linux-ext4 - Re: [PATCH] ext4: Ensure writecache to disk in no journal mode

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Message-ID: <4BACF174.50703@redhat.com>
Date:	Fri, 26 Mar 2010 12:40:04 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Ric Wheeler <rwheeler@...hat.com>
CC:	Dmitry Monakhov <dmonakhov@...nvz.org>,
	Surbhi Palande <surbhi.palande@...onical.com>,
	linux-ext4@...r.kernel.org, Theodore Tso <tytso@....edu>
Subject: Re: [PATCH] ext4: Ensure writecache to disk in no journal mode

On 03/26/2010 12:16 PM, Ric Wheeler wrote:
> On 03/26/2010 12:37 PM, Dmitry Monakhov wrote:
>> Surbhi Palande<surbhi.palande@...onical.com>  writes:
>>
>>    
>>> Ensure that in the no journal mode the write cache is flushed to the disk by
>>> calling a blkdev_issue_flush() which issues a WRITE_BARRIER if necessary.
>>>      
>> As soon as i understand, nojournal mode is assumed to be used for
>> fail-free block devices(raid + UPS). So we don't have to worry about
>> blkdev's wcache vs persistent storage correctness.

No, I don't think so - even with "fail-free" storage, a system crash
still results in an inconsistent filesystem; with nojournalling you've
made the decision to either fsck or re-mkfs after that event.

> I don't think that is a safe assumption. If users want that behavior, 
> they can mount with fs without barriers...

yes, I agree with Ric - even if you don't have journalling, the proper
sequence of sync calls should still result in data permanently on disk
by default.  (though I think using mount -o nobarrier for this purpose,
in absence of journalling, overloads the option a little...)

-Eric
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