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

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Message-Id: <1291740643-sup-2494@think>
Date:	Tue, 07 Dec 2010 11:52:43 -0500
From:	Chris Mason <chris.mason@...cle.com>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Mark Lord <kernel@...savvy.com>,
	James Bottomley <james.bottomley@...e.de>,
	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

Excerpts from Christoph Hellwig's message of 2010-12-07 04:27:49 -0500:
> On Fri, Nov 19, 2010 at 10:21:35AM -0500, Mark Lord wrote:
> > >I really hate to rely on this third party hearsay (from all sides), and
> > >have implement TRIM support in qemu now.  I'll soon install win7 and
> > >will check out the TRIM patters myself.
> > 
> > Excellent!
> 
> I did a Windows 7 installation under qemu today, and the result is:

Great, thanks for testing this.

> 
>  - it TRIMs the whole device early during the installation
>  - after that I see a constant stream of small trims during the
>    installation.  It's using lots of non-contiguous ranges in a single
>    TRIM command, with sizes down to 8 sectors (4k) for a single range.
>  - after installation there's is some background-trimming going on
>    even when doing no user interaction with the VM at all.
>  - removing files leads to an instant stream of TRIMs, again vectored
>    and of all sizes down to 4k.  Note that the TRIMs are a lot more
>    instant than even with btrfs and -o discard, which delays most
>    TRIMs until doing a sync.

Btrfs will do some small trims right when the block is freed, especially
in fsync heavy workloads but this is a suboptimal thing I want to fix.

The code tries to gather a whole transaction worth of trims and do them
after the commit is done.

-chris
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