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

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Date:	Fri, 16 Sep 2011 15:12:19 +0300
From:	Amir Goldstein <amir73il@...il.com>
To:	Yongqiang Yang <xiaoqiangnk@...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 3:00 PM, Yongqiang Yang <xiaoqiangnk@...il.com> wrote:
> 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.
>

First, I was never suggesting to change the name of the feature in the
code, only
the way it is presented by tune2fs -l.
Second, you are confusing exclude_bitmap with cow_bitmap (or
active_snapshot_cow_bitmap).
The bits in exclude_bitmap are set for *ALL* blocks of *ALL* snapshots.
So "snapshot_bitmap" or "snapshots_bitmap" is an appropriate name.

Amir.
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