RAPID-Cache --- A Reliable and Inexpensive Write Cache for High-performance Storage Systems


Modern RAID systems make extensive use of non-volatile RAM (NVRAM) write caches to mask the effects of the small write penalty. A single-copy NVRAM cache creates a single point of failure in a highly reliable RAID system while a dual-copy NVRAM cache is very expensive because of the high cost of NVRAM. This paper presents a new cache architecture for RAID systems called RAPID-Cache for Redundant, Asymmetrically Parallel, and Inexpensive Disk Cache . A typical RAPID-Cache consists of two redundant write buffers on top of a RAID. One of the buffers is a primary cache made of RAM or NVRAM and the other is a backup cache containing a two level hierarchy: a small NVRAM buffer on top of a log disk. The small NVRAM buffer combines small write data and writes them into the log disk in large sizes. By exploiting the locality property of I/O accesses and taking advantages of Log-structured File Systems, the backup cache has nearly equivalent write performance as the primary RAM cache. The read performance of the backup cache is not as critical because normal read operations are performed through the primary RAM cache and reads from the backup cache happen only during error recovery periods. The RAPID-Cache presents an asymmetric architecture with a fast-write-fast-read RAM being a primary cache and a fast-write-slow-read NVRAM-disk hierarchy being a backup cache. Such an asymmetric cache allows cost-effective designs for very large write caches to completely mask the effects of small write penalty for high-end RAID systems that would otherwise have to use dual-copy, costly NVRAM caches. It also makes it possible to implement reliable write caching for low-end RAID systems since the RAPID-Cache makes use of inexpensive disks to perform reliable caching. Four different configurations of the RAPID-Cache are studied in detail in this paper by means of simulations and analytical models. Our results show that the RAPID-Cache has significant reliability/cost advantages over conventional single NVRAM write caches and has great cost advantages over dual-copy NVRAM caches. The RAPID-Cache architecture opens a new dimension for RAID system designers to exercise trade-offs among performance, reliability and cost.


"RAPID-Cache --- A Reliable and Inexpensive Write Cache for Disk I/O Systems", in proceedings of the 5th International Symposium on High Performance Computer Arch itecture (HPCA-5). Orlando, Florida. Jan. 1999. Download the full HPCA-5 paper in Postscript format (IEEE Copyrighted) (117 KB gzipped PS file).

A more comprehensive version of the paper will appear in IEEE Transaction on Parallel and Distributed Systems. To download the full paper in PDF format, click here.
"RAPID-Cache --- A Reliable and Inexpensive Write Cache for Disk I/O Systems", Technical Report No. 1198-0001, Dept. of Electrical and Computer Engineering, University of Rhode Island, Nov. 1998. This is an expended version of the above HPCA-5 paper. Download the the full report in Postscript format (154 KB gzipped PS file) or the full report in PDF format

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RAPID-Cache is sponsored by National Science Fundation.


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