Re: [HACKERS] snapbuild woes

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.

Anyway, I have repeated the same old pgbench-test, assuming inital data 
copy should be working.

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.

Something is not right.


thanks,


Erik Rijkers