linux-ext4 - Re: [PATCH] e2fsck: Discard free data and inode blocks.

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Message-ID: <alpine.LFD.2.00.1010221958440.3007@dhcp-lab-213.englab.brq.redhat.com>
Date:	Fri, 22 Oct 2010 20:01:53 +0200 (CEST)
From:	Lukas Czerner <lczerner@...hat.com>
To:	Andreas Dilger <adilger.kernel@...ger.ca>
cc:	Ric Wheeler <ricwheeler@...il.com>,
	Lukas Czerner <lczerner@...hat.com>,
	linux-ext4@...r.kernel.org, tytso@....edu, sandeen@...hat.com
Subject: Re: [PATCH] e2fsck: Discard free data and inode blocks.

On Fri, 22 Oct 2010, Andreas Dilger wrote:

> On 2010-10-22, at 08:46, Ric Wheeler wrote:
> > On 10/22/2010 10:32 AM, Lukas Czerner wrote:
> >> 
> >> Also there is a big win, when discard
> >> also zeroes data, because in that case we can just skip inode table
> >> initialization (zeroing) without any need of in-kernel lazyinit code
> >> enabled. And we get all this for free. It was introduced with Sandeens
> >> patch:
> >> 
> >> http://marc.info/?l=linux-ext4&m=128234048208327&w=2
> >> 
> >> So, I would rather leave it on by default.
> > 
> > You cannot 100% depend on discard zeroing blocks - that is not a universal requirement of devices that support it. Specifically, for ATA devices, I think that there are optional bits that specify how a device will behave when you read from a trimmed region.
> 
> That patch also checks for the zeroing feature.  When this patch was first under discussion, I proposed that we validate that the device is actually zeroed by doing a write a non-zero block to the disk and then calling discard+zero for that region, and reading back the block and verifying it.
> 
> Eric wasn't convinced that was necessary, maybe you can convince him more...
> 
> Cheers, Andreas

One of the counter arguments was, that some devices does not preserve
this behavior through power cycles. I think Ted was the one talking
about that.

-Lukas
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