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

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Message-ID: <20140506152129.GA31730@thunk.org>
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
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