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Byte-addressable, non-volatile memory (NVM) presents an opportunity to rethink the entire system stack. We present Twizzler, an operating system redesign for this near-future. Twizzler removes the kernel from the I/O path, provides programs with memory-style access to persistent data using small (64 bit), object-relative cross-object pointers, and enables simple and efficient long-term sharing of data both between applications and between runs of an application. Twizzler provides a clean-slate programming model for persistent data, realizing the vision of Unix in a world of persistent RAM.We show that Twizzler is simpler, more extensible, and more secure than existing I/O models and implementations by building software for Twizzler and evaluating it on NVM DIMMs. Most persistent pointer operations in Twizzler impose less than 0.5 ns added latency. Twizzler operations are up to 13× faster than Unix, and SQLite queries are up to 4.2× faster than on PMDK. YCSB workloads ran 1.1–2.9× faster on Twizzler than on native and NVM-optimized SQLite backends.

The increasing availability of byte-addressable non-volatile memory on the system bus provides an opportunity to dramatically simplify application interaction with persistent data. However, software and hardware leverage different abstractions: software operating on persistent data structures requires“global” pointers that remain valid after a process terminates, while hardware requires that a diverse set of devices all have the same mappings they need for bulk transfers to and from memory, and that they be able to do so for a potentially heterogeneous memory system. Both abstractions must be implemented in a way that is efficient using existing hardware.We propose to abstract physical memory into an object space, which maps objects to physical memory, while providing applications with a way to refer to data that may have a lifetime longer than the processes accessing it. This approach reduces the coordination required for access to multiple types of memory while improving hardware security and enabling more hardware autonomy. We describe how we can use existing hardware support to implement these abstractions, both for applications and for the OS and devices, and show that the performance penalty for this approach is minimal.

New byte-addressable non-volatile memory (BNVM) technologies such as phase change memory (PCM) enable the construction of systems with large persistent memories, improving reliability and potentially reducing power consumption. However, BNVM technologies only support a limited number of lifetime writes per cell and consume most of their power when flipping a bit’s state during a write; thus, PCM controllers only rewrite a cell’s contents when the cell’s value has changed. Prior research has assumed that reducing the number of words written is a good proxy for reducing the number of bits modified, but a recent study has suggested that this assumption may not be valid. Our research confirms that approaches with the fewest writes often have more bit flips than those optimized to reduce bit flipping.To test the effectiveness of bit flip reduction, we built a framework that uses the number of bits flipped over time as the measure of “goodness” and modified a cycle-accurate simulator to count bits flipped during program execution. We implemented several modifications to common data structures designed to reduce power consumption and increase memory lifetime by reducing the number of bits modified by operations on several data structures: linked lists, hash tables, and red-black trees. We were able to reduce the number of bits flipped by up to 3.56× over standard implementations of the same data structures with negligible overhead. We measured the number of bits flipped by memory allocation and stack frame saves and found that careful data placement in the stack can reduce bit flips significantly. These changes require no hardware modifications and neither significantly reduce performance nor increase code complexity, making them attractive for designing systems optimized for BNVM.

Removing hard drives from a data center may expose sensitive data, such as encryption keys or passwords. To prevent exposure, data centers have security policies in place to physically secure drives in the system, and securely delete data from drives that are removed. Despite advances in security technology and best practices, implementation of these security measures is often done incorrectly. We anticipate that physical security will fail, and fixing the issue after the failure is costly and ineffective.We propose Inkpack, a protocol that prevents an attacker from reading data from a drive removed from the data center even if the attacker has the user key linked to the data. An implementation of this protocol encrypts data, and secret splits the key over a number of drives. Recovering the key requires communicating with other drives, thereby denying access to the data if a few drives have been removed. Inkpack also requires the system to verify the validity of individual drives before normal operation. A prototype created within the Ceph storage system executed individual key split, key rebuild, and drive validation operations in 100–150 μs. We also show that our protocol is sensitive to small data write overheads, demonstrating potential performance gains if implemented on smart solid state storage devices, and propose a solution to increase performance.

The first year of one's superintendency is a year like no other. Even those educators who've worked for years in the wings fulfilling high-level duties in the central office find one eye-opener after another when they alone are at the top of the organization's leadership ladder. At the start of the summer in 2010, The School Administrator invited four school leaders who were about to embark on their first superintendency to participate in a writing assignment. They agreed to keep notes or a personal journal of significant interactions and developments throughout the 2010-2011 school year. This article presents their accounts of how their rookie season transpired.