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Date:	Thu, 31 Mar 2011 20:58:07 -0700
From:	Amir Goldstein <>
To:	Trond Myklebust <>
Cc:	Chris Mason <>,
	lsf-pc <>,
	Joel Becker <>,
	linux-fsdevel <>,
	Ext4 Developers List <>,
	Josef Bacik <>
Subject: Re: [Lsf-pc] [LSF/FS TOPIC] Ext4 snapshots status update

Sent from my iPhone

On 31/03/2011, at 17:10, Trond Myklebust <>  

> On Wed, 2011-03-30 at 14:08 +0200, Amir Goldstein wrote:
>> On Wed, Mar 30, 2011 at 1:50 PM, Chris Mason  
>> <> wrote:
>>> Excerpts from Amir Goldstein's message of 2011-03-30 00:16:45 -0400:
>>>> On Wed, Mar 30, 2011 at 2:34 AM, Joel Becker <>  
>>>> wrote:
>>>>> On Wed, Mar 23, 2011 at 10:19:38PM +0200, Amir Goldstein wrote:
>>>>>> On Fri, Feb 4, 2011 at 2:20 AM, Joel Becker  
>>>>>> <> wrote:
>>>>>>> On Fri, Feb 04, 2011 at 12:33:39AM +0200, Amir Goldstein wrote:
>>>>>>>       I've already got a design for a front-end snapshot  
>>>>>>> program that
>>>>>>> implements a policy on top this generic behavior.  This design  
>>>>>>> would
>>>>>>> cover both first-class and hidden style snapshots, because it  
>>>>>>> assume
>>>>>>> snapshots are in a distinct namespace.  I haven't gotten  
>>>>>>> around to
>>>>>>> implementing it yet, but btrfs and other snapshottable  
>>>>>>> filesystems were
>>>>>>> part of the design goal.
>>>>>> Any chance of getting a copy of that design of yours, to get a  
>>>>>> head start
>>>>>> for LSF?
>>>>>       Yeah, I owe it to you.  It wasn't a written-down thing, it  
>>>>> was a
>>>>> hammered-out-in-our-heads thing among some ocfs2 developers.   
>>>>> I'm going
>>>>> to braindump here to get us going.  First, I'll speak to your  
>>>>> points.
>>>>>> Here are some other generic snapshot related topics we may want  
>>>>>> to discuss:
>>>>>> 1. Collaborating the use of inode flags COW_FL, NOCOW_FL,  
>>>>>> suggested by Chris.
>>>>>       I'm unsure where these fit, perhaps because I missed the
>>>>> discussion between Chris and you.  ocfs2 has the inode flag
>>>>> OCFS2_REFCOUNTED_FL to signify a refcount tree is attached to  
>>>>> the inode.
>>>>> This is ocfs2's structure for maintaining extent reference  
>>>>> counts.  Is
>>>>> your COW_FL the same?  Or is it a permission flag?  NOCOW_FL  
>>>>> sounds
>>>>> like: "Set this flag on the inode and it will prevent CoW."
>>>> I don't have a use for COW_FL, since my snapshots are volume  
>>>> level snapshots.
>>>> I intend to use NOCOW_FL to mark an inode as an "island" of NOCOW
>>>> blocks in the volume.
>>>> Maybe Chris or Josef can elaborate of the flags intended use in  
>>>> btrfs.
>>> NOWCOW_FL in btrfs means to directly overwrite blocks (and not do  
>>> crcs)
>>> unless the block has another reference.  If there is another  
>>> reference,
>>> we COW once to honor the snapshot and then continue in NOCOW mode.
>>> I'm kind of worried about your NOCOW island idea, maybe we can  
>>> talk more
>>> about that next week.  It seems like it will lead to a lot of admin
>>> surprises.
>> Yes, that's something to talk about.
>> My desire for NOCOW comes from lack of sub volume granularity
>> in ext4 snapshots.
>> My NOCOW design states that NOCOW flag cannot be toggled on a  
>> regular file.
>> like a snapshot file, a NOCOW file must be born and die NOCOW, to  
>> avoid
>> admin surprises. NOCOW directories (which ARE COWed) are were NOCOW
>> files are born.
>> Using this scheme, an admin can exclude->include->exclude directory  
>> sub trees
>> from snapshots.
> OK. I'd like to schedule a general talk about the state of snapshots  
> and
> future improvements. I'm assuming you would like to lead the debate.
Sure, I can do that.


> Cheers
>  Trond
> -- 
> Trond Myklebust
> Linux NFS client maintainer
> NetApp
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