I have to say he’s wrong when he says there’s no reason for EVER using O_DIRECT. One HUGE reason is performance for large sequential writes when using fast med

I have to say he’s wrong when he says there’s no reason for EVER using O_DIRECT. One HUGE reason is performance for large sequential writes when using fast media and slow processors. Specifically, when the write speed of the media is in the same ballpark as the effective memcpy() speed of the processor itself, which, believe it or not, is very possible today (but was probably more unlikely in 2007) when working with embedded systems and modern NVMe media. Consider a system with an SoC like the NXP T2080 that has a high speed internal memory bus, DDR3, and PCIe 3.0 support. The processor cores themselves are relatively slow PowerPC cores, but the chip has a ton of memory bandwidth. Now assume you had a contiguous 512 MiB chunk of data to write to an NVMe drive capable of a sustained write rate of 2000 MB/s. The processor core itself can barely move 2000 MB/s of data, so it’s clear why direct IO would perform better since you’re telling the drive to pull the data directly from the buffer instead of memcpy-ing into into an intermediate kernel buffer first. With direct IO, you can perform zero-copy writes to the media. This is why I’m able to achieve higher sequential write performance on some modern MVMe drives than most benchmark sites report, all while using a 1.2 GHz PowerPC system.