On 29/05/17 20:59, Andres Freund wrote:
> On May 29, 2017 11:58:05 AM PDT, Petr Jelinek <petr.jeli...@2ndquadrant.com> 
> wrote:
>> On 27/05/17 17:17, Andres Freund wrote:
>>> On May 27, 2017 9:48:22 AM EDT, Petr Jelinek
>> <petr.jeli...@2ndquadrant.com> wrote:
>>>> Actually, I guess it's the pid 47457 (COPY process) who is actually
>>>> running the xid 73322726. In that case that's the same thing
>> Masahiko
>>>> Sawada reported [1]. Which basically is result of snapshot builder
>>>> waiting for transaction to finish, that's normal if there is a long
>>>> transaction running when the snapshot is being created (and the COPY
>> is
>>>> a long transaction).
>>> Hm.  I suspect the issue is that the exported snapshot needs an xid
>> for some crosscheck, and that's what we're waiting for.  Could you
>> check what happens if you don't assign one and just content the error
>> checks out?   Not at my computer, just theorizing.
>> I don't think that's it, in my opinion it's the parallelization of
>> table
>> data copy where we create snapshot for one process but then the next
>> one
>> has to wait for the first one to finish. Before we fixed the
>> snapshotting, the second one would just use the ondisk snapshot so it
>> would work fine (except the snapshot was corrupted of course). I wonder
>> if we could somehow give it a hint to ignore the read-only txes, but
>> then we have no way to enforce the txes to stay read-only so it does
>> not
>> seem safe.
> Read-only txs have no xid ...

That's what I mean by hinting, normally they don't but building initial
snapshot in snapshot builder calls GetTopTransactionId() (see
SnapBuildInitialSnapshot()) which will assign it xid.

  Petr Jelinek                  http://www.2ndQuadrant.com/
  PostgreSQL Development, 24x7 Support, Training & Services

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