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    SubjectRe: io performance...
    On Mon, Jan 16 2006, Jeff V. Merkey wrote:
    > Max Waterman wrote:
    >
    > >Hi,
    > >
    > >I've been referred to this list from the linux-raid list.
    > >
    > >I've been playing with a RAID system, trying to obtain best bandwidth
    > >from it.
    > >
    > >I've noticed that I consistently get better (read) numbers from kernel
    > >2.6.8
    > >than from later kernels.
    >
    >
    > To open the bottlenecks, the following works well. Jens will shoot me
    > for recommending this,
    > but it works well. 2.6.9 so far has the highest numbers with this fix.
    > You can manually putz
    > around with these numbers, but they are an artificial constraint if you
    > are using RAID technology
    > that caches ad elevators requests and consolidates them.
    >
    >
    > Jeff
    >
    >

    >
    > diff -Naur ./include/linux/blkdev.h ../linux-2.6.9/./include/linux/blkdev.h
    > --- ./include/linux/blkdev.h 2004-10-18 15:53:43.000000000 -0600
    > +++ ../linux-2.6.9/./include/linux/blkdev.h 2005-12-06 09:54:46.000000000 -0700
    > @@ -23,8 +23,10 @@
    > typedef struct elevator_s elevator_t;
    > struct request_pm_state;
    >
    > -#define BLKDEV_MIN_RQ 4
    > -#define BLKDEV_MAX_RQ 128 /* Default maximum */
    > +//#define BLKDEV_MIN_RQ 4
    > +//#define BLKDEV_MAX_RQ 128 /* Default maximum */
    > +#define BLKDEV_MIN_RQ 4096
    > +#define BLKDEV_MAX_RQ 8192 /* Default maximum */

    Yeah I could shoot you. However I'm more interested in why this is
    necessary, eg I'd like to see some numbers from you comparing:

    - The stock settings
    - Doing
    # echo 8192 > /sys/block/<dev>/queue/nr_requests
    for each drive you are accessing.
    - The kernel with your patch.

    If #2 and #3 don't provide very similar profiles/scores, then we have
    something to look at.

    The BLKDEV_MIN_RQ increase is just silly and wastes a huge amount of
    memory for no good reason.

    --
    Jens Axboe

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