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Date:	Fri, 26 Aug 2011 13:06:48 -0700
From:	Justin Maggard <jmaggard10@...il.com>
To:	ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: New resize interface implementation

On Wed, Aug 17, 2011 at 12:28 AM, Yongqiang Yang <xiaoqiangnk@...il.com> wrote:
> On Tue, Aug 16, 2011 at 8:22 AM, Justin Maggard <jmaggard10@...il.com> wrote:
> > Does this patch set combined with your e2fsprogs patch add 64-bit resize
> > support now, or does it just make it easier to add later?
> YES. e2fsprgos's patch is ready too.

So I finally got around to gather the hardware and patching all the
software components to try out this 64-bit expansion code.  The first
thing I noticed is that there is still a check to make sure the block
count is 32 bits.  However, I can get around it by specifying a size
string (something like "20T") rather than a block count, in which case
it will actually try the expansion.

> > If I am making a 64-bit ext4 filesystem today (20TB), and hoping to resize
> > it next year to 30TB what features should I set?  In my searching it sounded
> > like maybe I would need meta_bg, but it is not compatible with the default
> > resize_inode.
> You can understand meta_bg here http://linuxsoftware.co.nz/wiki/ext4.
> Now, ext4 with meta_bg does not support resize.  It is in ext4's TODO list.
> The feature you should set is resize_inode.
>
> > Also, if I am making a <16TB filesystem today, should I turn on the 64-bit
> > flag in order to expand to >16TB in the future?
> Yes.  You should turn on 64 bit feature.  If the block number is 32
> bit, the size it can support is 2^32 * 2^(log blocksize),  4K
> blocksize as an example, it maximum size of a filesystem is 2^32 *
> 2^12 = 2^44 = 16TB.

I think this is where the real problem is with this 64-bit resize
support.  With the 64-bit flag set, the most I can expand by online is
just 8TB over the life of the filesystem, because my reserved GDT
blocks get used up twice as fast as with a 32-bit filesystem.  Is
there any way around this?

-Justin
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