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Re: [MariaDB/server] Proper locking for mysql.gtid_slave_pos truncation (84b437d)



Thanks for your prompt reply!

On Wed, Dec 18, 2019 at 08:38:31AM +0100, Kristian Nielsen wrote:
> Sergey Vojtovich <notifications@xxxxxxxxxx> writes:
> > ATTN @dr-m, @andrelkin, @SachinSetiya, @knielsen 
> So IIUC, this is about incorrect usage of ha_truncate() in
> rpl_slave_state::truncate_state_table().
> This is used only for
>   SET GLOBAL gtid_slave_pos = "..."
> when all slave threads are stopped and nothing else is accessing the
> gtid_pos table.
It is a table, so any client connection can be accessing it any time?

> So it's fine to use ha_truncate() if that can be done easily (and
> correctly). But it would also be fine just to loop and delete all rows one
> by one in a normal transaction, if that is simpler. gtid_slave_pos is a
> small table, there are normally only a few rows per active replication
> domain.
It is good to have this alternative with less strict locking. I'll leave it
up to Andrei to decide if he wants to implement it.

> I'm not myself very familiar with details of metadata locking etc. around
> ha_truncate().
I think original code worked well initially. Then we got some InnoDB
improvement, which made original code not valid.

> But looking at the code now, I don't understand why it only truncates one
> table? If --gtid-pos-auto-engines is in effect, there could be multiple
> tables... shouldn't they all be cleared when setting the gtid_slave_pos
> variable? If so, maybe the delete-rows-one-by-one approach is in any case
> preferable over ha_truncate, since it can then be done transactionally, to
> not leave an inconsistent gtid_slave_pos state if one truncate fails and
> another succeeds?


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