MRAMFS: a compressing file system for non-volatile RAM

File systems using non-volatile RAM (NVRAM) promise great improvements in file system performance over conventional disk storage. However, current technology allows for a relatively small amount of NVRAM, limiting the effectiveness of such an approach. We have developed a prototype in-memory file system which utilizes data compression on inodes, and which has preliminary support for compression of file blocks. Our file system, MRAMFS, is also based on data structures tuned for storage efficiency in non-volatile memory. This prototype allows us to examine how to use this limited resource more efficiently. Simulations show that inodes can be reduced to 15-20 bytes each at a rate of 250,000 or more inodes per second. This is a space savings of 79-85% over conventional 128-byte inodes. Our prototype file system shows that for metadata operations, inode compression does not significantly impact performance, while significantly reducing the space used by inodes. We also note that a naive block-based implementation of file compression does not perform acceptably either in terms of speed or compression achieved.

1. Introduction

We have developed a prototype file system, mramfs, intended for use with byte-addressable non-volatile RAM (NVRAM). It is intended to explore the value of metadata and small-file compression for such a file system. It differs from common in-memory file systems in that it is not tied to volatile main memory through tight integration with the VFS caches like ramfs or tmpfs [25], and it is tuned for byte-addressable NVRAM rather for flash memory's particular constraints. Its support for compression notwithstanding, mramfs is most closely comparable in function to running a disk file system such as ext2fs or ReiserFS on a RAM disk or emulated block device. However, with compression and structures tuned specifically for random access memory, it offers greater space efficiency and potentially better performance.