Re: Funny WAL corruption issue

Hi, Chris.

> 10 авг. 2017 г., в 15:09, Chris Travers <chris(dot)travers(at)gmail(dot)com> написал(а):
>
> Hi;
>
> I ran into a funny situation today regarding PostgreSQL replication and wal corruption and wanted to go over what I think happened and what I wonder about as a possible solution.
>
> Basic information is custom-build PostgreSQL 9.6.3 on Gentoo, on a ~5TB database with variable load. Master database has two slaves and generates 10-20MB of WAL traffic a second. The data_checksum option is off.
>
>
> The problem occurred when I attempted to restart the service on the slave using pg_ctl (I believe the service had been started with sys V init scripts). On trying to restart, it gave me a nice "Invalid memory allocation request" error and promptly stopped.
>
> The main logs showed a lot of messages like before the restart:
> 2017-08-02 11:47:33 UTC LOG: PID 19033 in cancel request did not match any process
> 2017-08-02 11:47:33 UTC LOG: PID 19032 in cancel request did not match any process
> 2017-08-02 11:47:33 UTC LOG: PID 19024 in cancel request did not match any process
> 2017-08-02 11:47:33 UTC LOG: PID 19034 in cancel request did not match any process
>
> On restart, the following was logged to stderr:
> LOG: entering standby mode
> LOG: redo starts at 1E39C/8B77B458
> LOG: consistent recovery state reached at 1E39C/E1117FF8
> FATAL: invalid memory alloc request size 3456458752
> LOG: startup process (PID 18167) exited with exit code 1
> LOG: terminating any other active server processes
> LOG: database system is shut down
>
> After some troubleshooting I found that the wal segment had become corrupt, I copied the correct one from the master and everything came up to present.
>
> So It seems like somewhere something crashed big time on the back-end and when we tried to restart, the wal ended in an invalid way.

We have reported the same thing [1] nearly a year ago. Could you please check with pg_xlogdump that both WALs (normal from master and corrupted) are exactly the same until some certain LSN?

[1] https://www.postgresql.org/message-id/20160614103415.5796.6885%40wrigleys.postgresql.org

>
> I am wondering what can be done to prevent these sorts of things from happening in the future if, for example, a replica dies in the middle of a wal fsync.
> --
> Best Wishes,
> Chris Travers
>
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