I agree that correctness is job #1, including in this "corner case" of accidentally leaving a hole unpunched when sending/receiving.
We should add a kernel tunable to ignore the hole birth. This is low-effort and low-risk, and allows those "in the know" to immediately resolve their problem. I would be open to having this be set to disable hole birth behavior by default until we can implement the more complete solution described below. I know we have improved the performance of handling holes in send/receive since the hole birth feature was first introduced, and I appreciate the data points that turning it off doesn't seem to hurt. But I'd like to see/do a more thorough analysis before throwing it out altogether (though it would be a relief if we could do so, given how much headache this feature has caused). In particular, incremental sends of a large zvol with lots of large, discontiguous holes will result in lots of FREE records without the hole_birth feature. In the worst case, each modified block could result in 8KB of data and 156KB of FREE records (312 bytes/record * 512, if every other block is a hole in a 128KB indirect block (which has 1024 block pointers)). So hopefully most cases are not close to the worst one :-) As a more complete, permanent solution, I would like to propose this as the first use case of a new type of feature flag: the "transparent, fully backward compatible feature flag". The problem this solves is the need to know last time old software wrote to pool, because old software does not maintain certain data structures (e.g. does not correctly maintain hole BP’s birth times - this could also be used for other things like additional space accounting). In this case, there was a bug that caused hole BP’s birth times to not be maintained correctly. Holes with birth times that were written with the buggy software can not be trusted, but we can trust holes that were written with newer software. By adding a fully-backwards-compatible feature flag to the new (fixed) software, we can say “since TXG X, we have only used new software that correctly maintains the holes birth times, and therefore we can trust hole birth times that are > X (and when sending from a TXG > X, birth times of 0 can be trusted)". Note: no user action is required (e.g. “enabling” the feature), and the feature is not listed in the UI, and is not documented. In other words, this is purely an implementation detail. Here's a brief design of how this could be implemented: New on-disk structures: - TXG range (mintxg, maxtxg) in which only new software has written to pool - stored per backwards-compatible feature - stored in object in MOS new behaviors: - when open pool: - if object does not mention this feature, or if this feature’s maxtxg != last written txg, store new range (mintxg = maxtxg = current_txg) - when sync each txg: - assert maxtxg == current_txg - 1 - store maxtxg = current_txg - query: when was feature enabled: return “mintxg”. We know that new software has been continuously in use since this txg. --matt On Wed, Aug 31, 2016 at 4:51 PM, Bryan Cantrill <[email protected]> wrote: > > 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/182179/27166212-fc0e512f> | > 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
