linux-kernel - Re: [patch 6/7] blk: add plug for blkdev_issue

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Message-ID: <20120313171414.GJ29169@redhat.com>
Date:	Tue, 13 Mar 2012 13:14:14 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	"Martin K. Petersen" <martin.petersen@...cle.com>
Cc:	Shaohua Li <shli@...ionio.com>, linux-kernel@...r.kernel.org,
	linux-raid@...r.kernel.org, neilb@...e.de, axboe@...nel.dk
Subject: Re: [patch 6/7] blk: add plug for blkdev_issue_discard

On Tue, Mar 13, 2012 at 01:04:58PM -0400, Martin K. Petersen wrote:
> >>>>> "Vivek" == Vivek Goyal <vgoyal@...hat.com> writes:
> 
> Vivek> On Mon, Mar 12, 2012 at 11:04:18AM +0800, Shaohua Li wrote:
> >> In raid 0 case, a big discard request is divided into several small
> >> requests in chunk_size unit. Such requests can be merged in low layer
> >> if we have correct plug added. This should improve the performance a
> >> little bit.
> 
> Vivek> Martin posted a patch to remove the support for allowing merging
> Vivek> of discard requests. But this seems to be a reasonable use case
> Vivek> for allowing mering discard requests. CCing Martin.
> 
> Merging discard requests is hard given how we need to prepare the
> command payload at the bottom of the stack. The current upstream merge
> code pretends to be working but it actually doesn't. That's why I want
> it dead and buried.
> 
> I have some changes pending (that I need for the REQ_COPY support) that
> will make merging of non-rw requests easier to deal with. But that's a
> kernel release cycle away...

So first we will get rid of mering discard request and then enable after
one release cycle once REQ_COPY support is in?

Thanks
Vivek
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