips.

Hynix advertises that its memory performs wear-leveling on-chip -- a technique used to even out memory cell usage so the flash memory does not write particular cells more than others. Typically, if bad blocks are detected in this wear leveling, they are removed from the address table, resulting in fewer blocks in the drive volume.

ASUS seems confident the isolated reports are not systemic. In conversation with Kristopher Kubicki, from Dailytech, ASUS spokesman Randy Chang told that the defect rate for the solid state storage devices on the Eee are lower than that of similar devices with traditional rotational media. In addition, Chang assures us that any defective SSD problems, including shrinking volume sizes, are covered by ASUS warranty.

Indeed, a quick search on the ASUS forums details only a handful of new posts about defective drives.

Current 8GB Eee models use Mini PCI-Express SSD cards instead of onboard chips. These can be removed and replaced by ASUS staff quickly if the drive fails. ASUS would not comment about why it chose to not solder its storage directly onto the motherboard, saving precious space, but a clever forum poster points out that its easier to replace defective solid-state memory when its in in a PCIe adaptor.

One thing is clear though, with ASUS pledging to sell nearly four million Eees this year, the company is fairly confident in the ability of NAND memory and high write scenarios. Apple and Lenovo both pledge to sell millions more NAND-based notebooks this year too: the Airbook and X300.


Anonymous said...

This article is the tip of the iceberg, but great job.

ASUS SSD life expectance depends how much free space remains on the SSD, plus a small percentage of the total size of files written over a period of time. Complicated.

Double SSD lifetime by doubling the free space on the SSD. Simple.

Say what? Yep, it is that simple.

Why? Research "dynamic" vs "static" SSD wear leveling. Consider, with dynamic wear leveling, the effects of re-writing a single file on an SSD disk with X (small) MB of free space. Consider the impact of writing other files, allowing the single file to migrate to Y MB other free space. Good luck.

Tiago Marques said...

Indeed. I have checked that some implementations of wear levelling do move the data in the background - so the drive doesn't get a performance hit - while it also tries to maintain ALL the blocks at the same wear level. In the case, SSDs are very durable drives.

Since most manufacturers(of the SSD controller) don't disclose the principles of ther wear-levelling algorithms, it's mostly impossible to know which one are good and which one's aren't.

Along with a list of those manufacturers, I would like to also publish a follow up to this one, like you suggested.
That will take some time, as I'm still cooking some articles and revamping the whole website.

Best regards.

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