ionice does not have any effect on un-synced writes (i.e. normal writes)?

Question

When I dump a large MySQL database (its dump weighs around 10GB) - it appears on the disk almost immediately, but then, later, when the kernel decides to flush it to the disk, the server almost stalls and other IO requests take a lot more time to complete even though mysqldump is run with ionice -c3, so the use of ionice has no real effect.

Artem

When you write data into the page cache, there is no field to store the IO priority. So ionice will have no effect. Do I have that right?

My latest kernel version is 4.18.16-200.fc28.x86_64.

Answer 1

Yes.

In theory, ionice allows you to prioritize a command's IO the way that nice(1) theoretically prioritizes its CPU usage. This would be a handy way to allow, say, a big but relatively important compile to grind away in the background without getting in the way of your interactive use of the machine.

(Why yes, I do recompile Firefox from source every so often.)

[...]

The next problem is that even when running directly on a disk, ionice does nothing to de-prioritize asynchronous write IO. This is, well, most of the write IO that most programs will do. Ionice may slow down synchronous writes (I don't have a test program) and it definitely works for reads, but that's it.

https://utcc.utoronto.ca/~cks/space/blog/linux/IoniceNotes