Micron M500DC (480GB & 800GB) Reviewby Kristian Vättö on April 22, 2014 2:35 PM EST
Endurance Ratings: How They Are Calculated
One of the questions I quite often face is about the manufacturers' endurance ratings. Go back two or three years and nobody had any endurance limits in their client SSDs but every SSD released in the past year or so has an endurance limitation associated with it. Why did that happen? Let's open up the situation a bit.
A few years ago, many enterprises would just go and buy regular consumer SSDs and use them in their servers. Generally there is nothing wrong with that because there are scenarios where enterprises can get by with client-grade hardware, but the problem was that a share of the enterprises knew that the drives weren't durable enough for their needs. However, they also knew that if they wore out the drive before the warranty ran out, the manufacturer would have to replace it.
Obviously that wasn't very good business for the manufacturers because for one drive sold, more than one had to be given away for free. At the same time less customers were buying the more expensive, high profit enterprise drives. Without disrupting the client market by either increasing prices or reducing quality, the manufacturers decided to start including a maximum endurance rating, which would invalidate the warranty if exceeded.
The equation for endurance is rather simple. All you need to take into account is the capacity of the drive, the P/E cycles of the NAND and the wear leveling and write amplification factors. When all that is put into an equation, it looks like this:
Notice that the correct term for TBW is TeraBytes Written, not TotalBytes Written although both are fairly widely used. The hardest part in calculating the TBW is figuring out the wear leveling and write amplification factors because these are workload depedent. Hence manufacturers often use a worst case 4KB random write scenario to come up with the TBW figure as this ensures that the end-user cannot have a more demanding workload with higher write amplification.
For the uninitiated, the wear leveling factor (WLF) in this context means the maximum stress that the wear leveling method would put onto the most heavily cycled block compared to the average number of cycles. A factor of two would mean that the most heavily cycled block would have twice the number of cycles compared to the average. Write amplification factor (WAF), on the other hand, refers to the ratio of host and NAND writes. A factor of two would in this case mean that for every megabyte that the host writes, two megabytes are written to the NAND. These two factors go hand in hand in the sense that a small WLF results in higher WAF because the drive will do more internal reorganization operations to cycle all blocks equally, which consumes NAND writes.
The interesting part about TBWs is that they actually give us a way to estimate the combined wear leveling and write amplification factor of the drive. In the case of 120GB M500DC, that would be a surprising 0.72x. Obviously you can go lower than 1x without using some form of compression but the 120GB M500DC actually has 192GiB of NAND onboard that extends the endurance. If we used that figure to calculate the combined WLF and WAF, it would be 1.24x, which is much more reasonable. For some reason the JEDEC spec defines the capacity as the usable capacity even for endurance calculations but in the end it doesn't matter what figure you change as they are all related to each other (e.g. with 120GB used as the capacity, the P/E cycles could be higher than 3,000 because the over-provisioned NAND adds cycles).
Ultimately none of the manufacturers are willing to disclose the exact details of how they calculate their endurance ratings but at the high-level this is how it's done according to JEDEC's standards. Furthermore, I wouldn't rule out the possibility that some OEMs artificially lower the ratings for their consumer drives just to make sure they are not used by enterprises. In the end, there isn't really a way for us to find out whether the TBW is accurate or not since the efficiency factors are not easily measurable by third parties like us.
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Solid State Brain - Tuesday, April 22, 2014 - linkEnterprise SSDs usually have their endurance rated at 3 months of residual data retention capability, vs 1 year for consumer models. Since data retention time decreases with NAND wear, this allows manufacturers to claim almost "for free" a higher endurance than what the P/E limit for consumer NAND memory would suggest, even though it might be the exact same memory (but different requirements).
Most likely, the rated endurance for these drives is at a much higher number of P/E cycles than 3000.
Kristian Vättö - Tuesday, April 22, 2014 - link"Most likely, the rated endurance for these drives is at a much higher number of P/E cycles than 3000."
