Re: CFQ will be the new default IO scheduler - why?

From: Arjan van de Ven
Date: Mon Jul 24 2006 - 12:31:24 EST

> > Should there be a default scheduler per filesystem? As some filesystems
> > may perform better/worse with one over another?
> It's currently perDevice, and should probably be extended to perMount.


per mount is going to be "not funny". I assume the situation you are
aiming for is the "3 partitions on a disk, each wants its own elevator".
The way the kernel currently works is that IO requests the filesystem
does are first flattened into an IO for the entire device (eg the
partition mapping is done) and THEN the IO scheduler gets involved to
schedule the IO on a per disk basis.
The 2.4 kernel did this the other way around, and it was really a bad
idea (no fairness, less optimal scheduling all around due to less
visibility into what the disk is really doing, several hardware
properties such as TCQ depth that affect scheduling IOs are truely per
disk not per partition etc etc)

So I don't think it's likely that per mount is really an option right

Arjan van de Ven

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