logoalt Hacker News

duped • last Monday at 1:40 AM • 2 replies • view on HN

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.

Replies

inkyoto • last Monday at 5:11 AM

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.

➕ show 1 reply
adgjlsfhk1 • last Monday at 2:35 AM

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