* Why does raid0 set max_hw_sectors as chunk size but the other raid types doesn't?
@ 2016-05-30 11:55 Joey Liao
2016-06-02 4:38 ` NeilBrown
0 siblings, 1 reply; 3+ messages in thread
From: Joey Liao @ 2016-05-30 11:55 UTC (permalink / raw)
To: linux-raid
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?
What's the purpose to limit the max_hw_sectors in raid0?
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???
- 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);*/
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: Why does raid0 set max_hw_sectors as chunk size but the other raid types doesn't?
2016-05-30 11:55 Why does raid0 set max_hw_sectors as chunk size but the other raid types doesn't? Joey Liao
@ 2016-06-02 4:38 ` NeilBrown
2016-06-02 9:30 ` Joey Liao
0 siblings, 1 reply; 3+ messages in thread
From: NeilBrown @ 2016-06-02 4:38 UTC (permalink / raw)
To: Joey Liao, linux-raid
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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);*/
> --
> To unsubscribe from this list: send the line "unsubscribe linux-raid" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at http://vger.kernel.org/majordomo-info.html
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^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: Why does raid0 set max_hw_sectors as chunk size but the other raid types doesn't?
2016-06-02 4:38 ` NeilBrown
@ 2016-06-02 9:30 ` Joey Liao
0 siblings, 0 replies; 3+ messages in thread
From: Joey Liao @ 2016-06-02 9:30 UTC (permalink / raw)
To: NeilBrown; +Cc: linux-raid
Hi NeilBrown,
Thanks for your kindly reply.
Originally, I doubt that would it affect the performance or not, but I
agree with your point.
Thanks.
2016-06-02 12:38 GMT+08:00 NeilBrown <nfbrown@novell.com>:
> 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);*/
>> --
>> To unsubscribe from this list: send the line "unsubscribe linux-raid" in
>> the body of a message to majordomo@vger.kernel.org
>> More majordomo info at http://vger.kernel.org/majordomo-info.html
^ permalink raw reply [flat|nested] 3+ messages in thread
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2016-05-30 11:55 Why does raid0 set max_hw_sectors as chunk size but the other raid types doesn't? Joey Liao
2016-06-02 4:38 ` NeilBrown
2016-06-02 9:30 ` Joey Liao
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