Re: Timeline in the light of Synchronous replication - Mailing list pgsql-hackers

I believe we should come up with a universal solution that will solve potential future problems as well (for example,
ifin sync replication, we decide to send writes to standbys in parallel to writing on local disk).<br /><br /> The
idealthing would be to have an id that is incremented on every failure, and is stored in the WAL. Whenever a standby
connectsto the primary, it should send the point p in WAL where streaming should start, plus the id. If the id is the
sameat the primary at point p, things are good. Else, we should tell the standby to either create a new copy from
scratch,or delete some WALs.<br /><br />@David<br />> One way to get them in sync without starting from scratch is
touse<br />> rsync from A to B.  This works in the asynchronous case, too. :)<br /><br />The problem mainly is
detectingwhen one can rsync/stream and when not.<br /><br />Regards<br /><br /><br /><br /><div class="gmail_quote">On
Mon,Oct 18, 2010 at 1:57 AM, Dimitri Fontaine <span dir="ltr"><<a
href="mailto:dimitri@2ndquadrant.fr">dimitri@2ndquadrant.fr</a>></span>wrote:<br /><blockquote class="gmail_quote"
style="margin:0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;"><div class="im">Fujii
Masao<<a href="mailto:masao.fujii@gmail.com">masao.fujii@gmail.com</a>> writes:<br /> > But, even though we
willhave done that, it should be noted that WAL in<br /> > A might be ahead of that in B. For example, A might crash
rightafter<br /> > writing WAL to the disk and before sending it to B. So when we restart<br /> > the old master
Aas the standby after failover, we should need to delete<br /> > some WAL files (in A) which are inconsistent with
theWAL sequence in B.<br /><br /></div>The idea to send from master to slave the current last applied LSN has<br />
beentalked about already. It would allow to send the WAL content in<br /> parallel of it's local fsync() on the master,
thestandby would refrain<br /> from applying any WAL segment until it knows the master is past that.<br /><br /> Now,
givensuch a behavior, that would mean that when A joins again as a<br /> standby, it would have to ask B for the
currentlast applied LSN too,<br /> and would notice the timeline change. Maybe by adding a facility to<br /> request
thelast LSN of the previous timeline, and with the behavior<br /> above applied there (skipping now-known-future-WALs
inrecovery), that<br /> would work automatically?<br /><br /> There's still the problem of WALs that have been applied
before<br/> recovery, I don't know that we can do anything here. But maybe we could<br /> also tweak the CHECKPOINT
mecanismnot to advance the restart point<br /> until we know the standbys have already replayed anything up to the<br
/>restart point?<br /><font color="#888888"><br /> --<br /> Dimitri Fontaine<br /><a href="http://2ndQuadrant.fr"
target="_blank">http://2ndQuadrant.fr</a>    PostgreSQL : Expertise, Formation et Support<br
/></font></blockquote></div><br/>