Prof. Dr. Frank Bellosa

Daniel Habicht

Abstract
Non-volatile memory is becoming of growing importance. There is great need for very fast and low latency, yet persistent memory. As flash storage cannot keep up with that, there has been rising research interest in Persistent Memory (PM). However, traditional PM is expensive and complex. Hybrid SSDs on the other hand combine traditional volatile memory with persistent flash storage. They are able to support both a block-granular and a fine-granular interface using Compute Express Link (CXL). The fine-granular interface allows the Operating System (OS) to map pages into the hybrid SSD’s memory, which is persistent as it is backed by flash storage. This direct mapping allows bypassing the OS page cache and makes writeback superfluous as the data is already in a memory with persistence guarantees.
While there is already research on extending the concept of Direct Access (DAX) to make it work well with hybrid SSDs, up until now that required application support. This decreases usability and slows down adoption, as major companies tend to be slow with implementing new features, especially if there is no production hardware yet. On the other hand, sales of new hardware are expected to be low while there is not great use for them.
We introduce Transparent DAX Upgrades, a way to automatically upgrade relevant file ranges to hybrid SSDs. With our work we lift this requirement of needing application support by tracking file ranges and automatically upgrading those that we consider to have strong persistence requirements. Particularly, this means file ranges that are often written back due to system calls such as fsync. An upgrade hereby refers to migrating pages from the OS page cache to the memory on a hybrid SSD.
In our testing we measure a speedup of up to 24.47× for RocksDB. We further benchmark Valkey and PostgreSQL and observe 7.29× and 2.53× speedups, respectively. By using our kernel patch, users can get large performance improvements for their applications without needing to configure anything.

BibTex:

@bachelorthesis{dietzler25TransparentDAXUpgrades,
  author = {Daniel John Dietzler},
  title = {Transparent DAX Upgrades for CXL-based hybrid SSDs},
  type = {Bachelor Thesis},
  year = 2025,
  month = oct# "13",
  school = {Operating Systems Group, Karlsruhe Institute of Technology (KIT), Germany}
 }