linux-ext4 - Re: [PATCH] libext2fs: reserve exclude bitmap fields in group descriptor

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Date:	Fri, 16 Sep 2011 19:43:19 +0800
From:	Yongqiang Yang <xiaoqiangnk@...il.com>
To:	Amir Goldstein <amir73il@...il.com>
Cc:	"Ted Ts'o" <tytso@....edu>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	"Darrick J. Wong" <djwong@...ibm.com>,
	Sunil Mushran <sunil.mushran@...cle.com>,
	Andi Kleen <andi@...stfloor.org>,
	Mingming Cao <cmm@...ibm.com>,
	Joel Becker <jlbec@...lplan.org>, linux-ext4@...r.kernel.org,
	Coly Li <colyli@...il.com>
Subject: Re: [PATCH] libext2fs: reserve exclude bitmap fields in group descriptor

On Fri, Sep 16, 2011 at 2:45 PM, Amir Goldstein <amir73il@...il.com> wrote:
> On Fri, Sep 16, 2011 at 12:57 AM, Ted Ts'o <tytso@....edu> wrote:
>> On Thu, Sep 15, 2011 at 01:08:34PM -0600, Andreas Dilger wrote:
>>> In that light, why not continue to use an inode to map the exclude bitmap
>>> blocks, where the bitmap offset is (group * blocksize), instead of
>>> explicitly listing all of the blocks in the group descriptor?  This is
>>> how the buddy bitmap works in memory only, but it could be done for the
>>> exclude bitmap on disk.
>>
>
> And this exactly is how the exclude inode works, except only the DIND
> block is used
> for mapping, just like the resize inode.
>
>> I seem to recall the use of an inode to map the exclude bitmap added a
>> huge amount of complexity to the snapshot patches.  Amir, am I
>> remembering this correctly?
>>
>
> No, I am not sure this is accurate.
> I think after we over viewed the e2fsprogs snapshots patch set, you
> has 2 observations:
> 1. the largest part (in lines of code) of the e2fsprogs snapshot patch set
>    is related to the exclude inode/bitmap code.
> 2. it reminded you of resize inode too much and you didn't like that
> 3. There was also the issue of whether or not to allow the removal of
> the exclude inode/bitmap
>    and then re-allocation would not be in optimal layout
>
> In retrospect, after Yongqiang has implemented the alternative exclude
> bitmap patch set
> for e2fsprogs, I can say:
> 1. The alternative patch set is not smaller
> 2. It is a lot more elegant and deals with correct layout of exclude
> bitmap (next to block bitmap)
> 3. It will be able to deal with 64bit fs (unlike exclude/resize inode)
> and 64bit resize
>
> Yongqiang, do you have anything else to add to the exclude inode vs.
> group desc issue?
Nope, regarding resize group desc is better than exclude inode.  For
meta_bg, group desc is much more welcome.


Yongqiang.
>
> Amir.
>



-- 
Best Wishes
Yongqiang Yang
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