On 2/2/15 9:37 PM, Slava Mudry wrote:
On Mon, Feb 2, 2015 at 5:52 PM, Jim Nasby <Jim.Nasby@xxxxxxxxxxxxxx
<mailto:Jim.Nasby@xxxxxxxxxxxxxx>> wrote:
On 2/2/15 7:36 PM, Jim Nasby wrote:
Currently the fact that it needs to go back to old tables
and FTS them
every 2B transactions (or rely on autovacuum for this) and
you can't do
anything about it (like permanently freeze the tables) seems
like a big
scalability issue. Does it not?
Unfortunately it's not terribly easy to fix this. The problem is
if we
try to play games here, we must have a 100% reliable method for
changing
relfrozenxid as soon as someone inserts a new tuple in the
relation. It
might be possible to tie this into the visibility map, but no
one has
looked at this yet.
Perhaps you'd be willing to investigate this, or sponsor the work?
I'll see what I can do. Will talk to folks at pgDay in a month.
Oh, there is another possibility that's been discussed: read-only
tables. If we had the ability to mark a table read-only, then a
VACUUM FREEZE on such a table would be able to set that table's
relfrozenxid to FrozenTransactionId and prevent any further attempts
at vacuuming. This might be easier than trying to do something
automatic.
I think if we could log "last update/delete/insert" timestamp for a
table - we could use that to freeze tables that are not changed.
A timestamp wouldn't work; you need to have an exact XID.
Even if it did work you still have the same problem: there's a huge,
hairy race condition between what vacuum is trying to do and any DML.
I also wonder how pg_database.datfrozenxid is set? Is it equal to the
oldest pg_class.relfrozenxid for that database?
Correct.
I ask because I am willing to give a try and update relfrozenxid for the
tables that are never updated and frozen. Currently we are looking at
8-hour downtime to vacuum the whole db in single-user mode. High
availability is more important that data loss in my case. [I still don't
want to lose data, but it won't be the end of world if it happens].
Why are you trying to go into single user mode? There's no reason to do
that.
Forcing relfrozenxid to 2 might work, but you're certainly playing with
fire.
--
Jim Nasby, Data Architect, Blue Treble Consulting
Data in Trouble? Get it in Treble! http://BlueTreble.com
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