linux-ext4 - Re: [RFC] ext4: Semantics of delalloc,data=ordered

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Date:	Mon, 16 Jun 2008 14:22:24 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Jan Kara <jack@...e.cz>
CC:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	cmm@...ibm.com, tytso@....edu, linux-ext4@...r.kernel.org,
	adilger@....com
Subject: Re: [RFC] ext4: Semantics of delalloc,data=ordered

Jan Kara wrote:
>>>   Imagine you have a file with blocks 1 and 3 allocated and block 2 is a
>>> hole. You write blocks 1-3. Block 2 isn't allocated because of delalloc.
>>> Now if inode is already in the current transaction's list, during commit
>>> writes to blocks 1 and 3 will land on disk but write to block 2 will
>>> happen only after pdflush finds it.
>> And that should be fine with data=ordered mode right ?. Because since
>> block 2 is not yet allocated we don't have associated meta-data. So
>> even if we crash we have meta-data pointing to 1 and 3 and not 2. The
>> problem is only when we write the meta-data pointing to block 2 and not
>> block 2 itself ?.
>   Yes, it is correct. I may be just surprising (we didn't do things like
> this in data=ordered mode before).

Will it even be surprising?  "fill-in-hole; crash;" today may give you
the same thing, right?  It's just that with delalloc it might be a
bigger window in time for this to happen?

-Eric
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