Subject: Re: Coallesce disk I/O (was RE: Interactive responsiveness under
To: Gordon Waidhofer <>
From: Stephan Uphoff <>
List: tech-perform
Date: 01/26/2004 18:52:37
> Disk I/O performance can be **dramatically** improved
> by I/O coallescing. My experience with a proprietary
> operating system proved this many times over. Linux
> coallesces disk I/Os.

I agree - really useful for a filesystem log and small nfs type write requests.
When I initially wrote the file system for Network Storage Solution
they had a proprietary kernel and I could use scatter/gather for
I/O coalescing.

Porting to NetBSD we had Chris Jepeway implement clustering using 
(architecture specific)
MMU tricks.

NSS allowed Chris Jepeway to post some of the clustering code.

However since the code only works on i386 (and probably a few other 
interest in the code was low.
Chris was busy with other work and never got around to make it arch 


> When there are multiple bufs (struct bufs) on the
> disk queue doing the same command (read/write) and
> accessing consecutive disk addresses, the bufs are
> turned into one I/O and use scatter/gather. Error
> recovery amounts to bursting the chain and retrying
> the bufs individually. It works best if I/Os can be
> scheduled in batches, ala strategy_list(bp) rather
> than one at a time ala strategy(bp).
> There was a reference in the "LFS on BSD" paper
> (Seltzer, et al) to a Berkeley project around
> the same time that added I/O coallescing to
> stock FFS. The performance rivaled LFS. I've
> never been able to find a paper on that project.
> Anybody know more?
> Is anybody working on coallesced disk I/Os?
> I've only given it a cursory look. I want
> to find a way to add coallescing without
> tearing-up the whole struct buf subsystem.
> It works best if done very near the device
> driver. I'm hoping to find a way for the
> device drivers to do some sort of "just in time"
> coallescing. That way, the struct bufs and
> queues would be fine the way they are.
> No new data structures need be added or maintained.
> It'll be the later part of the year before
> I can really wade in.
> 	-gww