linux-ext4 - Re: confused by delayed allocation and ordered journal

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Date:	Sat, 25 Dec 2010 18:43:27 +0200
From:	Amir Goldstein <amir73il@...il.com>
To:	"Ted Ts'o" <tytso@....edu>
Cc:	Yongqiang Yang <xiaoqiangnk@...il.com>, linux-ext4@...r.kernel.org
Subject: Re: confused by delayed allocation and ordered journal

On Sat, Dec 25, 2010 at 4:39 PM, Ted Ts'o <tytso@....edu> wrote:
> On Sat, Dec 25, 2010 at 09:15:28PM +0800, Yongqiang Yang wrote:
>>
>> I found that if a block'allocation is delayed, and is not allocated
>> when journal flushes it, then journal just redirties it and return in
>> journal_submit_data_buffers.
>>
>> If I understand right, how to guarantee that the journal mode is ordered?
>
> The primary goal of ordered mode is to make sure that stale data is
> not exposed after a crash.  To the extent that delayed allocation also
> achieves this goal, it's fine.  The fact that ext3 forced data blocks
> out as part of its jbd commit function was always an implementation
> detail.

Hi Ted,

FYI, Yongqiang is researching move-on-write of extent mapped files
data and opportunistic de-fragmentation on rewrite
and this is the context of his question.

As I mentioned to you on Plumbers the ordering requirements for move-on-write
are a bit stronger than "not exposing un-initialized data".
In the process of move-on-write, the extent must always map written data blocks
and traditional 'ordered' mode can be used to guaranty that.

Yongqiang,

I believe in the case of delalloc move-on-rewrite, on write_begin(),
you only need to reserve blocks (in-memory)
and on writepages(), you do the actual move-on-write, modifying the
extent from one valid data blocks range to another.

>
> In the long run we'll be getting rid of ordered mode even more so, by
> writing the data block first, and only then updating the file system
> metadata.  At that point there will be no ordered flushing at all, and
> in fact ordered mode will go away as a journal mode supported by ext4.
>

And that can simplify move-on-write implementation as well.
But until that happens, snapshots will require 'ordered' mode.

Amir.
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