Bandwidth QoS for disks?

[Stefan Hoffmeister]

Hi,

is there anything in the kernel that would allow an application to
*declare* that it needs disk read (or possibly) write bandwidth of n
KB/s?

The kernel would make every attempt to deliver that bandwidth, even
allowing starvation of other less important clients, except for some
5-10% headroom for emergency? I realise that the kernel cannot
*guarantee* that bandwidth, but some kind of priorization scheme would
help often enough, I guess.

I just lost another CD-R due to cron with lots and lots of on-disk
seeking kicking in, killing all that bandwidth cdrecord needed - and I
don't have one of these new and fancy burn-proof things (and, yes, I
should have suspended cron and friends, but I am only human and
computers are meant to made my life easier).

Sure, I could instruct cdrecord to increase its own read-ahead cache
from 4 MB to, say, 128 MB. But read-ahead cache != "QoS" (except for
volume of data == size of read-ahead cache), only a lame attempt at,
well, being helpful in an imperfect world.