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

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Message-Id: <386E61B0-BF4D-4F96-9541-A614F63DE808@dilger.ca>
Date:	Fri, 22 Oct 2010 11:50:40 -0600
From:	Andreas Dilger <adilger.kernel@...ger.ca>
To:	Ric Wheeler <ricwheeler@...il.com>
Cc:	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 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





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