Measuring The 256 GB Vector's Write Endurance
Endurance: Kind Of A Big Deal
Write endurance is a term often thrown around in discussions of solid-state storage because we all worry about that point when an SSD is no longer able to reliably hold our data (even though very few of us have actually seen such a thing happen).
If you have an SSD in your notebook or mainstream desktop, write endurance isn't something that should keep you up at night. It's highly improbable that you'll ever write enough data per day to exhaust the rated life space of the NAND flash cells composing your drive. Both Micron and Intel estimate that the average desktop user writes between 7-10 GB worth of information per day. Even if you use an exaggerated number, basic math assures us that you almost can't render your SSD useless within its warranty period. If you were to run into a reliability issue, it'd be far more likely to come from some sort of firmware-oriented bug.
How the rated write endurance of NAND changes at each new lithography node is interesting to track, though, as are the ways vendors address the physical realities of NAND wear-out. OCZ tells us that the Vector leverages 25 nm ONFi 2.x-compliant NAND from IMFT. However, we've seen this flash rated at 3000 and 5000 P/E cycles, and we're curious to know which specification applies to the Vector.
Our estimates come from monitoring each drive's media wear indicator (referred to as the MWI), which counts down from 100 to 1. Because the number of program-erase cycles a NAND cell can withstand is finite, the MWI is designed to facilitate a rough estimate of endurance.
In theory, once you reach the end of the counter, all of the memory's rated P/E cycles are exhausted. That's not to say something bad happens when you hit the bottom, but nobody wants to entrust irreplaceable data to a drive operating on borrowed time, either. Naturally, enterprises place a lot of importance on the MWI because it represents a "safe zone."
Fewer Cycles, In Theory; Does That Matter?
|Sequential Workload, Queue Depth=1, 2 MB|
|Row 0 - Cell 0||Intel SSD 320||OCZ Vector||Samsung 840 Pro|
|NAND Type||Intel 25 nm MLC||Intel 25 nm MLC||Samsung 21 nm MLC|
|RAW NAND Capacity||320 GB||256 GB||512 GB|
|IDEMA Capacity (User-Accessible)||300 GB||256 GB||512 GB|
|P/E Cycles Observed (IDEMA)||5460||2965||2512|
|P/E Cycles Observed (Raw)||5119||2965||2512|
|Host Writes per 1% of MWI||16.38 TB||7.59 TB||12.86 TB|
Our analysis places the endurance of a 256 GB Vector somewhere around 759 TB using sequential writes. If the MWI is to be believed, a bit of math ([Host Writes per 1% of MWI * 100] / Capacity) tells us we're dealing with NAND rated at 3000 P/E cycles. This isn't the best stuff we've seen, but we're really not worried about it either, particularly at a 256 GB capacity point. If we assume you perform 10 GB of writes per day, it’d take more than 200 years to wear down the drive's NAND cells using our workload.
But the story doesn't end there. Endurance ratings apply to each flash cell. But because higher-capacity SSDs simply have more on-board flash, it takes longer to write across all of their cells, yielding a higher endurance rating. Now, somewhat strangely, OCZ gives us just one endurance number for all of the drives in its Vector family: 36.5 TB of writes. This comes out to about 20 GB of writes per day for five years (the duration of the warranty).
Naturally, this raises some questions. OCZ's 36.5 TB figure is a lot less than the 759 TB we derived, and 20 GB of writes per day is a lot more than the 7-10 GB we consider to be "average."
Our endurance rating tries to isolate the endurance of the NAND itself by using a pattern whereby host writes match NAND writes. In the real-world, host writes consist of multiple transfer sizes at various queue depths. As a result, the amount of data you write on the host tends to be a fraction of what's actually written to the NAND. This phenomenon is know as write amplification (NAND writes/host writes). On the 256 GB Vector, a 759 TB endurance rating with a write amplification of 1x is equivalent to 36.5 TB at a write amplification of ~20x.
Rarely does write amplification get that high, though. Here's an example after writing a mixture of random and sequential data:
On an SSD 520, write amplification reaches as high as 2.9x when it's faced with completely incompressible files. In a best-case scenario, SandForce's technology compresses the data so that NAND writes are represented by just 17% of the space requested by the host.
|Impact of Random Accesses On Write Endurance|
|Workload Ratio: 35% 128 KB Sequential, 65% 4 KB Random128 KB Sequential: 66% Reads, 34% Writes4 KB Random: 66% Reads, 34% WritesFull Span, QD=1, ~3 Hours||Write Amplification|
|Intel SSD 520, Incompressible Data||2.9x|
|Intel SSD 520, Compressible Data||0.17x|
Clearly, OCZ's Barefoot 3 controller does not employ compression the same way SandForce's logic does. In theory, then, write amplification is, at best, 1x across a large sequential write. The real-world isn't limited to one type of access pattern, though. You end up with a mix of sequential and random I/O. Even when we mix things up a bit, we see write amplification as high as 2.8x. Even in that situation, it'd take ~40 years to exhaust the 256 GB Vector's endurance, assuming 10 GB of writes per day. Since ratings still scale with capacity, even the 128 GB Vector should demonstrate 20 years of endurance. Providing you're not intentionally taxing the drive the way we do, you shouldn't have any issues within the five-year warranty OCZ provides.