linux-ext4 - Re: [PATCH 37/37] ext5: define new subtype to add features and reduce testing complexity

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Date:	Tue, 6 May 2014 17:30:23 +0200 (CEST)
From:	Lukáš Czerner <lczerner@...hat.com>
To:	"Theodore Ts'o" <tytso@....edu>
cc:	"Darrick J. Wong" <darrick.wong@...cle.com>,
	linux-ext4@...r.kernel.org
Subject: Re: [PATCH 37/37] ext5: define new subtype to add features and reduce
 testing complexity

On Tue, 6 May 2014, Theodore Ts'o wrote:

> Date: Tue, 6 May 2014 11:21:29 -0400
> From: Theodore Ts'o <tytso@....edu>
> To: Lukáš Czerner <lczerner@...hat.com>
> Cc: Darrick J. Wong <darrick.wong@...cle.com>, linux-ext4@...r.kernel.org
> Subject: Re: [PATCH 37/37] ext5: define new subtype to add features and reduce
>      testing complexity
> 
> On Tue, May 06, 2014 at 02:50:39PM +0200, Lukáš Czerner wrote:
> > meta_bg - Just makes group descriptors to be spread across the file
> > 	system. It has been around for some time and I am not sure
> > 	why this is not a default already. It should also increase
> > 	the limit of the file system size but I am not sure whether
> > 	this is still true with flex_bg ?
> 
> meta_bg signiicantly slows down mount operations (and in general, any
> operation where we need to read in the block group descriptors ---
> i.e., dumpe2fs, e2fsck, etc.)
> 
> The strategy for meta_bg is that it's something that we enable as we
> need it, as part of an online or off-line resize.  That way, we keep
> the block groups contiguous for as long as possible.  Once the resize
> inode has been exhausted (which _will_ happen when the file system
> size grows beyond 16T), the resize operation will turn off the
> resize_inode feature and then enable the meta_bg feature.
> 
> And this is all working today, with the latest kernel and e2fsprogs;
> so there's no reason to enable meta_bg as part of mke2fs operation,
> and a good reason not to enable it by default, but to let resize2fs
> turn it on when it makes sense to do so.
> 
>         	    		       - Ted

Perfect, thanks for explanation. I was not very sure about meta_bg
myself.

Thanks!
-Lukas