linux-kernel - Re: [patch v2 2/6] blk: dont allow discard request merge temporarily

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Date:	Thu, 22 Mar 2012 10:33:50 +0800
From:	Shaohua Li <shli@...nel.org>
To:	"Martin K. Petersen" <martin.petersen@...cle.com>
Cc:	Vivek Goyal <vgoyal@...hat.com>, linux-kernel@...r.kernel.org,
	linux-raid@...r.kernel.org, neilb@...e.de, axboe@...nel.dk
Subject: Re: [patch v2 2/6] blk: dont allow discard request merge temporarily

2012/3/22 Martin K. Petersen <martin.petersen@...cle.com>:
>>>>>> "Shaohua" == Shaohua Li <shli@...nel.org> writes:
>
> Shaohua> Enabling discard merge is required for device with slow discard
> Shaohua> (and very helpful for raid),
>
> Before RAID support there really wasn't much point. That's one of the
> reasons the current discard merge code has survived despite being
> completely broken.
Exactly.

> As for how I'd like to handle contiguous discard merges going forward
> here's a proof of concept. It's on top of my write same tree so you may
> have to noodle a bit.
>
> It is crucially important that we never merge a command that can't be
> processed by the device. So I'd like to do something like this:
This is great to consolidate the check with this patch. But looks this
isn't enough. As far as I know the current discard merge is broken
because blk_update_request() can't handle such request because
we override the first BIO to add payload. I had a quick hack and post
out yesterday. It works in my SATA ssd, but I really didn't know if there
is something missing in SCSI side, can you share anything here?

Thanks,
Shaohua
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