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

Eric Sandeen
2012-10-22T20:28:28Z

On 10/22/12 3:24 PM, Ben Myers wrote:
> Hey Eric,
>
> On Mon, Oct 22, 2012 at 11:22:44AM -0500, Eric Sandeen wrote:
>> On 10/22/12 10:56 AM, Ben Myers wrote:
>>> 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?
>>
>> Well, I started putting in some feature flag detection. So in a super-perfect world
>> we could:
>>
>> * Dump to to V4 only if the PROJID32 flag is set on the fs being dumped, else V3
>
> It might be better to just do v4 always (except if -K is specified). I guess I
> don't understand what the additional level of complexity buys us.

Because then "V4" signifies "top 16 bits are set in projid"

>> * Refuse / warn if restoring a < V4 dump onto a fs w/ the PROJID32 set
>
> Lets see if I understand...
>
> In this case we're warning the user because we are concerned that they may have
> been running with projid32 and dumped the filesystem with an older xfsdump
> which didn't capture the top 16 bits of the projid. The restore of the
> filesystem would result in the loss of those top 16 bits.

right

>> * Refuse / warn if restoring a >= V4 dump onto a non-PROJID32 fs
>
> In this case we're warning the user because... the filesystem would drop the
> top 16 bits of the valid project ids in the dump?

well, I think dump would continue but would issue failures/warnings when
it tries to set the high bits of projid. Failing to set attrs isn't fatal
to the restore process, IIRC.

>> * Extend the -K option to allow specifying/forcing arbitrary dump levels
>
> This would allow the newer xfsdump to create a v3 dump level.

yeah.

>> But:
>>
>> * No released kernel yet reports the feature flag, so we don't have guarantees
>> about feature presence
>> - So going to V4 only if flag found will silently fail prior to kernel 3.7,
>> since no feature flag is available there.
>> - so then we should default to V4 regardless of feature flag test
>> ~ so then we can't make restore of V4 require the feature flag on the fs
>> * Older dump/restore already has the silent failure problem
>> * By the time you are restoring a <V4 dump onto a PROJID32 fs, it's unlikely
>> that you have further options anyway (you're possibly in recovery, can't go
>> back & make V4 dumps to restore from)
>
> xfsdump/restore aren't just for those recovery situations: some people might
> dump on one machine and pipe it to another host during an upgrade.

fair enough.

>> * Warning on >= V4 dump to non-PROJID32 fs only makes sense if V4 is contingent
>> on feature set/recognized at dump time but we cant' reliably detect that.
>> * projid32bit is a non-default option today, so it's an infrequent case...
>
> It sounds like the super-perfect world needs to have kernel support for
> reporting whether a filesystem has the projid32 feature flag set. Are there
> other options for getting that data? xfs_db?

I suppose, but do you really want xfsdump to shell out to xfs_db, or .. ?

> In this somewhat-less-than-perfect world, I think it might be ok to extend the
> -K option to have some backward compatability in the near term, and then work
> toward improved warnings once your fsgeom series hits upstream. Maybe I'm
> missing something.

I think it's too many knobs with too many caveats, and too much work, for very
little gain. Bear in mind this dump/restore bug has never even been reported
in the wild.

-Eric

> Regards,
> Ben