Re: Re: [patch]raid5: fix directio regression

["Jianpeng Ma" <majianpeng@gmail.com>]

On 2012-08-09 09:32 NeilBrown <neilb@suse.de> Wrote:
>On Thu, 9 Aug 2012 09:20:05 +0800 "Jianpeng Ma" <majianpeng@gmail.com> wrote:
>
>> On 2012-08-08 20:53 Shaohua Li <shli@kernel.org> Wrote:
>> >2012/8/8 Jianpeng Ma <majianpeng@gmail.com>:
>> >> On 2012-08-08 10:58 Shaohua Li <shli@kernel.org> Wrote:
>> >>>2012/8/7 Jianpeng Ma <majianpeng@gmail.com>:
>> >>>> On 2012-08-07 13:32 Shaohua Li <shli@kernel.org> Wrote:
>> >>>>>2012/8/7 Jianpeng Ma <majianpeng@gmail.com>:
>> >>>>>> On 2012-08-07 11:22 Shaohua Li <shli@kernel.org> Wrote:
>> >>>>>>>My directIO randomwrite 4k workload shows a 10~20% regression caused by commit
>> >>>>>>>895e3c5c58a80bb. directIO usually is random IO and if request size isn't big
>> >>>>>>>(which is the common case), delay handling of the stripe hasn't any advantages.
>> >>>>>>>For big size request, delay can still reduce IO.
>> >>>>>>>
>> >>>>>>>Signed-off-by: Shaohua Li <shli@fusionio.com>
>> >>>> [snip]
>> >>>>>>>--
>> >>>>>> May be used size to judge is not a good method.
>> >>>>>> I firstly sended this patch, only want to control direct-write-block,not for reqular file.
>> >>>>>> Because i think if someone used direct-write-block for raid5,he should know the feature of raid5 and he can control
>> >>>>>> for write to full-write.
>> >>>>>> But at that time, i did know how to differentiate between regular file and block-device.
>> >>>>>> I thik we should do something to do this.
>> >>>>>
>> >>>>>I don't think it's possible user can control his write to be a
>> >>>>>full-write even for
>> >>>>>raw disk IO. Why regular file and block device io matters here?
>> >>>>>
>> >>>>>Thanks,
>> >>>>>Shaohua
>> >>>> Another problem is the size. How to judge the size is large or not?
>> >>>> A syscall write is a dio and a dio may be split more bios.
>> >>>> For my workload, i usualy write chunk-size.
>> >>>> But your patch is judge by bio-size.
>> >>>
>> >>>I'd ignore workload which does sequential directIO, though
>> >>>your workload is, but I bet no real workloads are. So I'd like
>> >> Sorry,my explain maybe not corcrect. I write data once which size is almost chunks-size * devices,in order to full-write
>> >> and as possible as to no pre-read operation.
>> >>>only to consider big size random directio. I agree the size
>> >>>judge is arbitrary. I can optimize it to be only consider stripe
>> >>>which hits two or more disks in one bio, but not sure if it's
>> >>>worthy doing. Not ware big size directio is common, and even
>> >>>is, big size request IOPS is low, a bit delay maybe not a big
>> >>>deal.
>> >> If add a acc_time for 'striep_head' to control?
>> >> When get_active_stripe() is ok, update acc_time.
>> >> For some time, stripe_head did not access and it shold pre-read.
>> >
>> >Do you want to add a timer for each stripe? This is even ugly.
>> >How do you choose the expire time? A time works for harddisk
>> >definitely will not work for a fast SSD.
>> A time is like the size which is arbitrary.
>> How about add a interface in sysfs to control by user? 
>> Only user can judge the workload, which sequatial write or random write.
>
>This is getting worse by the minute.  A sysfs interface for this is
>definitely not a good idea.
>
>The REQ_NOIDLE flag is a pretty clear statement that no more requests that
>merge with this one are expected.  If some use cases sends random requests,
>maybe it should be setting REQ_NOIDLE.
>
>Maybe someone should do some research and find out why WRITE_ODIRECT doesn't
>include REQ_NOIDLE.  Understanding that would help understand the current
>problem.
>
>NeilBrown
>
Hi neil:
	Thanks your suggestion.
Direct-write can set REQ_NOIDLE because only finish this write-operation the next can do.
But direct-write(struct dio) can break up to some bios(struct bios).
For those bios, they have releationSo they may not set REQ_NOIDLE unless the last bio.
I think this may increase the performance, because random-direct-write at most only one bio?