linux-ext4 - Re: [PATCH][RFC] resize2fs and uninit

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Message-id: <20090916190831.GH2537@webber.adilger.int>
Date:	Wed, 16 Sep 2009 13:08:31 -0600
From:	Andreas Dilger <adilger@....com>
To:	Will Drewry <redpig@...aspill.org>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: [PATCH][RFC] resize2fs and uninit_bg questions

On Sep 16, 2009  11:24 -0500, Will Drewry wrote:
> I have a two questions with an accompanying patch for clarification.
> 
> resize2fs is uninit_bg aware, but when it is expanding an ext4
> filesystem, it will always zero the inode tables.  Is it safe to mimick
> mke2fs's write_inode_table(.., lazy_flag=1) and leave the new block
> groups' inode tables marked INODE_UNINIT, BLOCK_UNINIT and _not_ zero
> out the inode table if uninit_bg is supported?
> 
> If it is okay, then it means offline resizing upwards can be just as
> fast as mke2fs.  I've attached a patch which is probably incomplete.
> I'd love feedback as to the feasibility of the change and/or patch
> quality.
> 
> As a follow-on, would it be sane to add support like this for
> online resizing.  From a cursory investigation, it looks like
> setup_new_block_groups() could be modified to not zero itables
> if uninit_bg is supported, and INODE_ZEROED could be replaced
> with ΒG_*_UNINIT.  However, I'm not sure if that is a naive
> view.  I'm happy to send along a patch illustrating this change
> if that'd be helpful or welcome.

The question is why you would want to risk disk corruption vs.
the (likely not performance critical) online resize?

> Any and all feedback is appreciated -- even if it just for me
> to look at the archives/link/etc.
> 
> diff --git a/resize/resize2fs.c b/resize/resize2fs.c
> index 1a5d910..9fcc3b9 100644
> --- a/resize/resize2fs.c
> +++ b/resize/resize2fs.c
> @@ -497,8 +497,7 @@ retry:
>  
>  		fs->group_desc[i].bg_flags = 0;
>  		if (csum_flag)
> -			fs->group_desc[i].bg_flags |= EXT2_BG_INODE_UNINIT |
> -				EXT2_BG_INODE_ZEROED;
> +			fs->group_desc[i].bg_flags |= EXT2_BG_INODE_UNINIT;

This shouldn't be unconditional, since most users will want the
safety of having zeroed inode tables.

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.

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