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The Conquest file system: better performance through a disk/persistent-RAM hybrid design
Wang A., Kuenning G., Reiher P., Popek G. ACM Transactions on Storage2 (3):309-348,2006.Type:Article
Date Reviewed: Feb 27 2007

As dynamic random access memory (DRAM) gets cheaper, larger memories are typically used as buffers to hide input/output (I/O) latency to disk. The Conquest file system is a novel approach for the more effective use of cheap DRAM. It is designed so that battery-backed DRAM serves as a persistent store for small files and file system services, while the slower disks serve as a store for large files.

Rather than adapting existing file system solutions, as in the case of random access memory (RAM) file systems or RAM-based disk emulators, the authors make a case for the need to redesign a file system optimized for persistent RAMs. The Conquest file system has a simpler datapath to small files and metadata in memory that bypasses the I/O buffer and disk management found in conventional disk-based file systems. The performance evaluation of Conquest shows up to a 19-times improvement in memory performance compared to file systems designed for disks, supporting the need for file system redesign to better exploit memory performance.

Based on a variety of prior studies of file access patterns and file size distribution, the strategy for delegating files to a storage medium has a file size threshold. Files with sizes below the threshold are delegated to persistent RAM, while those larger than the threshold are stored on disk. The performance gain achieved with Conquest for workloads that exercise both disk and memory supports this simple design decision. The large file layout on disk is optimized for sequential rather than random access, making Conquest disk access optimal for multimedia files, a significant component of current and future workloads.

An implementation of Conquest as a loadable module in the Linux 2.4.2 kernel is available; however, due to issues such as lower reliability and lack of a garbage collector implementation, persistent DRAM must be cost-effective before it can be deployed.

This paper is definitely worth reading for operating system designers. It demonstrates a successful redesign of a system component, the Conquest file system, after re-evaluating underlying assumptions, namely, file system optimizations for disks, in the context of changes to the system organization, namely, a memory-rich storage hierarchy.

Reviewer:  Suma Adabala Review #: CR133975 (0801-0075)
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Storage Hierarchies (D.4.2 ... )
 
 
Directory Structures (D.4.3 ... )
 
 
Measurements (D.4.8 ... )
 
 
File Systems Management (D.4.3 )
 
 
Performance (D.4.8 )
 
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