24 November 2012

Still, Don't Be A Dupe

Yet another dustup over SandForce and deduplication over at AnandTech. The fisticuffs are in the comments. What's still amusing is that the combatants still haven't figured out that dedupe is on-the-fly/ad-hoc 5NF storage. Ask them about relational databases, and they roll their eyes.
For example, if you have two very similar photos which are 5MB each, the controller may not write 10MB. Instead, it will only write let's say 8MB to the NAND because some of the data is duplicate and the whole idea of deduplication is to minimize NAND writes.

If you go and delete one of these photos, the OS sends a TRIM command that tells the LBA is no longer in use and it can be deleted. What makes SandForce more complicated is the fact that the photos don't necessarily have their own LBAs, so what you need to do is to check that the LBA you're about to erase is not mapped to any other LBA. Otherwise you might end up erasing a portion of the other photo as well.

While it's still up in the air whether SandForce controllers actually do dedupe, rather than just compression, the observation is correct. Not that any of these folks is able to make the mental leap. Kids! They never figure anything out until it's too late.

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