direct-io

I wrote this for Node.js, which is a native binding in C, exposing cross platform functionality: https://github.com/ronomon/direct-io Although if it's a new project and you're used to C, I would recommend also taking a good look at Zig (https://ziglang.org/), because it's so explicit about alignment compared to C, and makes alignment a first-class part of the type system, see this other comment of mine that goes into more detail: https://news.ycombinator.com/item?id=25801542 Something that will also help, is setting your minimum IO unit to 4096 bytes, the Advanced Format sector size, because then your Direct IO system will just work, regardless of whether sysadmins swap disks of different sector sizes from underneath you. For example, a minimum sector size of 4096 bytes will work not only for newer AF disks but also for any 512 byte secto

How would you implement https://github.com/ronomon/direct-io with WebAssembly? Native modules exist because people sometimes need to drop down to work with the platform directly.

I agree with using O_DSYNC to surface the error to the write call, rather than waiting until the fsync call, which is often not checked by the user. I did some testing recently [1] with O_DIRECT + O_DSYNC and found some surprising performance results, on Linux it can be similar to O_DIRECT + fsync() after every write for hard drives. But as soon as you are doing grouped writes, performance is almost always better by using O_DIRECT + fsync() after the end of the group. For SSD drives though, O_DIRECT + O_DSYNC can be faster than O_DIRECT + fsync() after the end of the group, if you are pipelining your IO, e.g. you encrypt and checksum the next batch of sectors while you wait for the previous batch of checksummed and encrypted sectors to be written out. Because SSDs are so much faster, you can actually afford to slow down the write a little

I released a native add on [1] for Node.js earlier this week for querying a block device file descriptor for block size. It uses different ioctl calls depending on the platform and works on FreeBSD, Linux, macOS and Windows. It also provides cross-platform aligned buffers for DMA. Note that block size can be logical or physical sector size. IIRC st_blksize will typically report logical sector size (usually 512 bytes), but physical sector size (usually 4096 bytes) is often larger for Advanced Format drives, and is what you want to be using for optimal alignment. Any multiple of 4096 bytes is usually a safe bet for block size. Note that some SSD drives have 8192 or 16384 bytes as a "physical sector" page size / block size. [1] https://github.com/ronomon/direct-io