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Re: [PATCH 0/3] xfsdump: more projid32bit fixes

To: Eric Sandeen <sandeen@xxxxxxxxxxx>
Subject: Re: [PATCH 0/3] xfsdump: more projid32bit fixes
From: Ben Myers <bpm@xxxxxxx>
Date: Mon, 22 Oct 2012 10:56:04 -0500
Cc: Eric Sandeen <sandeen@xxxxxxxxxx>, xfs-oss <xfs@xxxxxxxxxxx>
In-reply-to: <508559DE.3060203@xxxxxxxxxxx>
References: <50788C50.40600@xxxxxxxxxx> <508559DE.3060203@xxxxxxxxxxx>
User-agent: Mutt/1.5.20 (2009-06-14)
Hey Eric,

On Mon, Oct 22, 2012 at 09:36:14AM -0500, Eric Sandeen wrote:
> On 10/12/12 4:32 PM, Eric Sandeen wrote:
> > I recently sent a patch for 32-bit project IDs for xfsdump, to properly
> > restore the top 16 bits, which otherwise get lost.  This forced a new
> > dump format version 4 (we were currently at 3).
> > 
> > One thing missing is that we should not restore a dump with 32-bit
> > project IDs onto a filesystem w/o that format; the restore will fail
> > to restore the top 16 bits (but otherwise it returns success; attribute
> > setting failures are not fatal (!?))
> > 
> > Also, 32-bit project ID is a bit uncommon; bumping the format (and making
> > older restore incompatible) is a bit draconian.
> > 
> > 3 patches here:
> > 
> > 1/3: extend fs info call to get fs flags as well
> > 2/3: default back to V3 and go to V4 only if the projid32 flag is set
> > 3/3: fail restore if the target XFS fs doesn't have projid32 set
> > 
> > I have to say, I'm not super happy with this.  I have nagging fear
> > of feature-flag-itis, and I'm not sure how extensible this is as newer
> > versions may appear.  But anyway, here's a place to start.
> > 
> > (p.s. anybody have wkendall's new email?)  ;)
> 
> I spoke with Bill, and he actually didn't feel that a new version was
> needed for the projid32 fix.  I'd like to get some discussion here,
> and reach an agreement.  *NOT* bumping the version simplifies a whole
> lot of things.

Can you post a list of things that it simplifies?  That would help with the
discussion.

> Here's what I'd said to Bill:
> 
> >> If we restore old dumps w/ new xfsdump, nothing special is needed;
> >> 0 gets restored for the top 16 bits (vs. garbage, which WOULD be
> >> bad).
> >> 
> >> So bumping the version really only prevents old restore from
> >> restoring newer dumps.
> >> 
> >> If I *didn't* bump the version, then old restore would work, and
> >> would simply not restore the top 16 bits - just like an old
> >> dump+restore option did.
> 
> And Bill replied:
> 
> > Had a look at xfsdump, and I agree, there's no need to bump the format
> > version. Nice of someone to leave some zeroed pad bytes next to the
> > project id. 
> 
> so what are people's thoughts?  Moving to a new version has complexity
> & compatibility consequences...

Initially I think that moving to the new version was the right thing to do.
It's good for users to have some warning when there are consequences of
upgrading to a new release of xfsdump.

A new dump format version will make old xfs_restore fail with an error if a v4
dump is encountered, rather than do the restore and chop off the top 16 bits of
the project id silently.  I think that for a user it is reasonable to expect to
be notified if project ids are going to be lost.  Looks like there is no way
for an older xfsrestore to force an override, so if there is to be some level
of compatability between versions, newer xfsdump would have to still be able to
create v3 dumps.

What other complexity/compat issues are there?

Regards,
        Ben

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