Re: [HACKERS] logical replication busy-waiting on a lock

On 29/05/17 21:21, Petr Jelinek wrote:
> On 29/05/17 20:59, Andres Freund wrote:
>>
>>
>> On May 29, 2017 11:58:05 AM PDT, Petr Jelinek <petr.jelinek@2ndquadrant.com> wrote:
>>> On 27/05/17 17:17, Andres Freund wrote:
>>>>
>>>>
>>>> On May 27, 2017 9:48:22 AM EDT, Petr Jelinek
>>> <petr.jelinek@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.
> 

Looking at the code more, the xid is only used as parameter for
SnapBuildBuildSnapshot() which never does anything with that parameter,
I wonder if it's really needed then.

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