linux-ext4 - Re: [PATCH 1/2] fs: Do not dispatch FITRIM through separate super

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Message-ID: <4CF935FB.5010903@gmail.com>
Date:	Fri, 03 Dec 2010 13:24:59 -0500
From:	Ric Wheeler <ricwheeler@...il.com>
To:	Chris Mason <chris.mason@...cle.com>
CC:	James Bottomley <james.bottomley@...e.de>,
	Christoph Hellwig <hch@...radead.org>,
	Matthew Wilcox <matthew@....cx>,
	Josef Bacik <josef@...hat.com>,
	Lukas Czerner <lczerner@...hat.com>, tytso <tytso@....edu>,
	linux-ext4 <linux-ext4@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	sandeen <sandeen@...hat.com>
Subject: Re: [PATCH 1/2] fs: Do not dispatch FITRIM through separate super_operation

On 11/18/2010 12:55 PM, Chris Mason wrote:
> Excerpts from James Bottomley's message of 2010-11-18 12:19:10 -0500:
>> On Thu, 2010-11-18 at 09:29 -0500, Christoph Hellwig wrote:
>>> On Thu, Nov 18, 2010 at 07:19:58AM -0700, Matthew Wilcox wrote:
>>>> I guess I was assuming that, on receiving a FALLOC_FL_PUNCH_HOLE, a
>>>> filesystem that was TRIM-aware would pass that information down to the
>>>> block device that it's mounted on.  I strongly feel that we shouldn't
>>>> have two interfaces to do essentially the same thing.
>>>>
>>>> I guess I'm saying that you're going to have to learn about TRIM :-)
>>> Did you actually look Lukas FITRIM code (not the slight reordering here,
>>> but the original one).  It's the ext4 version of the batched discard
>>> model, that is a userspace ioctl to discard free space in the
>>> filesystem.
>>>
>>> hole punching will free the blocks into the free space pool.  If you do
>>> online discard it will also get discarded, but a filesystem that has
>>> online discard enabled doesn't need FITRIM.
>> Not stepping into the debate: I'm happy to see punch go to the mapping
>> data and FITRIM pick it up later.
>>
>> However, I think it's time to question whether we actually still want to
>> allow online discard at all.  Most of the benchmarks show it to be a net
>> lose to almost everything (either SSD or Thinly Provisioned arrays), so
>> it's become an "enable this to degrade performance" option with no
>> upside.
> I think we want to keep it.  In general we've (except for hch) spent
> almost zero time actually tuning online discard, and the benchmarking
> needs to be redone with the shiny new barrier code.
>
> -chris
>

Very belated response - I agree that we should keep the online discard support 
in (but off by default).

Some of the devices we have tested perform well with it and I expect that 
hardware vendors will get better now that we have the support for them to test with.

Ric


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