linux-ext4 - Re: What happened to data=guarded?

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Date:	Tue, 11 Aug 2009 13:04:50 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Jan Kara <jack@...e.cz>
CC:	Andi Kleen <andi@...stfloor.org>, Frans Pop <elendil@...net.nl>,
	Chris Mason <chris.mason@...cle.com>,
	"Theodore Ts'o" <tytso@....edu>, linux-kernel@...r.kernel.org,
	linux-ext4@...r.kernel.org
Subject: Re: What happened to data=guarded?

Jan Kara wrote:
>> Frans Pop <elendil@...net.nl> writes:
>>
>>> On Tuesday 11 August 2009, Chris Mason wrote:
>>>> On Tue, Aug 11, 2009 at 03:35:36PM +0200, Frans Pop wrote:
>>>>> Somewhat unrelated, but what happened to the data=guarded patches
>>>>> Chris Mason proposed back in April?
>>>> I missed 2.6.31 but plan on sending for 2.6.32.  I promised to send
>>>> along a forward port of the patches a while back, but I finally have
>>>> one in testing here.  It should go out shortly.
>>> Good to hear. I've so far stayed with data=ordered as I think I'd prefer 
>>> data=guarded over data=writeback. I'll certainly give it a try when it's 
>>> available.
>> Same here. data=writeback already cost me a few files after crashes here :/
>   In this regard, data=guarded need not be better than data=writeback.
> We push out the data in guarded mode as late as in writeback mode
> (that's where the performance benefit comes from ;). The difference is
> that we increase i_size only after data are safely on disk so we cannot
> expose old data.
>   So security-wise, guarded mode is as safe as ordered mode but in other
> aspects its more like data=writeback.

Yes, I think the people anxiously waiting for data=guarded may be sadly
surprised at their 0-length files.

For those who understand the data=writeback tradeoffs it'll be very
useful in terms of more consistent results (easily-detectable 0-size or
short files, vs. randomly corrupted data sprinkled around) but it's not
going to be "data=ordered, but faster!"

-Eric

> 								Honza

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