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JFTL: A flash translation layer based on a journal remapping for flash memory

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Published:09 February 2009Publication History
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Abstract

In flash memory-based storage, a Flash Translation Layer (FTL) manages the mapping between the logical addresses of a file system and the physical addresses of the flash memory. When a journaling file system is set up on the FTL, the consistency of the file system is guaranteed by duplications of the same file system changes in both the journal region of the file system and the home locations of the changes. However, these duplications inevitably degrade the performance of the file system. In this article we present an efficient FTL, called JFTL, based on a journal remapping technique. The FTL uses an address mapping method to write all the data to a new region in a process known as an out-of-place update. Because of this process, the existing data in flash memory is not overwritten by such an update. By using this characteristic of the FTL, the JFTL remaps addresses of the logged file system changes to addresses of the home locations of the changes, instead of writing the changes once more to flash memory. Thus, the JFTL efficiently eliminates redundant data in the flash memory as well as preserving the consistency of the journaling file system. Our experiments confirm that, when associated with a writeback or ordered mode of a conventional EXT3 file system, the JFTL enhances the performance of EXT3 by up to 20%. Furthermore, when the JFTL operates with a journaled mode of EXT3, there is almost a twofold performance gain in many cases. Moreover, the recovery performance of the JFTL is much better than that of the FTL.

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  1. JFTL: A flash translation layer based on a journal remapping for flash memory

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            Elliot Jaffe

            Flash memory is the basis for many new products and devices. It is found in universal serial bus (USB) keys, MP3 players, and solid-state disks. Flash memory devices are presented as block devices, on which the local operating system (OS) implements a file system. Yet, flash memory is not exactly equivalent to existing hard disks. The major problem is that flash memory has relatively limited rewrite capacity. Furthermore, each block must be erased before it can be rewritten. A software module called a flash translation layer (FTL) hides these issues from the OS, enabling it to treat the device as a commodity hard disk. This paper describes JFTL, an extended FTL that is optimized for journaling file systems such as ext3, ReiserFS, and XFS. The basic concept is to avoid write-in-place operations in the flash device. Instead, blocks can be updated via a simple address translation, thus avoiding duplication of blocks and rewriting operations. As a side effect, JFTL reduces the number of disk writes and improves overall performance. The paper provides good exposure to issues related to flash memory devices. It represents a convergence between modern file system designs and hardware implementations. It should be of interest to those concerned with device drivers, file systems, and solid-state memory. Online Computing Reviews Service

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