Subject: Re: Disk scheduling policy (Re: NEW_BUFQ_STRATEGY)
To: Jason Thorpe <>
From: Thor Lancelot Simon <>
List: tech-kern
Date: 12/01/2003 19:23:58
On Mon, Dec 01, 2003 at 04:14:54PM -0800, Jason Thorpe wrote:
> Really, it's not clear that the elevator sort buys you much anyway, 
> when you're talking to raw disks, because disks don't really expose 
> their real geometry anymore.

Well, I have yet to encounter one that randomly shuffles blocks
such that sorting in order of increasing block number would be
_harmful_.  Indeed, since we can see more information about future
requests than the disk can, it's not clear to me why the elevator
sort is not highly beneficial.

> I would also like to see a disk sorting algorithm that could coalesce 
> adjacent writes or reads into single requests (perhaps enqueueing an 
> uber-buf that pointed to a list of sub-bufs that were treated as s/g 
> elements, or something).  As part of this, I'd really like to add a 

I thought I suggested this to you about a year ago and you were rather
strongly opposed, due to the VM system tricks involved in the obvious
way one would do this given the rest of our current implementation?  I
am _definitely_ in favor of request coalescing in disksort.

 Thor Lancelot Simon	                            
   But as he knew no bad language, he had called him all the names of common
 objects that he could think of, and had screamed: "You lamp!  You towel!  You
 plate!" and so on.              --Sigmund Freud