For Flash and SSD - what is the definition of Valid Page Count?

Writing and Erasing NAND

Before we can understand the various maintenance algorithms that the controller employs to keep your SSD neat and tidy, we need to understand a few basics about how we read and write data to a NAND chip. Data is stored in a unit called a “page,”

excerpt from: http://web.stanford.edu/~rajivag/papers/wa_paper.pdf

Whereas the reclaiming policy that selects the blocks to garbage-collect is usually based only on the amount of free space to be gained, the policy defined in [2] also included the time elapsed since the last writing of the block with data. In general, the objective is to minimize the number of valid pages in the blocks selected for garbage collection, thereby minimizing the number of read and write operations resulting from garbage collection.

From what I can understand a valid page is a page NOT marked for deletion in an SSD block. Everything I have read says that SSD "garbage collection" is what is keeping a count on this, in order to make the SSD efficient as possible for future writes to the SSD. I would guess that "number of valid pages" is what you are calling a valid page count.

Musselman

Posted 2016-03-29T20:17:28.653

Reputation: 560

Musselman - @musselman - thank you so much for your input. I did not realize that a data unit is also a page. So, (I think) after garbage collection, the valid page count is the count of usable pages that can receive data. I should have added in my original question that I am a technical writer. So, what you brainiac-engineers learned in your first year of college, I struggle with. Often times it is because I lack the understanding of the elementary elements - like 'a data is stored in units called pages'. - thanks again. – Linda Lawson-Bruton – 2016-03-31T17:49:27.157

The Google result mentioning OpenVMS is actually a red herring. The book just happens to have the words count, valid and page in close enough proximity to trigger the algorithm.

– Burgi – 2016-03-29T20:50:31.617

What research have you found so far? – Burgi – 2016-03-29T20:56:29.273

Hi Burgi - I have scoured all of my company documents and tried in every way to find the definition on the internet. It's frustrating because it seems to be self-explanatory 'Valid Page Count'. A count of all the valid pages. But, what I do not understand is what makes a page 'valid'. – Linda Lawson-Bruton – 2016-03-31T01:36:19.577

For Flash and SSD - what is the definition of Valid Page Count?

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