Re: snapbuild woes

On 22/02/17 11:29, Erik Rijkers wrote:
> On 2017-02-22 03:05, Petr Jelinek wrote:
>>
>> So to summarize attached patches:
>> 0001 - Fixes performance issue where we build tons of snapshots that we
>> don't need which kills CPU.
>>
>> 0002 - Disables the use of ondisk historical snapshots for initial
>> consistent snapshot export as it may result in corrupt data. This
>> definitely needs backport.
>>
>> 0003 - Fixes bug where we might never reach snapshot on busy server due
>> to race condition in xl_running_xacts logging. The original use of extra
>> locking does not seem to be enough in practice. Once we have agreed fix
>> for this it's probably worth backpatching. There are still some comments
>> that need updating, this is more of a PoC.
>>
>
> I am not not entirely sure what to expect. Should a server with these 3
> patches do initial data copy or not? The sgml seems to imply there is
> not inital data copy. But my test does copy something.
>

Not by itself (without the copy patch), those fixes are for snapshots.

>
> With
>> 0001-Skip-unnecessary-snapshot-builds.patch
>> 0002-Don-t-use-on-disk-snapshots-for-snapshot-export-in-l.patch
>> 0003-Fix-xl_running_xacts-usage-in-snapshot-builder.patch
>
> the consistent (but wrong) end state is always that only one of the four
> pgbench tables, pgbench_history, is replicated (always correctly).
>
> Below is the output from the test (I've edited the lines for email)
> (below, a,b,t,h stand for: pgbench_accounts, pgbench_branches,
> pgbench_tellers, pgbench_history)
> (master on port 6972, replica on port 6973.)
>
> port
> 6972 a,b,t,h: 100000 1 10 347
> 6973 a,b,t,h: 0 0 0 347
>
> a,b,t,h: a68efc81a 2c27f7ba5 128590a57 1e4070879 master
> a,b,t,h: d41d8cd98 d41d8cd98 d41d8cd98 1e4070879 replica NOK
>
> The md5-initstrings are from a md5 of the whole content of each table
> (an ordered select *)
>
> I repeated this a few times: of course, the number of rows in
> pgbench_history varies a bit but otherwise it is always the same: 3
> empty replica tables, pgbench_history replicated correctly.
>

That's actually correct behaviour without the initial copy patch, it
replicates changes, but since the 3 tables only get updates there is
nothing to replicate as there is no data downstream. However inserts
will of course work fine even without data downstream.

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