I was surprised by this recently when optimizing a build process that uses an intermediate write-to-disk step. I replaced the intermediate filesystem API with an in-memory one and it was not measurably faster. Not even by a single millisecond.

How much data were you writing? If you don't fill the OS's page cache and the SSD controller's DRAM cache, and you're not blocking on fsync() or O_DIRECT or some other explicit flushing mechanism, then you're not going to see much of a difference in throughput.

Only a few MB written then read, so that is a likely explanation.

Ahh, thanks, I didn't know about the ramdisk. Very interesting about I/O not being the bottleneck, though.

Unfortunately there are still cases where local disk I/O can be a serious bottleneck that you do have to be aware of:

1. Cloud VMs sometimes have very slow access even to devices advertised as local disks, depending on which cloud

2. Windows

The latter often catches people out because they develop intuitions about file I/O speed on Linux, which is extremely fast, and then their app runs 10x slower on Windows due to opening file handles being much slower (and sometimes they don't sign their software and virus scanners make things much slower again).

Apparently, the cause of the long standing windows disk IO problem was discovered a month or so ago, and MS were said to be working on a fix.

Whether it'll be constrained to Win 11 is yet to be seen.

That's interesting. A project at work is affected by Windows slow open() calls (wrt to Linux/Mac) but we haven't found a strong solution rather than "avoid open() as much as you can".

It's likely Windows Defender, which blocks on read I/O to scan files. Verify by adding a folder to its exclusion list, though this isn't helpful if it's a desktop app. The difference is most noticeable when you're reading many small files.

Really? Where did you hear that? Conventional wisdom is based on a post by an MS employee years ago that described the performance issues as architectural.

MS does have something called "dev drive" in Win11. Dev Drive is what ReFS turned into and is an entirely different filesystem that isn't NTFS, which is better optimized for UNIX-style access patterns. The idea is that core Windows remains slow, but developers (the only people who care about file IO performance apparently) can format another partition and store their source/builds there.