SanDisk NAND:

Written by on November 2, 2014 in Gadgets, Technology - No comments

In our 240GB sample the SLC cache size is 10GB and since sixteen 128Gbit (16GiB) NAND dies are needed for the raw NAND capacity of 256GiB, the cache per die works out to be 625MB. I am guessing that in reality there is 32GiB of TLC NAND running in SLC mode (i.e. 2GiB per die), which would mean 10.67GiB of SLC, but unfortunately SanDisk could not share the exact block sizes of TLC and MLC with us for competitive reasons.

The performance benefits of the SLC mode are obvious. A TLC block requires multiple iterations to be programmed because the distribution of the voltage states is much narrower, so there is less room for errors, which needs a longer and more complex programming process.

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I ran HD Tach to see what the performance is across all LBAs. With sequential data the threshold for On Chip Copy seems to be about 8GB because after that the performance drops from 400MB to ~230MB/s. For average client workloads that is more than enough because users do not usually write more than ~10GB per day and with idle time nCache 2.0 will also move data from SLC to TLC to ensure that the SLC cache has enough space for all incoming writes.

The improved performance is not the only benefit of nCache 2.0. Because everything gets written to the SLC portion first, the data can then be written sequentially to TLC. That minimizes write amplification on the TLC part, which in turn increases endurance because there will be less redundant NAND writes. With sequential writes it is typically possible to achieve write amplification of very close to 1x (i.e. the minimum without compression) and in fact SanDisk claims write amplification of about 0.8x for typical client workloads (for the TLC portion, that is). That is because not all data makes it to the TLC in the first place – some data will be deleted while it is still in the SLC cache and thus will not cause any wear on the TLC. Remember, TLC is generally only good for about 500-1,000 P/E cycles, whereas SLC can easily surpass 30,000 cycles even at 19nm, so utilizing the SLC cache as much as possible is crucial for endurance with TLC at such small lithographies.

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Like the previous nCache 1.0, the 2.0 version is also used to cache the NAND mapping table to prevent data corruption and loss. SanDisk does not employ any power loss protection circuitry (i.e. capacitors) in client drives, but instead the SLC cache is used to flush the mapping table from the DRAM more often, which is possible due to the higher endurance and lower latency of SLC. That obviously does not provide the same level of protection as capacitors do because all writes in progress will be lost during a power failure, but it ensures that the NAND mapping table will not become corrupt and turn the drive into a brick. SanDisk actually has an extensive whitepaper on power loss protection and the techniques that are used, so those who are interested in the topic should find it a good read.

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