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Proposal/RFC: new metadata-specific UUID for V5 supers

To: xfs-oss <xfs@xxxxxxxxxxx>
Subject: Proposal/RFC: new metadata-specific UUID for V5 supers
From: Eric Sandeen <sandeen@xxxxxxxxxxx>
Date: Mon, 27 Apr 2015 17:10:25 -0500
Delivered-to: xfs@xxxxxxxxxxx
Wanted to float this & see what people think...

Today, it's impossible (FSVO impossible) to change a V5 filesystem's
UUID, because that UUID is stamped into every bit of metadata.  If
not impossible, it's unimplemented and any implementation would be
terribly inefficient.

With 20/20 hindsight, an sb_meta_uuid field, separate from the sb_uuid
field that is userspace-visible, would have avoided this problem.

We can certainly add it, but it'd need to be an incompat change, because
old kernels would see this as catastrophic mismatching of every bit
of metadata on a V5 superblock filesystem.

Other than that little problem (o_O) it'd be trivial to implement.

Thoughts?

Thanks,
-Eric

p.s. Thanks to Zach for making me think about this, but I'm not
blaming him for my decision to propose it.  ;)

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