linux-ext4 - Re: On-disk field assignments for metadata checksum and snapshots

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Message-ID: <CAGBYx2Yzb=zt-Bay8iTzkFD6SJu+GW_qgw2+QXD-djnnqdq_uw@mail.gmail.com>
Date:	Fri, 16 Sep 2011 20:00:52 +0800
From:	Yongqiang Yang <xiaoqiangnk@...il.com>
To:	Amir Goldstein <amir73il@...il.com>
Cc:	"Ted Ts'o" <tytso@....edu>, "Darrick J. Wong" <djwong@...ibm.com>,
	linux-ext4@...r.kernel.org,
	Andreas Dilger <adilger.kernel@...ger.ca>
Subject: Re: On-disk field assignments for metadata checksum and snapshots

On Fri, Sep 16, 2011 at 7:48 PM, Amir Goldstein <amir73il@...il.com> wrote:
> On Fri, Sep 16, 2011 at 2:35 PM, Yongqiang Yang <xiaoqiangnk@...il.com> wrote:
>> On Fri, Sep 16, 2011 at 2:52 PM, Amir Goldstein <amir73il@...il.com> wrote:
>>> On Thu, Sep 15, 2011 at 8:57 PM, Ted Ts'o <tytso@....edu> wrote:
>>>> On Thu, Sep 15, 2011 at 07:01:47PM +0300, Amir Goldstein wrote:
>>>>> > +       {       E2P_FEATURE_COMPAT, EXT2_FEATURE_COMPAT_EXCLUDE_BITMAP,
>>>>> > +                       "snapshot" },
>>>>>
>>>>> The feature name should read "exclude_bitmap".
>>>>> This is a COMPAT feature which should be set by default on every mkfs.ext4.
>>>>> The "has_snapshot" (or "snapshots") feature is a RO_COMPAT feature, which
>>>>> can be turned on and off, depending on whether you want to enable or
>>>>> disable snapshots.
>>>>
>>>> Yeah, I was trying to use a name which would be a bit more descriptive
>>>> than "exclude_bitmap", but I agree that will get very confusing with
>>>> the "has_snapshot" feature name.
>>>>
>>>>                                                - Ted
>>>>
>>>
>>> maybe we can simply call it snapshot_bitmap, if it pleases everyone ;-)
>> active_snapshot_bitmap will be clearer.:-)
>
> clearer to who?
To code reviewers, actually bits in exclude bitmap are set only if the
blocks are used by active/current snapshot inode.

Yongqiang.
> this name is printed by "tune2fs -l"
> potential readers of this output, who did not read the documentation
> first, are just as likely to know what
> "exclude_bitmap" means as when they read "resize_inode" or "flex_bg",
> but at least "snapshot_bitmap"
> will give them a clue.
>
> Amir.
>



-- 
Best Wishes
Yongqiang Yang
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