So, I have a different perspective. Given that the failure mode here is
severe (namely, silent data corruption) I think our collective disposition
needs to be to ignore hole birth on zfs send by default. To claim that
this is heavy handed (that is, that the hole birth optimization is so
essential to the performance of so many that disabling it would be
debilitating) is also to effectively claim that this condition is common
and that silent data corruption is therefore rampant -- which should in
turn strengthen our resolve to eliminate it, regardless the cost. For
whatever it's worth, for our use case we believe the optimization afforded
by tracking hole birth is broadly immaterial -- and that disabling this
optimization has similarly little effect on our systems.
Either way, the system should be correct first -- especially given the
acute failure mode -- and hole birth should be ignored by zfs send until we
know that this work is entirely correct.
- Bryan
On Wed, Aug 31, 2016 at 8:58 AM, Paul Dagnelie <[email protected]> wrote:
> I think that the best approach, from my perspective, is to have an
> off-by-default flag that lets you ignore the effects of hole birth. While
> we're doing that, we should keep trying to fix all of these issues; we've
> made good progress, and I suspect there aren't many issues left. I think
> this approach does at least an ok job of addressing the needs of everyone
> affected:
> 1) Developers can keep developing, and this approach has a fairly low
> implementation cost
> 2) Users new to ZFS will get to use the feature by default, and will be
> unaffected by any previous hole_birth bugs, because they have no data from
> before the fixes
> 3) Users who already have ZFS but didn't have corrupt data will get to use
> the feature by default, and also will be unaffected by earlier bugs because
> their data is not corrupt.
> 4) Users who already have ZFS and do have corrupt data are probably aware
> of the issue; they will hopefully be paying enough attention to modify the
> flag when they upgrade to the new version. They won't be able to use
> hole_birth until all their old data falls out of whatever retention
> policies they have set up, or they recreate it.
>
> A flag (undocumented?) to zfs send that forced the use of hole_birth might
> not be a bad idea, because it would let group 4 take advantage of the
> feature on new data they create. The only people this approach doesn't
> help are users who have corrupted data, but aren't aware of it; if we think
> that's a large population, we might want to take a different approach, like
> the "hole_birth_fixed" featureflag.
>
> On Wed, Aug 31, 2016 at 4:02 AM, Rich <[email protected]> wrote:
>
>> Hi all,
>> I wanted to prod this discussion back to life again, given the existence
>> of two more hole_birth flaws in the intervening period, namely Illumos
>> #7176 and ZoL #4996.
>>
>> So, if we'd opted for a feature flag to mark "fixed" before this point,
>> we'd be in a position of needing another one (or a mechanism to push the
>> enabled_txg forward) to indicate that "actually, after THIS POINT, we don't
>> think there's any more known bugs."
>>
>> I dislike the idea of whack-a-mole on revisions that are "safe", or
>> manual intervention to detect that, but I'm not sure there's a nicer
>> solution than either that or defaulting to discarding hole_birth metadata
>> without an explicit flag to zfs send like -e or -L.
>>
>> If anyone has any nicer suggestions, or thoughts on how to better handle
>> this recurring problem, I'd love to have a better answer to this.
>>
>> Thanks,
>> - Rich
>>
>> On Fri, Jul 8, 2016 at 1:31 PM, Boris <[email protected]> wrote:
>>
>>> So, as we found out a short time ago, this is actually fixed in ZoL as
>>> well, but after the 0.6.5.7
>>> ------------------------------
>>> *From:* Rich <[email protected]>
>>> *Sent:* Friday, July 8, 2016 12:37:52 PM
>>> *To:* [email protected]
>>> *Cc:* developer; Developer Lists Illumos
>>>
>>> *Subject:* Re: [developer] Improvements to 6513 handling
>>>
>>> Hi Boris,
>>> I now have working code that implements this feature, and defaults to
>>> ignoring hole_birth data for sends if this feature is not enabled; I'm
>>> going to post patches for it after I've tested it on both ZoL and illumos.
>>>
>>> The latter portion of the above is easily changed, but "always correct
>>> and marginally less efficient for old data" seemed a better default than
>>> "usually correct".
>>>
>>> For a filesystem that prides itself on not allowing silent corruption,
>>> requiring manual detection and intervention for correctness seems
>>> unreasonable to me.
>>>
>>> - Rich
>>>
>>> On Fri, Jul 8, 2016 at 12:15 PM, Boris <[email protected]> wrote:
>>>
>>>> Hi, Rich,
>>>>
>>>>
>>>> I agree that unconditional switch using the tunable is heavy if done
>>>> 'as a matter of standard practice' as opposed to 'for a short time, to
>>>> fixup the known corrupted backups'.
