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Re: [PATCH 1/2] xfs: create new metadata UUID field and incompat flag

To: Brian Foster <bfoster@xxxxxxxxxx>
Subject: Re: [PATCH 1/2] xfs: create new metadata UUID field and incompat flag
From: Eric Sandeen <sandeen@xxxxxxxxxxx>
Date: Mon, 11 May 2015 11:52:52 -0500
Cc: Eric Sandeen <sandeen@xxxxxxxxxx>, xfs-oss <xfs@xxxxxxxxxxx>
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20150511161404.GA54361@xxxxxxxxxxxxxxx>
References: <554D1F56.7030209@xxxxxxxxxx> <554D2110.8030405@xxxxxxxxxxx> <20150511161404.GA54361@xxxxxxxxxxxxxxx>
On 5/11/15 11:14 AM, Brian Foster wrote:
> On Fri, May 08, 2015 at 03:48:16PM -0500, Eric Sandeen wrote:
>> > This adds a new superblock field, sb_meta_uuid.  If set, along with
>> > a new incompat flag, the code will use that field on a V5 filesystem
>> > to compare to metadata UUIDs, which allows us to change the user-
>> > visible UUID at will.  Userspace handles the setting or clearing
>> > of the incompat flag as appropriate, as the UUID gets written.
>> > 
>> > If the incompat flag is not set, copy the user-visible UUID into
>> > into the meta_uuid slot in memory; this is not written back to disk in
>> > this case.
>> > 
>> > The remainder of this patch simply switches verifiers, initializers,
>> > etc to use the new sb_meta_uuid field.
>> > 
>> > Signed-off-by: Eric Sandeen <sandeen@xxxxxxxxxx>
>> > ---
> This all looks fine to me. The only thing that confuses me is why we
> continue to use sb_uuid for the log. Eric points out on irc that we
> can't modify the uuid when the log is dirty, so we can't actually break
> anything in this manner. In other words, we effectively already handle a
> modified uuid with respect to the log.
> 
> That said, it seems inconsistent to me to create a metadata uuid field
> and not use it for all metadata. My expectation from such a change is to
> see sb_uuid now used only for user facing bits (e.g., mount and get geo
> related) and sb_meta_uuid used everywhere else. Instead, we use sb_uuid
> for the user facing bits, sb_meta_uuid for the internal bits that can't
> handle a uuid change and sb_uuid for the internal bits that can.
> 
> I suppose I could be convinced otherwise if there's context I'm missing,
> but otherwise this seems a bit confusing to me...
> 
> Brian

Yeah, I'm sympathetic to that argument.  I can look into using sb_meta_uuid
for the log too, if the incompat flag is set.

I had skipped it just because that use of the UUID predated the CRC changes.

-Eric

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