Abstract: As the store component of SSD, NAND flash requires erasing a block before writing pages in it, naming it write once memory. The lifetime of SSD is limited to the number of erasures issued to flash cells in SSD. Thus, reducing erasure counts is important for providing highly reliable SSD. This paper proposes a difference and compression based encoding scheme to enable second write to flash pages which are already written once of the SSD, decreasing the erasure time and increasing the lifetime of SSD. This scheme first calculates the differences between existing written data in a physical page and the new data to be written, then it compresses the differences information and encodes the compression result. The scheme writes the encoded data into pages which still hold writable bits, by which the twice-write of SSD is realized. The experiment results show that the proposed scheme effectively reduces erasure counts in frequent data update application.

Key words: solid state drives (SSD), erasure number, write twice

摘要： 作为SSD（solid state drives）的存储元件，NAND闪存在进行写之前，存储单元必须先进行擦除, 因此被称作写一次存储器。SSD的使用寿命受到存储单元的擦除次数的限制，因此减少擦除次数对于SSD的可靠性十分重要。提出了一种通过编码压缩后的差值信息的方法来对SSD中写过一次的页面进行二次写，从而减少SSD的擦除次数，延长使用寿命。首先计算物理页面中更新前后的数据的差值，然后将差值数据进行压缩，再将压缩后的数据进行编码后保存在写过的物理页中的可写位中，以此实现写过物理页的二次写。实验结果表明，对于数据更新为主的应用，该方法能够充分利用写过的物理页中的可写位，大幅减少SSD的擦除次数。

关键词: 固态硬盘（SSD）, 擦除次数, 二次写