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SSD life self monitoring question
Do any SSDs use any pages to monitor the expected life of the product.
Pages in various physical locations could be set to known values. These pages would not be refreshed by the usual periodic rewrites or moving. As the device had data written to it, additional pages would start to be used for monitoring. The addition pages would be selected by virtue of having already been rewritten an interesting number of times (Say 10%, 20%...100%, 110%... of the expected average rewrite lifetime for pages.) The pages being monitored would be checked every once in a while. If "enough" pages showed "enough" decay or needed "enough" error correction, then all of the pages that had been rewritten that many times or more and which hadn't been refreshed for the same length of time or more would have their data moved or refreshed into the same location. The SSD could be divided into areas depending on physical location on the device, and the "extra" rewrites done in each area based on monitored pages within the area. Simpler alternatives: 1. only refresh a page when read error rate exceeds typical value for pages 2. only refresh a page when read error rate indicates decay with be lost soon compared to typical values for pages 3. refresh everything that hasn't been refreshed in some amount of time. Perhaps this time is automatically changed based on experience for this particular device. Perhaps the time interval is based on the current total number of writes for this particular device. My question/proposal is about adding monitoring at a finer grain than the entire device. NOTE: The manufacturers keep everything secret, so I can't guess how much data loss rate would decrease, the read speed would increase, or if the average useable life in total data written by the user would increase or decrease. It might be the case that refreshing everything once a month would be enough to greatly decrease read error correction time and greatly reduce data loss, while at the same using only less than %10 of the life of a device.\ (10 year design life means 120 writes used. Typical MLC life numbers higher quality devices are 10 full write/day for 5 years = 365*10*5 = 1825 average writes of user capacity amount. Even if you reduce take into account over population, you still have an average of more than 1200 writes/cell available. (These devices might actually have an expected life of about 3000 writes/cell.) Lower quality devices typically are rated for 5 years and probably have a design life of 5 years also. These can be written 0.1/day. This would indicate that expected average writes/cell is only 180 or so, but I judging by the press, I think that the life expected average life is 700 so. 60 periodic rewrites might "waste" 1/3 to 1/10 of the device life. Thus, I think that finer grained monitoring could pay for these devices. I started thinking about this due to the Samsung 840 EVO Performance Drop that turns out to have been related to excessive time taken by read error recovery of "old" data, as indicated by trade publications. I haven't seen a press release or page at www.samsung.com that confirms that the problem is due to read error recovery, but here is a pointer to a description of the patch: https://www.samsung.com/global/busin...downloads.html at: "Samsung SSD 840 EVO Performance Restoration Software" |
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