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How Active Garbage Collection works. Reliable speeds and customized functions cater to the requirements of various embedded system applications. Featuring the USB 3. SSDs are an increasingly popular form of storage. Though lighter, faster, and more shock-resistant than traditional hard drives, the SSDs suffer from a higher cost and lower endurance. Garbage Collection is one important factor increasing durability and speed.
With SSDs, the controller writes information to a free space on the Flash. Based on the Garbage Collection algorithm, effective data will be transferred to Block C. Blocks A and B are then erased. After that, new data may be more efficiently written to the now freed up Blocks of A and B. This is the essence of Garbage Collection. This greatly downgraded performance. Flash first organizes free space and maintains spare blocks as buffer space.
They store data with floating gate transistors. There is, of course, a downside. SSDs read and write data as pages, but they erase data at a much larger block level. Blocks are usually made up of hundreds of pages. So, why not erase at the page level? Thanks for your article! Great detail on the SSDs and how they work. I knew some of the detail but only a percentage.
This article was much clearer at explaining why things move around so much. And thanks to LSI for letting you share your knowledge. I have a question. Is there a way to tell when garbage collection is completed. So, if I Trim a file, it is ready for GC. The application in mind is more concerned with security than performance. SSDs appear to out perform but how do we reassure people that GC is complete? Any thoughts? Thanks in advance and thanks for the articles!
I would assume military contractors could develop their own custom firmware but other than that someone who needs that kind of security should rather focus on encrypting such content-sensitive files directly. Hey Ken. Sorry I missed your question. Locodoco is correct about the user not knowing when the GC process is complete for a particular portion of the SSD. Moreover, GC is performed at different times for some controllers.
If the controller wants to optimize the life of the flash, it will perform GC as space is needed to prevent rewriting data which will be soon erased anyway. For situations where you must ensure the residual data is truly erased, you need to have a secure erase operation built into the FW.
Not all drives support this capability. Short of having a secure erase command built in, there are procedures where you can be fairly confident you have removed all prior data, but they would not meet any of the Mil Spec definitions. The questions I have regarding all this is, how long in terms of time minutes, hours, days, weeks, months etc would it take for a Non Trim enabled SSD to get round to carrying out GC?
Garbage collection is the same whether the drive supports TRIM or not, and it depends on how each drive manufacturer has programmed the GC calling to execute. GC takes place based on when the controller is architected to manage the unused space. TRIM is sent by the operating system to enable the controller to know what previously saved data is no longer required and will not need to be saved during the GC process.
This is a very good article. Hi Ken. Thank you very much for the wonderful explanation. But I have some confusion that I hope you could clarify. They both look pictorially identical to me except for the cell color of GC.
Does this mean that at this point, the SSD knows the data is invalid in both cases? Could you please better elaborate the difference between the two? Q2: In Figure 2 Row-4,Col-4 , the overprovisioning space reduces and free space remains constant. Is it because in your example overprovisioning is assigned from available free space? Could you please clarify?
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