Comparing disk and memory's resistance to operating system crashes

Abstract

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.