Re: Online enabling of checksums

On Sat, Mar 31, 2018 at 5:38 PM, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>
wrote:

> On 03/31/2018 05:05 PM, Magnus Hagander wrote:
> > On Sat, Mar 31, 2018 at 4:21 PM, Tomas Vondra
> > <tomas(dot)vondra(at)2ndquadrant(dot)com <mailto:tomas(dot)vondra(at)2ndquadrant(dot)com>>
> wrote:
> >
> > ...
> >
> > I do think just waiting for all running transactions to complete is
> > fine, and it's not the first place where we use it - CREATE
> SUBSCRIPTION
> > does pretty much exactly the same thing (and CREATE INDEX
> CONCURRENTLY
> > too, to some extent). So we have a precedent / working code we can
> copy.
> >
> >
> > Thinking again, I don't think it should be done as part of
> > BuildRelationList(). We should just do it once in the launcher before
> > starting, that'll be both easier and cleaner. Anything started after
> > that will have checksums on it, so we should be fine.
> >
> > PFA one that does this.
> >
>
> Seems fine to me. I'd however log waitforxid, not the oldest one. If
> you're a DBA and you want to make the checksumming to proceed, knowing
> the oldest running XID is useless for that. If we log waitforxid, it can
> be used to query pg_stat_activity and interrupt the sessions somehow.
>

Yeah, makes sense. Updated.

> > > And if you try this with a temporary table (not hidden in
> transaction,
> > > so the bgworker can see it), the worker will fail with this:
> > >
> > > ERROR: cannot access temporary tables of other sessions
> > >
> > > But of course, this is just another way how to crash without
> updating
> > > the result for the launcher, so checksums may end up being
> enabled
> > > anyway.
> > >
> > >
> > > Yeah, there will be plenty of side-effect issues from that
> > > crash-with-wrong-status case. Fixing that will at least make things
> > > safer -- in that checksums won't be enabled when not put on all
> pages.
> > >
> >
> > Sure, the outcome with checksums enabled incorrectly is a
> consequence of
> > bogus status, and fixing that will prevent that. But that wasn't my
> main
> > point here - not articulated very clearly, though.
> >
> > The bigger question is how to handle temporary tables gracefully, so
> > that it does not terminate the bgworker like this at all. This might
> be
> > even bigger issue than dropped relations, considering that temporary
> > tables are pretty common part of applications (and it also includes
> > CREATE/DROP).
> >
> > For some clusters it might mean the online checksum enabling would
> > crash+restart infinitely (well, until reaching MAX_ATTEMPTS).
> >
> > Unfortunately, try_relation_open() won't fix this, as the error comes
> > from ReadBufferExtended. And it's not a matter of simply creating a
> > ReadBuffer variant without that error check, because temporary tables
> > use local buffers.
> >
> > I wonder if we could just go and set the checksums anyway, ignoring
> the
> > local buffers. If the other session does some changes, it'll
> overwrite
> > our changes, this time with the correct checksums. But it seems
> pretty
> > dangerous (I mean, what if they're writing stuff while we're updating
> > the checksums? Considering the various short-cuts for temporary
> tables,
> > I suspect that would be a boon for race conditions.)
> >
> > Another option would be to do something similar to running
> transactions,
> > i.e. wait until all temporary tables (that we've seen at the
> beginning)
> > disappear. But we're starting to wait on more and more stuff.
> >
> > If we do this, we should clearly log which backends we're waiting
> for,
> > so that the admins can go and interrupt them manually.
> >
> >
> >
> > Yeah, waiting for all transactions at the beginning is pretty simple.
> >
> > Making the worker simply ignore temporary tables would also be easy.
> >
> > One of the bigger issues here is temporary tables are *session* scope
> > and not transaction, so we'd actually need the other session to finish,
> > not just the transaction.
> >
> > I guess what we could do is something like this:
> >
> > 1. Don't process temporary tables in the checksumworker, period.
> > Instead, build a list of any temporary tables that existed when the
> > worker started in this particular database (basically anything that we
> > got in our scan). Once we have processed the complete database, keep
> > re-scanning pg_class until those particular tables are gone (search by
> oid).
> >
> > That means that any temporary tables that are created *while* we are
> > processing a database are ignored, but they should already be receiving
> > checksums.
> >
> > It definitely leads to a potential issue with long running temp tables.
> > But as long as we look at the *actual tables* (by oid), we should be
> > able to handle long-running sessions once they have dropped their temp
> > tables.
> >
> > Does that sound workable to you?
> >
>
> Yes, that's pretty much what I meant by 'wait until all temporary tables
> disappear'. Again, we need to make it easy to determine which OIDs are
> we waiting for, which sessions may need DBA's attention.
>
> I don't think it makes sense to log OIDs of the temporary tables. There
> can be many of them, and in most cases the connection/session is managed
> by the application, so the only thing you can do is kill the connection.
>

Yeah, agreed. I think it makes sense to show the *number* of temp tables.
That's also a predictable amount of information -- logging all temp tables
may as you say lead to an insane amount of data.

PFA a patch that does this. I've also added some docs for it.

And I also noticed pg_verify_checksums wasn't installed, so fixed that too.

--
Magnus Hagander
Me: https://www.hagander.net/ <http://www.hagander.net/>
Work: https://www.redpill-linpro.com/ <http://www.redpill-linpro.com/>