Re: Why does raid0 set max_hw_sectors as chunk size but the other raid types doesn't?

[NeilBrown <nfbrown@novell.com>]

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On Mon, May 30 2016, Joey Liao wrote:

> Hi,
>
> I have no idea why does raid0_run() in raid0.c use
> blk_queue_max_hw_sectors() to set max_hw_sectors as the chunk size,
> but the other raid types doesn't?

git is your friend.... admittedly you need the 'history' git tree to go
back before 2.6.12, but it is available.

http://git.kernel.org/cgit/linux/kernel/git/history/history.git/commit/?id=f556ef000efc90a45a285f4f0b4fd70bb70f



>
> What's the purpose to limit the max_hw_sectors in raid0?

unfortunately the commit doesn't answer that question.  I think it was
to ensure requests larger than one chunk were not created.  If they were
they would just have to be split, so there is no much to gain.

>
> Is it related to the source code logic issue or the performance issue?
>
> Besides, I have an interesting observation. If I remove all the
> following queue limitation codes in raid0_run() of raid0.c, the block
> size in iostat is still the same as chunk size even the input block
> size is much larger than the chunk size. Why???

Because when you write to a RAID0 you *must* divide each request up into
chunk-sizes sub-requests, and send them to different devices.

NeilBrown


>
> -  blk_queue_max_hw_sectors(mddev->queue, mddev->chunk_sectors);
> -  blk_queue_max_write_same_sectors(mddev->queue, mddev->chunk_sectors);
> +  //blk_queue_max_hw_sectors(mddev->queue, mddev->chunk_sectors);
> +  //blk_queue_max_write_same_sectors(mddev->queue, mddev->chunk_sectors);
>
> -  blk_queue_io_min(mddev->queue, mddev->chunk_sectors << 9);
> -  blk_queue_io_opt(mddev->queue,
> -                 (mddev->chunk_sectors << 9) * mddev->raid_disks);
> +  //blk_queue_io_min(mddev->queue, mddev->chunk_sectors << 9);
> +  /*blk_queue_io_opt(mddev->queue,
> +              (mddev->chunk_sectors << 9) * mddev->raid_disks);*/
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