Re: [PATCH -RFC 0/2] mm/ext4: avoid data corruption when extending DIO write race with buffered read

Baokun Li
2023-12-05T13:19:11Z

On 2023/12/5 12:17, Theodore Ts'o wrote:
> On Mon, Dec 04, 2023 at 09:50:18PM +0800, Baokun Li wrote:
>> The problem is with a one-master-twoslave MYSQL database with three
>> physical machines, and using sysbench pressure testing on each of the
>> three machines, the problem occurs about once every two to three hours.
>>
>> The problem is with the relay log file, and when the problem occurs,
>> the middle dozens of bytes of the file are read as all zeros, while
>> the data on disk is not. This is a journal-like file where a write
>> process gets the data from the master node and writes it locally,
>> and another replay process reads the file and performs the replay
>> operation accordingly (some SQL statements). The problem is that
>> when replaying, it finds that the data read is corrupted, not valid
>> SQL data, while the data on disk is normal.
> You mentioned "scripts" --- are these locally developped scripts by
> any chance?
This refers to the sql commands to be replayed in the relay log file.
I don't know much about this file, but you can read the official
documentation.
https://dev.mysql.com/doc/refman/8.0/en/replica-logs-relaylog.html
> The procedure suggested in a few places that I looked up
> don't involve needing to read the replay log. For example from[1]:
>
> On the master server:
>
> root@repl-master:~# mysql -uroot -p;
> mysql> CREATE USER ‘slave’@’12.34.56.789‘ IDENTIFIED BY ‘SLAVE_PASSWORD‘;
> mysql> GRANT REPLICATION SLAVE ON . TO ‘slave’@’12.34.56.222 ‘;
> mysql> FLUSH PRIVILEGES;
> mysql> FLUSH TABLES WITH READ LOCK;
>
> This will make the master server read-only, with all pending writes
> flushed out (so you don't need to worry about the replay log), and
> then you move the data from the master to slave:
>
> root@repl-master:~# mysqldump -u root -p –all-databases –master-data > data.sql
> root@repl-master:~# scp data.sql root@12.34.56.222
>
> Then on the slave:
>
> root@repl-slave:~# mysql -uroot -p < data.sql
> root@repl-slave:~# mysql -uroot -p;
> mysql> STOP SLAVE;
>
> ... and then on the master:
>
> root@repl-master:~# mysql -uroot -p;
> mysql> UNLOCK TABLES;
>
> ... and back on the slave:
>
> root@repl-slave:~# mysql -uroot -p;
> mysql> START SLAVE;
>
> [1] https://hevodata.com/learn/mysql-master-slave-replication/
>
> ... or you could buy the product advertised at [1] which is easier for
> the database administrators, but results in $$$ flowing to the Hevo
> company. :-)
>
> In any case, I'm pretty sure that the official documented way of
> setting up a failover replication setup doesn't involve buffered reads
> of the replay file.
>
> It is certainly the case that mysqldump uses buffered reads, but
> that's why you have to temporary make the database read-only using
> "FLUSH TABLES WITH READ LOCK" before taking a database snapshot, and
> then re-enable database updates the "UNLOCK TABLES" SQL commands.
>
> Cheers,
>
> - Ted
Thank you very much for your detailed explanation!
But the downstream users do have buffered reads to read the relay log
file, as I confirmed with bpftrace. Here's an introduction to turning on
relay logging, but I'm not sure if you can access this link:
https://blog.csdn.net/javaanddonet/article/details/112596148

Thanks!
--
With Best Regards,
Baokun Li
.