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

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Message-ID: <20101118180557.GU22787@shareable.org>
Date:	Thu, 18 Nov 2010 18:05:57 +0000
From:	Jamie Lokier <jamie@...reable.org>
To:	Jeff Moyer <jmoyer@...hat.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@....edu,
	linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, sandeen@...hat.com
Subject: Re: [PATCH 1/2] fs: Do not dispatch FITRIM through separate super_operation

Jeff Moyer wrote:
> James Bottomley <James.Bottomley@...e.de> writes:
> 
> > 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
> 
> Define online discard, please.
> 
> > lose to almost everything (either SSD or Thinly Provisioned arrays), so
> > it's become an "enable this to degrade performance" option with no
> > upside.
> 
> Some SSDs very much require TRIMming to perform well as they age.  If
> you're suggesting that we move from doing discards in journal commits to
> a batched discard, like the one Lukas implemented, then I think that's
> fine.  If we need to reintroduce the finer-grained discards due to some
> hardware changes in the future, we can always do that.

"Growable" virtual disks benefit from it too, if it frees up a lot of space.

Windows has some ability to trim unused space in NTFS on virtual disks
for this reason; I'm not sure if it's an online or offline procedure.

Online trim may be slow, but offline would be awfully inconvenient
when an fs is big and needed for a live system, or when it's your root fs.

-- Jamie
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