I don't think that is necessarily the case. If you look at my calculations on the "Endurance Ratings" page, the combined WLF and WAF is already only 1.24x when using the raw NAND capacity to calculate the endurance at 3,000 P/E cycles. 1.24x is excellent, so I find it hard to believe that the NAND would be rated at higher than 3,000 cycles as the combined WLF and WAF would essentially be about 1.00 in that case (which is basically impossible without compression). Also, Micron specifically said that this is a 3,000 P/E cycle part.
Solid State Brain - Tuesday, April 22, 2014 - linkAs the endurance rating for enterprise drives is usually intended for a typical steady random workload (and no trim to help), the write amplification factor should be higher than the rather low value you used for your calculation. You can see that endurance figures (not just in this case, but most other other enterprise drives as well) start make more sense when actual P/E cycles for that usage/application are higher than their consumer counterparts.
Here's a prominent example. You could try the same calculation here. In this specification sheet for 2013 Samsung enterprise drives, which includes a model with TLC NAND, it's dead obvious that the rated P/E cycles limit of consumer drives (unofficially, rated 1000 cycles) doesn't apply for them, even though for the low end models they're most certainly using the same memory. You never really see a fixed P/E cycles limit for enterprise drives as in the end is the TBW figure that counts and the shorter data retention requirement for them helps boosting that even though there might actually not be any hardware difference at all.
apudapus - Tuesday, April 22, 2014 - linkThe specs you linked definitely show 1000 P/E cycles for all the NAND on all the drives, TLC and MLC. I used this formula: Total Bytes Written Allowed = NAND P/E cycles * Total Bytes Written per Day
Enterprise drives have lower data retention requirements because in the enterprise space, drives will be read and written to more frequently and will not be powered off for extended periods of time. Consumer drives on the other hand can have a lots of down time.
Solid State Brain - Tuesday, April 22, 2014 - linkPM843, TLC NAND rated 1000 P/E cycles on the consumer version. Let's take the 120GB model as an example.
Endurance with 100% sequential workloads: 207 TB
1000 P/E (NAND life @ 1 year of data retention, on the consumer version) * 128 GiB (physical NAND capacity) = 128000 GB = 125 TiB. This drive doesn't make use of data compression. With sequential workloads the best case write amplification would therefore be 1.0x. To reach the claimed 207 TiB of terabytes written of endurance, the NAND memory on this drive would need at the very least to endure 1000/125*207 = 1656 P/E cycles, again assuming the best case write amplification factor. One can expect this to be at least around 1.15-1.20x under real world scenarios, which would bring this figure to about 1900-2000 P/E cycles.
SM843, the enterprise version of the 840 Pro with 3000 P/E cycles MLC NAND. Again, let's take the 120GB version for reference.
Stated endurance with 100% sequential workloads: 1 PB
128 GiB physical capacity * 3000 P/E cycles = 375 TiB
Actual P/E cycles needed for 1 PB at 1.0x write amplification: 3000 * 1024/375 = 8192
Kristian Vättö - Wednesday, April 23, 2014 - linkLike I said, ultimately it's impossible to figure out where exactly the endurance is coming from. It's likely that the NAND could be rated higher thanks to the looser retention requirements (3 months vs 1 year) in the enterprise space but then again, figuring out the exact P/E cycle count isn't easy because we don't know the write amplification.
Solid State Brain - Wednesday, April 23, 2014 - linkIf you have spare time and still have the drives you could try applying a standard sustained 4kB random load for an extended period of time to figure out what the write amplification for these drives with that usage is. Marvell-based SSDs usually provide, in a way or another, both NAND writes and Host writes among their monitoring attributes, and with these data it's pretty straightforward to calculate it. Given the large OP area, I predict it will end up being somewhere around 2.5x.
Kristian Vättö - Wednesday, April 23, 2014 - linkI still have the drives but there are other products in the review queue. I'll see what I can do -- the process is rather simple as you outlined I've done some similar testing in the past too.
Kristian Vättö - Wednesday, April 23, 2014 - link*and I've done similar testing in the past too.
(Yes, we need an edit button)
apudapus - Wednesday, April 23, 2014 - linkOIC. My best guess is that the voltage thresholding (their ARM/OR) extends the life of the NAND.