Re: I/O Request [Elevator; Clustering; Scatter-Gather]

From: David Anderson (david-anderson2003@mail.com)
Date: Tue Mar 04 2003 - 09:11:04 EST

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Hi,

Thanks a lot for the reply...
I am using linux 2.4.

Got mislead with the statement in "Linux Device Drivers"

"Most high-performance disk controllers can do “scatter/gather” I/O as well, leading to
large performance gains."

Thanks and Regards,
David Anderson

----- Original Message -----
From: Jens Axboe <axboe@suse.de>
Date: Tue, 4 Mar 2003 14:50:00 +0100
To: David Anderson <david-anderson2003@mail.com>
Subject: Re: I/O Request [Elevator; Clustering; Scatter-Gather]

> On Tue, Mar 04 2003, David Anderson wrote:
> >
> > Hi, I have been going through some documentation that talks of
> > clustering, scatter-gather and elevator being used to improve
> > performance. I am confused between these :
> >
> > This is what I have understood : Elevator The job of the elevator is
> > to sort I/O requests to disk drives in such a way that the disk head
> > moving in the same direction for maximum performance. Have been able
> > to locate the code for the same.
> >
> > Clustering Combines multiple requests to adjecent blocks into a single
> > request. Have not been able to find the code which carries this out.
> > Any clue on where this is done in the linux source code ??
>
> Both actions are performed by the elevator in Linux. You did not mention
> which kernel you are looking at, for 2.4 you need to read
> drivers/block/ll_rw_blk.c and drivers/block/elevator.c. For 2.5, read
> the same files and drivers/block/deadline-iosched.c in addition.
>
> > Do Clustering of request and scatter-gather mean the same ?? Confused
> > to the core... Kindly help me ...
>
> No, the elevator clustering refers to clustering request that are
> contigious on disk. Scatter-gather may cluster sg entries that are
> contigious in memory.
>
> --
> Jens Axboe
>

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