On Thu, 11 Jan 2007 14:35:06 -0800 (PST)
dean gaudet <[email protected]> wrote:
> actually a global dirty_ratio causes interference between devices which
> should otherwise not block each other...
>
> if you set up a "dd if=/dev/zero of=/dev/sdb bs=1M" it shouldn't affect
> write performance on sda -- but it does... because the dd basically
> dirties all of the "dirty_background_ratio" pages and then any task
> writing to sda has to block in the foreground... (i've had this happen in
> practice -- my hack fix is oflag=direct on the dd... but the problem still
> exists.)
yeah. Plus your heavy-dd-to-/dev/sda tends to block light-writers to
/dev/sda in perhaps disproportionate ways.
This is on my list of things to look at. Hah.
> i'm not saying fixing any of this is easy, i'm just being a user griping
> about it :)
It's rather complex, I believe. Needs per-backing-dev dirty counts (already
in -mm) plus, I suspect, per-process dirty counts (possibly derivable from
per-task-io-accounting) plus some tricky logic to make all that work along
with global dirtiness (and later per-node dirtiness!) while meeting all the
constraints which that logic must satisfy.
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