## NVOverlay: Enabling Efficient and Scalable High-Frequency Snapshotting to NVM

NVOverlay is a fast technique to make fast snapshot from the DRAM or Cache to make them persistent. Meantime, it utilized tracking technique, which is common to the commercially available VMWare or Virtual Box on storage. Plus, it used NVM mapping to reduce the write amplification compared with the sota logged based snapshot.(by undo(write to NVM before they are updated) or redo may add the write amplification. To specify not the XPBuffer write amplification, but the log may adds more writing data)

So-called High-frequency snapshotting is to copy all the possible data in a millisecs interval when CPU load/store to DRAM. Microservice thread may require multiple random access to MVCC of data, especially for time series ones. To better debug the thread of these load/store, the copy contents process should be fast and scalable.

here OMC means overlay memory controller

The cache coherency is considered deeply. For scalability to 4U or 8U chassis, they add a tag walk to store the local LLC tag. We know that all the LLC slice is VIPT because they are shared. For the same reason, the tag can be shared but unique to one shared space.

For a distributed system-wide problem that have to sync epoch counters bettween VDs, they used a Lamport clock to maintain the dirty cache's integrity.