On 14.06.2013 14:06, Pavan Deolasee wrote:
On Fri, Jun 14, 2013 at 4:12 PM, Heikki Linnakangas<hlinnakan...@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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