Silicon Motion SM2256 Preview

I am using a small ssd for Windows that is backup weekly. Saved my butt last week. Speed is key. I can provide my own reliability.

Here's an idea:
While they're making 3d nand, why don't they add an extra layer for parity and then use the raid-5 algorithm?
e.g. 8 layers for data, adding 1 extra for parity. Not that much extra overhead, but data will be much more reliable.

15517825 said:

Here's an idea:
While they're making 3d nand, why don't they add an extra layer for parity and then use the raid-5 algorithm?
e.g. 8 layers for data, adding 1 extra for parity. Not that much extra overhead, but data will be much more reliable.

SSD/MMC controllers already use FAR more complex and rugged algorithms than plain parity. Parity only lets you detect single-bit errors. It only allows you to "correct" errors if you know where the error was, such as a drive failure in a RAID3/5 array. If you want to correct arbitrary errors without knowing their location beforehand, you need block codes and those require about twice as many extra bits as the number of correctable bit-errors you want to implement. (I say "about twice as many" because twice is the general requirement for uncorrelated, non-deterministic errors. If typical failures on a given media tend to be correlated or deterministic, then it becomes possible to use less than two coding bits per correctable error.)

15519330 said:

15517825 said:

Here's an idea:
While they're making 3d nand, why don't they add an extra layer for parity and then use the raid-5 algorithm?
e.g. 8 layers for data, adding 1 extra for parity. Not that much extra overhead, but data will be much more reliable.

SSD/MMC controllers already use FAR more complex and rugged algorithms than plain parity. Parity only lets you detect single-bit errors. It only allows you to "correct" errors if you know where the error was, such as a drive failure in a RAID3/5 array. If you want to correct arbitrary errors without knowing their location beforehand, you need block codes and those require about twice as many extra bits as the number of correctable bit-errors you want to implement. (I say "about twice as many" because twice is the general requirement for uncorrelated, non-deterministic errors. If typical failures on a given media tend to be correlated or deterministic, then it becomes possible to use less than two coding bits per correctable error.)

No you don't. Each sector has CRC right?
So if the sector read fails CRC, simply calculate CRC replacing each layer in turn with the raid parity bit.
All of them will be off except for one with the faulty bit.
And these CRC's can all be calculated in parallel so there would be 0 overhead with regard to time.
Easy huh?

15536005 said:

No you don't. Each sector has CRC right?
So if the sector read fails CRC, simply calculate CRC replacing each layer in turn with the raid parity bit.
All of them will be off except for one with the faulty bit.
And these CRC's can all be calculated in parallel so there would be 0 overhead with regard to time.
Easy huh?

If you know which drive/sector is bad thanks to a read error, your scheme is needlessly complicated: you can simply ignore ("erase") the known-bad data and calculate it by simply XORing all remaining volumes. But you needed the extra bits from the HDD's "CRC" to know that the sector was bad in the first place.

In the case of a silent error though, which is what you get if you have an even-count bit error when using parity alone, you have no idea where the error is or even that there ever was an error in the first place. That's why more complex error detection and correction block codes exist and are used wherever read/receive errors carry a high cost, such as performance, reliability, monetary cost or loss of data.

I know this article was about the SM2256, but the graphs really made the SanDisk PRO shine bright! In the latency tests, which content creators care about, nothing seemed to phase it, doing better than even the 850 Pro - consistently!

BUT, one significant critique I have was the density limitation. Everything here was based on the ~250 GB drives. Comparing a drive with the exact name, but in a different density, is akin to comparing two entirely different drives.

I realize producing the data can be time consuming, but having the same information at three density points would be extremely helpful for purchasing decisions - lowest, highest, and middle densities.