linux-ext4 - Re: e4defrag and immutable files

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Message-ID: <4C0EC034.6010603@zytor.com>
Date:	Tue, 08 Jun 2010 15:12:04 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Sunil Mushran <sunil.mushran@...cle.com>
CC:	tytso@....edu, linux-ext4@...r.kernel.org,
	Joel Becker <Joel.Becker@...cle.com>,
	Mark Fasheh <mfasheh@...e.com>
Subject: Re: e4defrag and immutable files

On 06/02/2010 11:02 AM, Sunil Mushran wrote:
> On 06/02/2010 12:28 AM, H. Peter Anvin wrote:
>> On 06/01/2010 12:49 PM, tytso@....edu wrote:
>>> On Tue, Jun 01, 2010 at 12:32:29PM -0700, Sunil Mushran wrote:
>>>>
>>>> We (ocfs2) are looking to add a new attribute to denote files that
>>>> have a fixed allocation on disk. But at the same time, allow writes
>>>> that do not change the allocation on disk. No truncating, extending,
>>>> filling holes, etc. We were thinking of calling it "Static" files.
>>>
>>> That's an interesting set of semantics, and it might make sense to
>>> conflate that with a local disk "don't move or defrag" the file
>>> option.  I'm not crazy with the name "static", since it could mean a
>>> number of other things in other contexts, but I admit I can't think of
>>> a better name.
>>>
>>
>> For what it's worth, this sort of seems to be what one would expect if 
>> a file is *both* "fixed" and "immutable".
> 
> "Immutable" means the contents do not change. But the file mappings
> could change.
> 
> "Fixed mapping" means the mappings do not change but contents
> could (as long as the ondisk mappings don't).
> 
> "Fixed metadata" means the entire inode (mappings included) cannot
> change but the contents could (as long as the ondisk mappings don't).
> (This does have the side effect of allowing writes without touching the
> mtime. Like XFS' invisible i/o.)
> 

Actually, if you're going to have three flags you might as well make
them orthogonal.  That is, separate "fixed contents", "fixed mappings",
"fixed metadata" -- and don't consider the mapping as metadata for this
purpose.

	-hpa
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