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

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Message-ID: <4CC1D815.2000808@redhat.com>
Date:	Fri, 22 Oct 2010 13:29:41 -0500
From:	Eric Sandeen <sandeen@...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
Subject: Re: [PATCH] e2fsck: Discard free data and inode blocks.

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...

*grin* I wouldn't -veto- it, but I think we have to maintain a balance
between defensive programming and trying to handle every possible piece
of buggy hardware by doing too many extra gyrations in the code...

-Eric

> Cheers, Andreas

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