SyncGC: A Synchronized Garbage Collection Technique for Reducing Tail Latency in Cassandra

Data-center applications running on distributed databases often suffer from unexpectedly high response time fluctuation which is caused by long tail latency. In this paper, we find that long tail latency of user writes is mainly created by the interference with garbage collection (GC) tasks running in various system layers. In order to address the tail latency problem, we propose a synchronized garbage collection technique, called SyncGC. By scheduling multiple GC instances to execute in sync with each other in an overlapped manner, SyncGC prevents user requests from being interfered with GC instances, thereby minimizing their negative impacts on tail latency. Our experimental results with Cassandra showthat SyncGC reduces the 99.99th-percentile tail latency and the maximum latency by 35% and 37%, on average, respectively.N

File Fragmentation in Mobile Devices: Measurement, Evaluation, and Treatment

Mobile devices, such as smartphones, have become a necessity in our daily life. However, users may notice that after being used for a long time, mobile devices begin to experience sluggish response. Based on an empirical study on a set of aged mobile devices, we identified that file fragmentation is among the key factors that contribute to the progressive degradation of response time. This study takes a three-step approach: First, we designed a set of reproducible file-system aging processes based on User-Interface (UI) script replay. Through the aging processes, we confirmed that file fragmentation quickly emerged, and SQLite files were among the most severely fragmented files. Second, based on the workloads of a selection of popular mobile applications, we observed that file fragmentation did impact on user-perceived latencies. Specifically, the launching time of Chrome on an aged file system was 79\% slower than it was on a pristine file system. Third, we evaluated existing treatments of file fragmentation, including space preallocation, persistent journal space, and file defragmentation to understand their efficacies and limitations. We also evaluated a state-of-the-art copyless defragmenter, janusd, to show its advantage over the existing methods. IEEE1

Evanesco: Architectural support for efficient data sanitization in modern flash-based storage systems

© 2020 Copyright held by the owner/author(s). Publication rights licensed to ACM.As data privacy and security rapidly become key requirements, securely erasing data from a storage system becomes as important as reliably storing data in the system. Unfortunately, in modern flash-based storage systems, it is challenging to irrecoverably erase (i.e., sanitize) a file without large performance or reliability penalties. In this paper, we propose Evanesco, a new data sanitization technique specifically designed for high-density 3D NAND flash memory. Unlike existing techniques that physically destroy stored data, Evanesco provides data sanitization by blocking access to stored data. By exploiting existing spare flash cells in the flash memory chip, Evanesco efficiently supports two new flash lock commands (pLock and bLock) that disable access to deleted data at both page and block granularities. Since the locked page (or block) can be unlocked only after its data is erased, Evanesco provides a strong security guarantee even against an advanced threat model. To evaluate our technique, we build SecureSSD, an Evanesco-enabled emulated flash storage system. Our experimental results show that SecureSSD can effectively support data sanitization with a small performance overhead and no reliability degradation.N