linux-ext4 - Re: [PATCH 2/2] Add batched discard support for ext4.

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Date:	Sat, 24 Apr 2010 11:03:27 -0400
From:	Greg Freemyer <greg.freemyer@...il.com>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	Ric Wheeler <rwheeler@...hat.com>,
	Lukas Czerner <lczerner@...hat.com>,
	Jeff Moyer <jmoyer@...hat.com>,
	Mark Lord <kernel@...savvy.com>, linux-ext4@...r.kernel.org,
	Edward Shishkin <eshishki@...hat.com>,
	Eric Sandeen <esandeen@...hat.com>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH 2/2] Add batched discard support for ext4.

On Sat, Apr 24, 2010 at 10:43 AM, Eric Sandeen <sandeen@...hat.com> wrote:
> Greg Freemyer wrote:
>> On Sat, Apr 24, 2010 at 9:48 AM, Ric Wheeler <rwheeler@...hat.com> wrote:
>>> On 04/24/2010 09:24 AM, Greg Freemyer wrote:
>
> ...
>
>>>> I know I've been arguing against this patch for the single SSD case
>>>> and I still think that use case should be handled by userspace as
>>>> hdparm/wiper.sh currently does.  In particular for those extreme
>>>> scenarios with JBOD SSDs, the user space solution wins because it
>>>> knows how to optimize the trim calls via vectorized ranges in the
>>>> payload.
>>>>
>>> I think that you have missed the broader point. This is not on by default,
>>> so you can mount without discard and use whatever user space utility you
>>> like at your discretion.
>>>
>>> ric
>>
>> Ric,
>>
>> I was trying to say the design should be driven by the large discard
>> range use case, not the potentially pathological small discard range
>> use case that would only benefit SSDs.
>>
>> Greg
>
> Bear in mind that this patch makes the discard range requests substantially
> -larger- than what mount -o discard does on ext4 today, in fact that was
> a main goal.
>
> If the kernel could assemble vectors of ranges and pass them down, I think it
> could be extended to use that as well.
>
> -Eric
>
Eric/Ric,

I was responding to the Lukas latest post which stated:

==
And also, currently I am rewriting the patch do use rbtree instead of the
bitmap, because there were some concerns of memory consumption. It is a
question whether or not the rbtree will be more memory friendly.
Generally I think that in most "normal" cases it will, but there are some
extreme scenarios, where the rbtree will be much worse. Any comment on
this ?
==

If one optimizes for large discard ranges and ignores the pathological
cases only beneficial to SSDs, then a rbtree wins.

Correct?

Greg
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