On Sun, Jun 10, 2012 at 11:10 PM, Magnus Hagander <mag...@hagander.net> wrote: > On Sun, Jun 10, 2012 at 4:08 PM, Fujii Masao <masao.fu...@gmail.com> wrote: >> On Sun, Jun 10, 2012 at 10:34 PM, Fujii Masao <masao.fu...@gmail.com> wrote: >>> On Sun, Jun 10, 2012 at 9:25 PM, Fujii Masao <masao.fu...@gmail.com> wrote: >>>> On Sun, Jun 10, 2012 at 7:43 PM, Magnus Hagander <mag...@hagander.net> >>>> wrote: >>>>> On Sat, Jun 9, 2012 at 2:51 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: >>>>>> Fujii Masao <masao.fu...@gmail.com> writes: >>>>>>> This seems a bug. I think we should prevent pg_basebackup from >>>>>>> becoming synchronous standby. Thought? >>>>>> >>>>>> Absolutely. If we have replication clients that are not actually >>>>>> capable of being standbys, there *must* be a way for the master >>>>>> to know that. >>>>> >>>>> I thought we fixed this already by sending InvalidXlogRecPtr as flush >>>>> location? And that this only applied in 9.2? >>>>> >>>>> Are you saying we picked pg_basebackup *in backup mode* (not log >>>>> streaming) as synchronous standby? >>>> >>>> Yes. >>>> >>>>> If so then yes, that is >>>>> *definitely* a bug that should be fixed. We should never select a >>>>> connection that's not even streaming log as standby! >>>> >>>> Agreed. Attached patch prevents pg_basebackup from becoming sync >>>> standby. Also this patch fixes another problem: currently only walsender >>>> which reaches STREAMING state can become sync walsender. OTOH, >>>> sync walsender thinks that walsender with higher priority will be sync one >>>> whether its state is STREAMING, and switches to potential sync walsender. >>>> So when the standby with higher priority connects to the master, we >>>> might have no sync standby until it reaches the STREAMING state. >>>> To fix this problem, the patch switches walsender's state from sync to >>>> potential *after* walsender with higher priority has reached the >>>> STREAMING state. >>>> >>>> We also should not select (1) background stream process forked from >>>> pg_basebackup and (2) pg_receivexlog as sync standby because they >>>> don't send back replication progress. To address this, I'm thinking to >>>> introduce new option "NOSYNC" in "START_REPLICATION" command >>>> as follows, and to change (1) and (2) so that they specify NOSYNC. >>>> >>>> START_REPLICATION XXX/XXX [NOSYNC] >>>> >>>> If the standby specifies NOSYNC option, it's never assigned as sync >>>> standby even if its name is in synchronous_standby_names. Thought? >>> >>> The standby which always sends InvalidXLogRecPtr back should not >>> become sync one. So instead of NOSYNC option, by checking whether >>> InvalidXLogRecPtr is sent, we can avoid problematic sync standby. >> >> We should not do this because Magnus is proposing the patch >> (http://archives.postgresql.org/pgsql-hackers/2012-06/msg00348.php) >> which breaks the above assumption at all. So we should introduce >> something like NOSYNC option. > > Wouldn't the better choice there in that case be to give a switch to > pg_receivexlog if you *want* it to be able to become a sync replica, > and by default disallow it? And then keep the backend just treating > InvalidXlogRecPtr as don't-become-sync-replica.
I don't object to making pg_receivexlog as sync standby at all. So at least for me, that switch is not necessary. What I'm worried about is the background stream process forked from pg_basebackup. I think that it should not run as sync standby but sending back its replication progress seems helpful because a user can see the progress from pg_stat_replication. So I'm thinking that something like NOSYNC option is required. Regards, -- Fujii Masao -- Sent via pgsql-admin mailing list (pgsql-admin@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-admin
