linux-ext4 - Re: mkfs.ext4 vs. e2fsck discard oddities

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Message-Id: <34CA4C93-194F-45F3-9A94-E181A7DFEBCE@mit.edu>
Date:	Wed, 29 Feb 2012 23:47:51 -0500
From:	Theodore Tso <tytso@....EDU>
To:	Lukas Czerner <lczerner@...hat.com>
Cc:	Theodore Tso <tytso@....edu>, Eric Sandeen <sandeen@...hat.com>,
	ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: mkfs.ext4 vs. e2fsck discard oddities

On Feb 29, 2012, at 2:12 AM, Lukas Czerner wrote:
> 
> The reason is (as I commented in the patch #2) that we will not discard
> BLOCK_UNINIT groups. We use BLOCK_UNINIT as a optimization measure to
> skip groups which are likely to be non-provisioned, because we have
> never written there anything since the mkfs.
> 
> If you create file system without discard, then obviously nothing is
> discarded, image is fully provisioned and e2fsck discard *only* initialized
> groups. So you'll end up with the bigger image, in case that your image was
> not sparse.

i still think it makes sense to have an option where we discard everything
including BLOCK_UNINIT blocks.   Mke2fs doesn't discard blocks by default
because of a fear of crappy SSD drives, and while that fear may be
overstated, assuming that all of the unused blocks will *always* have been
discarded at mkfs time isn't necessarily a good thing to assume.   I'll grant
that it might be a fine default, but there needs to be *some* way to discard
everything that's unused….

-- Ted

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