Laptop SSD Performance Degradation Problems

Flash SSDs in laptops have generated considerable excitement over the last year and are in use at both extremes of the laptop market. At the very low end, where only very small storage amounts can be funded, NAND Flash is below the below the disk price floor. Mechanical disks with all their complexity are very difficult to manufacture for less than $30 each. What this means is that for very small storage quantities, NAND Flash storage can actually be cheaper than mechanical disk drives even though the price per GB for Flash is larger. That’s why the One Laptop Per Child project uses NAND flash for persistent storage. At the high end of the market, NAND flash considerably more expensive than disk but, for the premium price, offers much higher performance, more resilience to shock and high G handling, and longer battery life.

Recently there have been many reports of high-end SSD laptop performance problems. Digging deeper, this is driven by two factors: 1) gen 1 SSDS produce very good read performance but aren’t particularly good on random write workloads, and 2) performance degradation over time. The first factor can be seen clearly in this performance study using SQLIO: The poor random write performance issue is very solvable using better Flash wear leveling algorithms, reserving more space (more on this later), and capacitor backed DRAM staging areas. In fact STEC ZeusIOPS is producing great performance numbers today, Fusion IO is reporting great numbers, and many others are coming. The first problem, that of poor random write performance, can be solved and these solutions will migrate down to the commodity drives.

The second problem, the performance degradation issue, is more interesting. There have been many reports of laptop dissatisfaction and very high return rates: Returns, technical problems high with flash-based notebooks. Dell has refuted these claims Dell: Flash notebooks are working fine but there are lingering anecdotal complaints of degrading performance. I’ve heard it enough myself that I decided to dig deeper. I chatted off the record with an industry insider on why SSDs appear to degrade over time. Here’s what I learned (released with their permission):

On a pristine NAND SSD made of quality silicon to ensure write amplification remaining at 1 [jrh: write amplification refers to the additional writes that are caused by a single write due to wear leveling and the Flash erase block sizes being considerably larger than the write page size – the goal is to get this as close to 1 as possible where 1 is no write amplification], given a not-so-primitive controller and reasonable over-provisioning (greater than 25%), a sparsely used volume (less than half full at any time) will not start showing perceptible degraded performance for a long time (perhaps as long as 5 years, the projected warranty period to be given to these SSD products).

If any of the above conditions is changed, the write amplification will quickly degrade ranging from 2 to 5, or even higher. That contributes to the early start of perceptible degraded write performance. That is, on a fairly full SSD you’d start having perceptible write performance problems more quickly, and so on.

Inexpensive (cheap?) SSD made of low-quality silicon will likely to have more read errors. Error correction techniques will still guarantee correct information being returned on reads. However, each time a read error is detected, the whole “block” of data will have to be relocated elsewhere on the device. A not-so-well designed controller firmware will worsen the read delay, due to poorly implemented algorithms and ill-conceived space layout that take longer to search for available space for the relocated data, away from the read error area.

If the read-error-data-relocation happens to collide with the negative conditions that plague the write performance above, you’d start seeing overall degraded performance very quickly.

Chkdsk may have contributed to the forced relocation of the data away from where read errors occurred, hence improving the SSD performance (for a while) until the above collisions happen. Perhaps the same when Defrag is used.

In short, performance degradation over time is unavoidable with SSD devices. It’s a matter of how soon it kicks in and how bad it gets; and it varies across designs.

We expect the enterprise class SSD devices to be as much as 100% over-provisioned (e.g., a 64GB SSD actually holds 128GB of flash silicon).

Summary: there are two factors in play. The first is that SSD write random performance is not great on low end parts so ensure you understand the random write I/O specification before spending on an SSD. The second one is more insidious in that, in this failure mode, the performance just degrade slowly over time. The best way to avoid this phenomena is to 2x over-provision. If you buy N bytes of SSD, don’t use more than ½N and consider either chkdsk or copying the data off, bulk erasing, and sequentially copying back on . We know over-provisioning is effective. The later techniques are unproven but seem likely to work. I’ll report supporting performance studies or vendor reports when either surface.


James Hamilton, Windows Live Platform Services
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