linux-kernel - Re: [PATCH 5/5] dm: implement REQ

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Message-ID: <4C6D51AF.3050008@kernel.org>
Date:	Thu, 19 Aug 2010 17:45:51 +0200
From:	Tejun Heo <tj@...nel.org>
To:	Kiyoshi Ueda <k-ueda@...jp.nec.com>
CC:	Mike Snitzer <snitzer@...hat.com>, jaxboe@...ionio.com,
	linux-fsdevel@...r.kernel.org, linux-scsi@...r.kernel.org,
	linux-ide@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-raid@...r.kernel.org, hch@....de, James.Bottomley@...e.de,
	tytso@....edu, chris.mason@...cle.com, swhiteho@...hat.com,
	konishi.ryusuke@....ntt.co.jp, dm-devel@...hat.com, vst@...b.net,
	jack@...e.cz, rwheeler@...hat.com, hare@...e.de, neilb@...e.de,
	rusty@...tcorp.com.au, mst@...hat.com,
	Mikulas Patocka <mpatocka@...hat.com>,
	"Jun'ichi Nomura" <j-nomura@...jp.nec.com>
Subject: Re: [PATCH 5/5] dm: implement REQ_FLUSH/FUA support

Hello,

On 08/19/2010 12:32 PM, Kiyoshi Ueda wrote:
>> I see but single pending flush and steady write streams w/o saturating
>> the mempool would be able to stall dm_wait_for_completeion(), no?  Eh
>> well, it's a separate issue, I guess.
> 
> Your understanding is correct, dm_wait_for_completion() for flush
> will stall in such cases for request-based dm.
> That's why I mentioned below in
> https://www.redhat.com/archives/dm-devel/2010-August/msg00026.html.
> 
>     In other words, current request-based device-mapper can't handle
>     other requests while a flush request is in progress.
>
> In flush request handling, request-based dm uses dm_wait_for_completion()
> to wait for the completion of cloned flush requests, depending on
> the fact that there should be only flush requests in flight owning
> to the block layer sequencing.

I see.  bio based implementation also uses dm_wait_for_completion()
but it also has DMF_QUEUE_IO_TO_THREAD to plug all the follow up bio's
while flush is in progress, which sucks for throughput but
successfully avoids starvation.

> It's not a separate issue and we need to resolve it at least.
> I'm still considering how I can fix the request-based dm.

Right, I thought you were talking about REQ_FLUSHes not sycnhronized
against barrier write.  Anyways, yeah, it's a problem.  I don't think
not being able to handle multiple flushes concurrently would be a
major issue.  The problem is not being able to process other
bios/requests while a flush is in progress.  All that's necessary is
making the completion detection a bit more fine grained so that it
counts the number of in flight flush bios/requests and completes when
it reaches zero instead of waiting for all outstanding commands.
Shouldn't be too hard.

Thanks.

-- 
tejun
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