linux-ext4 - Re: [PATCH, RFC] Don't do page stablization if !CONFIG_BLKDEV

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Date:	Thu, 8 Mar 2012 12:50:42 -0800
From:	Boaz Harrosh <bharrosh@...asas.com>
To:	Chris Mason <chris.mason@...cle.com>, "Ted Ts'o" <tytso@....edu>,
	Zach Brown <zab@...bo.net>, Eric Sandeen <sandeen@...hat.com>,
	<linux-fsdevel@...r.kernel.org>, <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH, RFC] Don't do page stablization if !CONFIG_BLKDEV_INTEGRITY

On 03/08/2012 12:37 PM, Chris Mason wrote:
> On Thu, Mar 08, 2012 at 12:20:26PM -0800, Boaz Harrosh wrote:
>> On 03/08/2012 10:09 AM, Chris Mason wrote:
>>>
>>> But, why are we writeback for a second or more?  Aren't there other
>>> parts of this we would want to fix as well?
>>>
>>> I'm not against only turning on stable pages when they are needed, but
>>> the code that isn't the default tends to be somewhat less used.  So it
>>> does increase testing burden when we do want stable pages, and it tends
>>> to make for awkward bugs that are hard to reproduce because someone
>>> neglects to mention it.
>>>
>>> IMHO it's much more important to nail down the 2 second writeback
>>> latency. That's not good.
>>>
>>
>> I think I understand this one. It's do to the sync nature introduced
>> by page_waiting in mkwrite.
> 
> Pages go from dirty to writeback for a few reasons.  Background
> writeout, or O_DIRECT or someone running sync
> 
> background writeout shouldn't be queueing up so much work that
> synchronous writeout has a 2 second delay.
> 
> If the latencies are coming from something that was run through
> fsync...well there's not too much we can do about that.  The problem is
> that our page_mkwrite call isn't starting the IO it is just waiting on
> it, so we can't bump the priority on it.
> 

I agree. I think the logger model is: write, than sync

Before they used to be waiting on the sync phase now their waiting
on write, when concurrent with sync. I would like to inspect this situation.
I agree with you that it's just shifting heaviness that is now hiding somewhere
else.

> -chris

Thanks
Boaz
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