I shudder to think about the impact of concurrent data structures fsync'ing on every write because the programmer can't reason about whether the data is in memory where a handful of atomic fences/barriers are enough to reason about the correctness of the operations, or on disk where those operations simply do not exist.

Also linear regions make a ton of sense for disk, and not just for performance. WAL-based systems are the cornerstone of many databases and require the ability to reserve linear regions.

▲ inkyoto 3 days ago | parent | next [-]

Linear regions are mostly a figment of imagination in real life, but they are a convenient abstraction and a concept.

Linear regions are nearly impossible to guarantee, unless the underlying hardware has specific, controller-level provisions.

  1) For RAM, the MCU will obscure the physical address of a memory page, which can come from a completely separate memory bank. It is up to the VMM implementation and heuristics to ensure the contiguous allocation, coalesce unrelated free pages into a new, large allocation or map in a free page from a «distant» location.

  2)  Disks (the spinning rust variety) are not that different.  A freed block can be provided from the start of the disk. However, a sophisticated file system like XFS or ZFS, and others like it, will make an attempt do its best to allocate a contiguous block.

  3) Flash storage (SSDs, NVMe) simply «lies» about the physical blocks and does it for a few reasons (garbage collection and the transparent reallocation of ailing blocks – to name a few). If I understand it correctly, the physical «block» numbers are hidden even from the flash storage controller and firmware themselves.

The only practical way I can think of to ensure the guaranteed contiguous allocation of blocks unfortunately involves a conventional hard drive that has a dedicated partition created just for the WAL. In fact, this is how Oracle installation worked – it required a dedicated raw device to bypass both the VMM and the file system.

When RAM and disk(s) are logically the same concept, WAL can be treated as an object of the «WAL» type with certain properties specific to this object type only to support WAL peculiarities.

▲

duped 3 days ago | parent [-]

Ultimately everything is an abstraction. The point I'm making is that linear regions are a useful abstraction for both disk and memory, but that's not enough to unify them. Particularly in that memory cares about the visibility of writes to other processes/threads, whereas disk cares about the durability of those writes. This is an important distinction that programmers need to differentiate between for correctness.

Perhaps a WAL was a bad example. Ultimately you need the ability to atomically reserve a region of a certain capacity and then commit it durably (or roll back). Perhaps there are other abstractions that can do this, but with linear memory and disk regions it's exceedingly easy.

Personally I think file I/O should have an atomic CAS operation on a fixed maximum number of bytes (just like shared memory between threads and processes) but afaik there is no standard way to do that.

	▲	inkyoto 3 days ago \| parent [-]
		I do not share the view that the unification of RAM and disk requires or entails linear regions of memory. In fact, the unification reduces the question of «do I have a contiguous block of size N to do X» to a mere «do I have enough memory to do X?», commits and rollbacks inclusive. The issue of durability, however, remains a valid concern in either scenario, but the responsibility to ensure durability is delegated to the hardware. Futhermore, commits and rollbacks are not sensitive to the memory linearity anyway; they are sensitive to durability of the operation, and they may be sensitive to the latency, although it is not a frequently occurring constraint. In the absence of a physical disk, commits/rollbacks can be implemented using the software transactional memory (STM) entirely in RAM and today – see the relevant Haskell library and the white paper on STM. Lastly, when everything is an object in the system, the way the objects communicate with each other also changes from the traditional model of memory sharing to message passing, transactional outboxes, and similar, where the objects encapsulate the internal state without allowing other objects to access it – courtesy of the object-oriented address space protection, which is what the conversation initially started from.

▲ adgjlsfhk1 3 days ago | parent | prev [-]

otoh, WAL systems are only necessary because storage devices present an interface of linear regions. the WAL system could move into the hardware.