linux-kernel - Re: [PATCH] null_blk: Register as a LightNVM device

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Date:	Thu, 12 Nov 2015 19:29:11 +0100
From:	Matias Bjørling <m@...rling.me>
To:	Jens Axboe <axboe@...com>, Christoph Hellwig <hch@...radead.org>
CC:	Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] null_blk: Register as a LightNVM device

On 11/12/2015 05:00 PM, Jens Axboe wrote:
> On 11/12/2015 08:58 AM, Christoph Hellwig wrote:
>> On Thu, Nov 12, 2015 at 08:54:48AM -0700, Jens Axboe wrote:
>>>> 300 lines of boilerplate for just setting up a few request_queues seem
>>>> wrong, can you show the actual patch you measured?
>>>
>>> I just took it from Matias' last posting:
>>>
>>> http://marc.info/?l=linux-kernel&m=144605858228534&w=2
>>
>> Well, that one has all these crazy completion methods which
>> are of no use for a blk-mq driver, so it should really be
>> compared without those.
>
> So we can cut it down a bit, it's still going to be the same boilerplate
> code that null_blk has, even with just mq completions. If it ended up
> rewriting null_blk to be something else entirely or full of ifdef
> sprinkles, I'd agree. But for adding a hundred lines of code to be able
> to test lightnvm perf, I think it's  a no-brainer to just add it to
> null_blk and not have a separate module.
>

As it is now, I prefer it part of null_blk, as long as it basically copy 
the core queueing structure. If null_nvm, we will have to maintain in 
two places. It'll be nice to keep it in one place.

The reason I would keep null_nvm, would be to add appropriate waiting 
times to simulate flash. However, I've now seen three implementations 
that utilized lightnvm for simulations, and it still doesn't scale to 
+1M IOPS that we need to actually compare it to a real device.
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