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Paging is not transparent in high-scale and high-performance databases when using direct I/O to disk. It is not possible to make it transparent on many larger s

Paging is not transparent in high-scale and high-performance databases when using direct I/O to disk. It is not possible to make it transparent on many larger servers even if you wanted to because the silicon does not support a large enough virtual address space. This is a pretty common design problem in databases, or anything with huge amounts of data and storage really. There were magic incantations that compilers informally respected to induce the correct behavior but it wasn’t always reliable and technically not a bug when it wasn’t. This provides an official and simple way to achieve the same effect without obscure magic.

loeg 3y ago
Are high performance databases embedding non-POD C++ objects in explicitly paged memory?
- jandrewrogers 3y ago
  Most of your runtime objects are expected to be pageable in databases designed to support very high storage density because it is necessary as a matter of robustness and persistability. Even single runtime metadata structures may not reasonably fit in RAM in some cases. There are constraints on these objects (e.g. no vtables) but they are typically not PODs. Even if they were PODs, changing the types and instances of types at a specific memory address without destroying the objects residing there can have side-effects. The compiler does not expect a live memory address to be overwritten with an arbitrary bucket of bits by a DMA operation. DMA doesn't respect lifetime and ownership models as understood by a compiler, so it is a bit of a Deus Ex Machina as far as the compiler is concerned and is assumed to never happen.
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