Sandforce Life Time Throttling

[johnw]

We saw with the Vertex 2 40GB it was writing at ~6.25MiB/sec with LTT, which works out to ~11 P/E cycles a day (with just a WA of 1.00x, which we deduced is too optimistic for 100% incompressible data). A three year warranty at that rate would work out to ~12,000 P/E cycles, which is what I was getting at when I said "LTT functioning isn't quite faithful to its intention"....it's intention to drag the life of the drive out to the end of the warranty period, but 12,000 P/E cycles is 2.4x what the NAND is rated at and seems optimistic for a warranty.

How did you determine that the SSD in question was set for a 3 year warranty and not a 1 year warranty?

By the way, I'm not sure what you are disagreeing with about the sawtooth implementation of throttling. I did NOT say that the average write speed would be less (or more) than the warranty slope. I guess you got confused by the statement that the slope of the the throttled part of the sawtooth is less than the warranty line slope. That is self-evidently true. But that proves nothing about the average write speed of the SSD, since it depends on the slope of both portions of the sawtooth, as well as the length of each edge of the teeth.

[zads]

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The main problem I see with the method you mentioned is that I suspect throttling is actually more of a sawtooth than a straight line. It is probably implemented to let the SSD write at full speed until it notices that the current state has dropped below the warranty line.
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If you get fine granularity with your monitoring, it is indeed slightly sawtooth but won't actually let you pass below the lifetime.

[johnw]

If you get fine granularity with your monitoring, it is indeed slightly sawtooth but won't actually let you pass below the lifetime.

How much above the "lifetime" line does the current state have to be for the throttling portion of the sawtooth to end and the full-speed portion of the sawtooth to be permitted? In other words, after you have run full-speed into the "lifetime" line, and the throttling kicks in, how long does the throttling continue until it checks again and finds that you are now above the "lifetime" line?

[Ao1]

The more I think about it the more I think that the implementation of LTT is flawed.

In theory LTT is supposed to prevent PE depletion below the life curve, but it did not appear to do that in the endurance test. It kicked in too late and it did not then restrict writes enough to protect the life curve.

Assume a default initial credit period of 2% and a 3 year warranty = 525 power on hours/ 22 days. The 2% credit is based on PE cycles that don't count towards the life curve.

In the endurance test I was able to incur 28,352GB of writes. Approx 4,096GB of those writes were 0fill. Let's assume that equates to 25,000GB of uncompressible data over 7 days or 168 hours. (150GB per power on hour).

I was able to use 2% PE credits and 9% of the PE life curve in 168 power on hours.

Anyway you look at it LTT should have prevented 9% depletion of PE cycles in 168 hours.

I also believe writes speeds should have been reduced to 3MB/s (or less) to prevent PE depletion based on the load that was being applied.

It will be interesting to see what happens with the V3. The V2 had a life curve based on 5,000 PE cycles. The V3 will have a life curve based on 3,000 PE cycles. Unless the V3 can better compress data or reduce WA LTT should kick in quicker.

[sergiu]

I was able to use 2% PE credits and 9% of the PE life curve in 168 power on hours.

Anyway you look at it LTT should have prevented 9% depletion of PE cycles in 168 hours.

I also believe writes speeds should have been reduced to 3MB/s (or less) to prevent PE depletion based on the load that was being applied.

I suspect OCZ choose to release the drive with much more credits, like 8%. It would make sense to have different configurations based on your capacity, specially to ensure that some sustained short time testing does not trigger the LTT. Also, I believe 3MB limit would have probably been achieved if drive would have been stressed more. Now how much more, that's another question. I would assume at least a week or two.

[Ao1]

I thought about that. There are a couple of options I guess. If NAND PE cycles are based on minimum specs you could "front load" an assumption of additional life without reducing the spec'd PE count for the life cycle curve.

The other (more likely) option would be to use 10% PE cycles for the credit period and then spread the remaining 90% over the life time curve. Use 20% and then spread 80% over the life time curve etc. [EDIT. i.e. On one hand you increase the period in which you will not be throttled, but on the other hand you increase the chances of being throttled further down the line]

In the endurance thread I got to 11.6TB before the MWI dropped to 99%.

Out of that 11.6TB approx 4TB was 0fill. If 0fill is around 10% NAND wear that would work out to 8TB

8TB = Credit plus 1% MWI.
If 1% = approx 2TB then the credit was for 6TB.

It certainly looks like OCZ did not use the default 2% and went much higher.

Whatever the credit period however once it expired it should not have let me go from 100% to 91% within 5 days. At that stage the PE cycles must have been below the life time curve.

[Timur]

10% NAND wear for 0fill seems a bit high?! How do you get to that number?

Good compression squeezes down the 256 mb (or bigger) ATTO testfile to less than 0.5 kb, even the non-stellar NTFS compression puts the whole testfile into a single cluster (=less than 4kb =less than 0.0002%). I expect the Sandforce compression to be better than NTFS.

[sergiu]

In the endurance thread I got to 11.6TB before the MWI dropped to 99%.

Out of that 11.6TB approx 4TB was 0fill. If 0fill is around 10% NAND wear that would work out to 8TB

8TB = Credit plus 1% MWI.
If 1% = approx 2TB then the credit was for 6TB.

It certainly looks like OCZ did not use the default 2% and went much higher.

Whatever the credit period however once it expired it should not have let me go from 100% to 91% within 5 days. At that stage the PE cycles must have been below the life time curve.

The anomaly in counting MWI was strange from beginning. What I assumed was the following scenario: if there is an initial 10% allowed to be written, then controller will behave normally until flash cells will achieve this level of wear. That would mean decreasing the wear level like in a normal usage. However, when this level is achieved, the nice guard named "lifetime throttle" will wake up and will enforce its rules if needed. This however does not explain the missing 6TB from MWI counting.