Re: Patch for fail-back without fresh backup
От | Heikki Linnakangas |
---|---|
Тема | Re: Patch for fail-back without fresh backup |
Дата | |
Msg-id | 51BAFC62.8060407@vmware.com обсуждение исходный текст |
Ответ на | Re: Patch for fail-back without fresh backup (Pavan Deolasee <pavan.deolasee@gmail.com>) |
Ответы |
Re: Patch for fail-back without fresh backup
(Greg Stark <stark@mit.edu>)
Re: Patch for fail-back without fresh backup (Tom Lane <tgl@sss.pgh.pa.us>) |
Список | pgsql-hackers |
On 14.06.2013 14:06, Pavan Deolasee wrote: > On Fri, Jun 14, 2013 at 4:12 PM, Heikki Linnakangas<hlinnakangas@vmware.com >> wrote: > >> Robert Haas pointed out in that thread that it has a problem with hint >> bits that are not WAL-logged, > > I liked that tool a lot until Robert pointed out the above problem. I > thought this is a show stopper because I can't really see any way to > circumvent it unless we enable checksums or explicitly WAL log hint bits. > >> but it will still work if you also enable the new checksums feature, which >> forces hint bit updates to be WAL-logged. > > Are we expecting a lot of people to run their clusters with checksums on ? > Sorry, I haven't followed the checksum discussions and don't know how much > overhead it causes. But if the general expectation is that checksums will > be turned on most often, I agree pg_rewind is probably good enough. Well, time will tell I guess. The biggest overhead with the checksums is exactly the WAL-logging of hint bits. >> Perhaps we could add a GUC to enable hint bits to be WAL-logged, >> regardless of checksums, to make pg_rewind work. > > Wouldn't that be too costly ? I mean, in the worst case every hint bit on a > page may get updated separately. If each such update is WAL logged, we are > looking for a lot more unnecessary WAL traffic. Yep, same as with checksums. I was not very enthusiastic about the checksums patch because of that, but a lot of people are willing to pay that price. Maybe we can figure out a way to reduce that cost in 9.4. It'd benefit the checksums greatly. For pg_rewind, we wouldn't actually need a full-page image for hint bit updates, just a small record saying "hey, I touched this page". And you'd only need to write that the first time a page is touched after a checkpoint. >> I think that's a more flexible approach to solve this problem. It doesn't >> require an online feedback loop from the standby to master, for starters. > > I agree. That's a big advantage of pg_rewind. Unfortunately, it can't work > with 9.3 and below because of the hint bits issue, otherwise it would have > been even more cool. The proposed patch is clearly not 9.3 material either. If anything, there's a much better change that we could still sneak in a GUC to allow hint bits to be WAL-logged without checksums in 9.3. All the code is there, it'd just be a new guc to control it separetely from checksums. - Heikki
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