Re: Should FLUSH TABLES WITH READ LOCK wait for updating transactions?

[Rob Wultsch]

On Thu, Jun 24, 2010 at 12:37 AM, Konstantin Osipov <kostja@stripped> wrote:
> * Baron Schwartz <baron@stripped> [10/06/24 11:35]:
>> > - To allow easy file system level backups of the databases
>> > - To prepare a database for a snapshot
>> > - (Optional) Flush things to disk, so that a restart is faster.
>> I have seen many people use it as a general-purpose write barrier.
>> For example, in Alex Davies's new High Availability Cookbook, he
>> suggests using it before SET GLOBAL read_only = 1, to be sure the
>> change has really taken effect for all connections when performing a
>> failover.
>
> Starting from 5.0, a useless suggestion.
> SET GLOBAL read_only is implemented by means of FTWRL.
>

I am not sure such a harsh judgment is justified.

FTWR causes connections to hang while read_only causes errors. I don't
think it takes too much mental gymnastics to see that as possibly
useful. I could imagine someone trying to avoid excessive errors by
setting FTWRL on the existing master, changing the connection
parameter (or moving an IP), killing connections if applicable on the
old master and then throwing RO on the old master.

The proposed behavior would be my preference for FTWR behavior.

-- 
Rob Wultsch
wultsch@stripped