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

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Message-ID: <20101118193255.GE1594@arch.trippelsdorf.de>
Date:	Thu, 18 Nov 2010 20:32:55 +0100
From:	Markus Trippelsdorf <markus@...ppelsdorf.de>
To:	Jamie Lokier <jamie@...reable.org>
Cc:	Jeff Moyer <jmoyer@...hat.com>,
	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

On 2010.11.18 at 18:05 +0000, Jamie Lokier wrote:
> 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.

You can call FITRIM from a running system. Infact I run it once per week
as a cron job on my (mounted) root fs.

-- 
Markus
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