One of the fundamental limits to high-performance, high-reliability file systems is memory's vulnerability to system crashes. Because memory is viewed as unsafe, systems periodically write data back to disk. The extra disk traffic lowers performance, and the delay period before data is safe lowers reliability. The goal of the Rio (RAM I/O) file cache is to make ordinary main memory safe for persistent storage by enabling memory to survive operating system crashes. Reliable memory enables a system to achieve the best of both worlds: reliability equivalent to a write-through file cache, where every write is instantly safe, and performance equivalent to a pure write-back cache, with no reliability-induced writes to disk. To achieve reliability, we protect memory during a crash and restore it during a reboot (a "warm" reboot). Extensive crash tests show that even without protection, warm reboot enables memory to achieve reliability close to that of a write-through file system. Adding protection makes memory even safer than a write-through file system while adding essentially no overhead. By eliminating reliability-induced disk writes, Rio performs 4-22 times as fast as a write-through file system, 2-14 times as fast as a standard Unix file system, and 1-3 times as fast as an optimized system that risks losing 30 seconds of data and metadata.

Memory is commonly viewed as an unreliable place to store permanent data (files) because it is perceived to be vulnerable to system crashes. Yet despite all the negative implications of memory's unreliability, no data exists that quantifies how vulnerable memory actually is to system crashes. This paper quantitatively compares the vulnerability of disk and memory to operating system crashes. We use software fault injection to induce a wide variety of operating system crashes in DEC Alpha workstations running Digital Unix, ranging from bit errors in the kernel stack to deleting branch instructions to C-level allocation management errors. We find that files on disk are rarely corrupted (1.1% corruption rate), which agrees with our intuition. We also find that, surprisingly files in memory are nearly as safe as files on disk. Only 10 of the 650 crashes we observed (1.5%) corrupt any files in memory. Our data contradicts the common assumption that operating system crashes often corrupt files in memory and suggests that memory can be used to store permanent data rather than needing to write it back to disk.