>>>>
>>>>
>>>> To clarify the earlier suggestions, people with data affected by the
>>>> bug can do two things:
>>>>
>>>>
>>>> 1) install the code with 6513 fix and the patch with the tunable,
>>>> then temporarily turn off the hole birth optimization, resend the
>>>> 'difference' (a selected subset of incrementals) affected by the problem,
>>>> then turn the optimization back on
>>>>
>>>> 2) install the code with 6513 fix without the patch, do a full send of
>>>> the affected snapshots
>>>>
>>>>
>>>> For 2) the non-incremental send would need to happen only once per the
>>>> affected snapshot lineage. Once the missed holes are re-instated with the
>>>> full send, the new fixed code will perform proper incremental sends.
>>>>
>>>>
>>>> 1) is potentially more optimal in terms of resources (network bw, etc.)
>>>>
>>>> 2) is potentially simpler from the operational standpoint, does not
>>>> require building/installing patched code, twiddling the tunables, etc.
>>>>
>>>>
>>>> Boris.
>>>>
>>>>
>>>>
>>>>
>>>> ------------------------------
>>>> *From:* Rich <[email protected]>
>>>> *Sent:* Friday, July 8, 2016 11:26:27 AM
>>>> *To:* developer
>>>> *Cc:* Developer Lists Illumos
>>>> *Subject:* Re: [developer] Improvements to 6513 handling
>>>>
>>>> Hi Boris,
>>>> A full send of the affected snapshots should be safe, AIUI - but that
>>>> means people would need to do non-incremental snapshot sends to be certain
>>>> of not hitting this bug, which becomes increasingly infeasible as your
>>>> datasets grow.
>>>>
>>>> If we're looking for the simplest solution without risk of data
>>>> corruption, unconditionally ignoring the hole_birth data for doing a zfs
>>>> send fits the bill, but seems a bit heavy-handed.
>>>>
>>>> This seemed like the best way to permit people to safely send older
>>>> datasets while also permitting use of the hole_birth data going forward.
>>>>
>>>> - Rich
>>>>
>>>> On Fri, Jul 8, 2016 at 11:03 AM, Boris <[email protected]> wrote:
>>>>
>>>>> Hi, Rich,
>>>>>
>>>>>
>>>>> perhaps there is a simpler solution here.
>>>>>
>>>>>
>>>>> I think for the datasets affected by this feature, a full (not
>>>>> incremental) send of the source snapshot that has some holes that have not
>>>>> been transmitted by the faulty incremental send code, should fix the
>>>>> issue,
>>>>> as far as the on-disk layout is concerned.
>>>>>
>>>>>
>>>>> Boris.
>>>>>
>>>>> ------------------------------
>>>>> *From:* Rich <[email protected]>
>>>>> *Sent:* Thursday, July 7, 2016 8:30:54 PM
>>>>> *To:* developer; Developer Lists Illumos
>>>>> *Subject:* [developer] Improvements to 6513 handling
>>>>>
>>>>> Hi all,
>>>>>
>>>>> So, ZFS on Linux just noticed it was getting bitten by what ultimately
>>>>> turned out to be Illumos #6513, partially filled holes losing birth time.
>>>>>
>>>>> Implementing that fix removes this problem for new data, but on all
>>>>> platforms, this doesn't help data already written on existing pools,
>>>>> getting munged silently in incremental sends forever.
>>>>>
>>>>> pcd pointed out that a relatively trivial workaround would be possible
>>>>> by simply ignoring the hole_birth metadata with something like a global
>>>>> tunable, but that seems too heavy-handed to me - either you're disabling
>>>>> the feature everywhere because you don't know when you can start trusting
>>>>> the birth times, or you're risking silent mangling of affected files
>>>>> forever.
>>>>>
>>>>> I'd like to suggest using a read-compatible feature, call it something
>>>>> like hole_birth_fix, in conjunction with the enabled_txg feature, to
>>>>> permit
>>>>> a reasonable default of ignoring hole_birth information before the
>>>>> hole_birth_fix feature was enabled, but still permitting use of it
>>>>> afterward.
>>>>>
>>>>> This has the unfortunate behavior of breaking write support if you
>>>>> enable hole_birth_fix and then try to go back to a prior codebase, but I
>>>>> can't think of a reasonable way to avoid this.
>>>>>
>>>>> I filed illumos #7175 to track this proposal - I'll happily write the
>>>>> code to implement this shortly.
>>>>>
>>>>> (Apologies if I've over-CCed or missed someone I should be asking for
>>>>> comment, I've not done this workflow before.)
>>>>>
>>>>> - Rich
>>>>>
>>>>
>>>>
>>>
>>
>
>
> --
> Paul Dagnelie
> *illumos-developer* | Archives
> <https://www.listbox.com/member/archive/182179/=now>
> <https://www.listbox.com/member/archive/rss/182179/21175001-8b3b9e0a> |
> Modify
> <https://www.listbox.com/member/?&;>
> Your Subscription <http://www.listbox.com>
>
-------------------------------------------
openzfs-developer
Archives: https://www.listbox.com/member/archive/274414/=now
RSS Feed: https://www.listbox.com/member/archive/rss/274414/28015062-cce53afa
Modify Your Subscription:
https://www.listbox.com/member/?member_id=28015062&id_secret=28015062-f966d51c
Powered by Listbox: http://www.listbox.com
