多階層儲存單元快閃記憶體安全刪除之 頁面覆寫策略

Abstract

近年來固態硬碟的技術日趨成熟，且與傳統硬碟相比具有較低的存取時間、較低的功率消耗、較佳的抗震能力等優點；因此固態硬碟的普及率逐漸的上升。然而，固態硬碟的更新方式又與傳統硬碟不同，它是採取異地更新的機制；此種更新機制會導致同一筆資料若有多次更新，則其舊有版本的資料會散落在固態硬碟中的多處並且不再被對應表所追蹤；除此之外固態硬碟在刪除檔案時，也僅是在對應表中將檔案所佔據的頁面標記為無效頁面。因為上述的特性，倘若我們以資訊安全的觀點來探討檔案的安全抹除機制，那麼適用於傳統硬碟的檔案安全抹除機制就未必全然適用於固態硬碟中。所以我們提出了Delete On Update、Delete On Trim、Adaptive Delete等三套以固態硬碟頁面覆寫為基礎的檔案安全抹除機制，除此之外我們所提出的覆寫機制不論針對強頁面或弱頁面都是可以完美覆寫，並不會干擾到配對頁面的。使用我們的方法具有檔案的安全抹除、抹除速度快速、不影響固態硬碟壽命這三項優勢。

Currently , SSD technology is getting mature , and compared to conventional one, it is advantageous with less time needed to store data, low power consumption and great shock- absorbent which leads to its prevalence. However, there are some differences to update between SSD and conventional one, it utilizes out of place update, by so doing , if the data is to be updated couple of times, then the old version of this data will be scattered in SSD and won’t be traced down by mapping table anymore, besides, it merely signifies the page which was taken up by this very data as invalid page when data is being trimmed in SSD. Due to abovementioned characteristics, if we are going to have a discussion on the basis of secure delete policy, then the one which is applicable to conventional disk is not necessarily suitable for SSD. Thus, we provide 3 options- Delete on update, Delete On Trim, OPT delete method which are based on overwriting SSD pages for security delete policy. In addition, the secure delete policy we provided can flawlessly overwrite strong pages or weak pages without disturbing pair pages. This is why by applying our policy, the system comes with the following 3 merits- safely delete data, faster to delete data and SSD life time won’t be affected .