As the world moves to digital storage for archival purposes, there is an increasing demand for reliable, lowpower, cost-effective, easy-to-maintain storage that can still provide adequate performance for information retrieval and auditing purposes. Unfortunately, no current archival system adequately fulfills all of these requirements. Tape-based archival systems suffer from poor random access performance, which prevents the use of inter-media redundancy techniques and auditing, and requires the preservation of legacy hardware. Many diskbased systems are ill-suited for long-term storage because their high energy demands and management requirements make them cost-ineffective for archival purposes. Our solution, Pergamum, is a distributed network of intelligent, disk-based, storage appliances that stores data reliably and energy-efficiently. While existing MAID systems keep disks idle to save energy, Pergamum adds NVRAM at each node to store data signatures, metadata, and other small items, allowing deferred writes, metadata requests and inter-disk data verification to be performed while the disk is powered off. Pergamum uses both intra-disk and inter-disk redundancy to guard against data loss, relying on hash tree-like structures of algebraic signatures to efficiently verify the correctness of stored data. If failures occur, Pergamum uses staggered rebuild to reduce peak energy usage while rebuilding large redundancy stripes. We show that our approach is comparable in both startup and ongoing costs to other archival technologies and provides very high reliability. An evaluation of our implementation of Pergamum shows that it provides adequate performance. 1

Non-volatile byte addressable memories are becoming more common, and are increasingly used for critical data that must not be lost. However, existing NVRAM-based file systems do not include features that guard against file system corruption or NVRAM corruption. Furthermore, most file systems check consistency only after the system has already crashed. We are designing PRIMS to address these problems by providing file storage that can survive multiple errors in NVRAM, whether caused by errant operating system writes or by media corruption. PRIMS uses an erasure-encoded log structure to store persistent metadata, making it possible to periodically verify the correctness of file system operations while achieving throughput rates of an order of magnitude higher than page-protection during small writes. It also checks integrity on every operation and performs on-line scans of the entire NVRAM to ensure that the file system is consistent. If errors are found, PRIMS can correct them using file system logs and extensive error correction information. While PRIMS is designed for reliability, we expect it to have excellent performance, thanks to the ability to do word-aligned reads and writes in NVRAM. 1

While NAND flash memories have rapidly increased in both capacity and performance and are increasingly used as a storage device in many embedded systems, their reliability has decreased both because of increased density and the use of multi-level cells (MLC). Current MLC technology only specifies the minimum requirement for an error correcting code (ECC), but provides no additional protection in hardware. However, existing flash file systems such as YAFFS and JFFS2 rely upon ECC to survive small numbers of bit errors, but cannot survive the larger numbers of bit errors or page failures that are becoming increasingly common as flash file systems scale to multiple gigabytes. We have developed a flash memory file system, RCFFS, that increases reliability by utilizing algebraic signatures to validate data and Reed-Solomon codes to correct erroneous or missing data. Our file system allows users to adjust the level of reliability they require by specifying the number of redundancy pages for each erase block, allowing them to dynamically trade off reliability and storage overhead. By integrating error mitigation with advanced features such as fast mounting and compression, we show, via simulation in NANDsim, that our file system can outperform YAFFS and JFFS2 while surviving flash memory errors that would cause data loss for existing flash file systems.

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