Re: Could RAM possibly be just 3-4 times faster than bare hdd writes and reads? or, is the Linux kernel doing its 'magic' in the bg? or, ...

[Reco]

        Hi.

On Wed, Jun 17, 2020 at 12:10:51PM -0700, David Christensen wrote:
> 2.  AIUI dd(1) uses asynchronous (buffered) I/O unless told otherwise.

You seem to confuse asynchronous and cached I/O too.

>From Linux kernel POV, *asynchronous* I/O is a pair of
io_submit/io_getevents syscalls, and dd does not do these regardless of
the options that are provided.
What dd does is a *synchronous* I/O (read/write syscalls or
pread64/pwrite64 for older kernels).

Whenever the I/O is cached (an output file is opened without
O_DIRECT|O_DSYNC flags) or not is orthogonal to whenever it's sync or
async. 

For the true async I/O you'll need something like fio, not dd.

Reco