晚點較好：延遲和聚集固態硬碟TRIM指令與發送時機管理方法

Abstract

固態硬碟比傳統硬碟擁有較低存取的時間、較低的功率消耗和較高抗震能力等優勢，且目前市售的固態硬碟技術逐漸成熟且價格逐漸下跌，固態硬碟已經漸漸在各種裝置中取代傳統硬碟成為一主流的儲存設備。但固態硬碟經長時間的大量使用後，固態硬碟的寫入效能會大幅下降，因為做垃圾回收成本變大許多。為了改善做垃圾回收的成本，TRIM指令之概念被提出來，准許作業系統通知固態硬碟哪些資料已被刪除不再被使用到。不過，我們發現固態硬碟處理TRIM指令需花不可忽略的時間，所以說如果作業系統發送TRIM指令的時機不當，就有很高的機會會延遲到後續讀或寫入指令，違反TRIM指令想要改善寫入效能的初衷。因此我們提出Delay TRIM的方法，希望藉由延遲和聚集TRIM指令，並找到夠大的固態硬碟閒置時間發送TRIM指令，兼顧TRIM指令降低垃圾回收成本、改善寫入效能之好處和減少TRIM指令延遲到後續讀或寫入指令之壞處。實驗結果顯示，使用我們的方法跟作業系統開起預設TRIM功能相比，對平均讀指令的反應時間改善了13%~27%且平均寫入指令的反應時間改善了5%~26%。

Solid state disks, compared to hard disk drives, have several advantages, such as lower access time, lower power consumption, and higher shock resistance. Recently, solid state disks become more affordable, and the technology of solid state disks has got mature. Solid state disks are replacing hard disk drives gradually from large data centers to portable laptops. However, high garbage collection overhead from heavy use over time reduces the write performance of solid state disks; and hence the concept of TRIM instruction has been introduced. A TRIM, a storage instruction, allows operating systems to inform solid state disks which part of data has been deleted and no longer in use. Although TRIM could relieve garbage collection overhead, it is also an instruction, with non-negligible process time. If operating system issues TRIMs in a bad timing, TRIMs would have a high possibility to delay the following read/write instruction. This is in conflict with TRIM’s goal, which is to improve write performance. As a result, we propose a delay TRIM method. By delaying and aggregating TRIMs and finding a long enough idle period to issue them, it not only minimizes TRIM’s possibilities of delaying the following read/write instructions, but also maximizes their advantages of garbage collection overhead reduction. Experiments show that our method, compared to operating system with default TRIM enabled, improves 13 ~ 27% in average read response time and 5 ~ 26% in average write response time.