Thread: Warm Standby question
Hi, (Note: I have never used log shipping before, I'm just interested in the concepts, so I'm might be missing a very important aspect) I was reading the blog entry about HA and warm standby: http://scale-out-blog.blogspot.com/2009/02/simple-ha-with-postgresql-point-in-time.html The image that explained how log shipping works, strikes me as being a bit too complex. <http://1.bp.blogspot.com/_26KnjtB2MFo/SYVDrEr1HXI/AAAAAAAAAEY/ncq_AW-Vv-w/s1600-h/pg_warm_standby.png> According to the picture it basically works like this: Master -> Copy master archive directory -> Copy to standby archive dir -> copy to pg_xlogs. When I look at this chain I'm asking myself, why do I need the two archive directories? Why can't the master copy the files directly into the pg_xlogs directory of the standby server? Thanks Thomas
Probably can. But you're talking about disabling off-host archiving. The whole point behind this is prevention in case a host hard drive fails... if it fails and you don't use off-host archiving then you've lost the files you need to rebuild the database along with the original database. Thanks, Scot Kreienkamp La-Z-Boy Inc. skreien@la-z-boy.com 734-242-1444 ext 6379 -----Original Message----- From: pgsql-general-owner@postgresql.org [mailto:pgsql-general-owner@postgresql.org] On Behalf Of Thomas Kellerer Sent: Sunday, February 01, 2009 7:47 AM To: pgsql-general@postgresql.org Subject: [GENERAL] Warm Standby question Hi, (Note: I have never used log shipping before, I'm just interested in the concepts, so I'm might be missing a very important aspect) I was reading the blog entry about HA and warm standby: http://scale-out-blog.blogspot.com/2009/02/simple-ha-with-postgresql-poi nt-in-time.html The image that explained how log shipping works, strikes me as being a bit too complex. <http://1.bp.blogspot.com/_26KnjtB2MFo/SYVDrEr1HXI/AAAAAAAAAEY/ncq_AW-Vv -w/s1600-h/pg_warm_standby.png> According to the picture it basically works like this: Master -> Copy master archive directory -> Copy to standby archive dir -> copy to pg_xlogs. When I look at this chain I'm asking myself, why do I need the two archive directories? Why can't the master copy the files directly into the pg_xlogs directory of the standby server? Thanks Thomas -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Thanks for the answer. I see the point with the backup :) Regards Thomas Scot Kreienkamp, 02.02.2009 16:19: > Probably can. But you're talking about disabling off-host archiving. > The whole point behind this is prevention in case a host hard drive > fails... if it fails and you don't use off-host archiving then you've > lost the files you need to rebuild the database along with the original > database. > > Thanks, > > Scot Kreienkamp > La-Z-Boy Inc. > skreien@la-z-boy.com > 734-242-1444 ext 6379 > -----Original Message----- > From: pgsql-general-owner@postgresql.org > [mailto:pgsql-general-owner@postgresql.org] On Behalf Of Thomas Kellerer > Sent: Sunday, February 01, 2009 7:47 AM > To: pgsql-general@postgresql.org > Subject: [GENERAL] Warm Standby question > > Hi, > > (Note: I have never used log shipping before, I'm just interested in the > > concepts, so I'm might be missing a very important aspect) > > I was reading the blog entry about HA and warm standby: > http://scale-out-blog.blogspot.com/2009/02/simple-ha-with-postgresql-poi > nt-in-time.html > > The image that explained how log shipping works, strikes me as being a > bit too > complex. > <http://1.bp.blogspot.com/_26KnjtB2MFo/SYVDrEr1HXI/AAAAAAAAAEY/ncq_AW-Vv > -w/s1600-h/pg_warm_standby.png> > > According to the picture it basically works like this: > > Master -> Copy master archive directory -> Copy to standby archive dir > -> copy > to pg_xlogs. > > When I look at this chain I'm asking myself, why do I need the two > archive > directories? > > Why can't the master copy the files directly into the pg_xlogs directory > of the > standby server? > > Thanks > Thomas > > >
On Feb 1, 2009, at 4:47 AM, Thomas Kellerer wrote: > Hi, > > (Note: I have never used log shipping before, I'm just interested in > the concepts, so I'm might be missing a very important aspect) > > I was reading the blog entry about HA and warm standby: > http://scale-out-blog.blogspot.com/2009/02/simple-ha-with-postgresql-point-in-time.html > > The image that explained how log shipping works, strikes me as being > a bit too complex. > <http://1.bp.blogspot.com/_26KnjtB2MFo/SYVDrEr1HXI/AAAAAAAAAEY/ncq_AW-Vv-w/s1600-h/pg_warm_standby.png > > > > According to the picture it basically works like this: > > Master -> Copy master archive directory -> Copy to standby archive > dir -> copy to pg_xlogs. > > When I look at this chain I'm asking myself, why do I need the two > archive directories? > > Why can't the master copy the files directly into the pg_xlogs > directory of the standby server? Well, on the master the implementation has to be resilient to issues with the archive_command and Postgres re-uses files in the pg_xlog directory. If they weren't copied into the archive_status directory then if a file couldn't be shipped for some reason you'd run the risk of the file getting re-used before it was successfully pushed to the standby. As to where they go on the standby, remember that log shipping is not just for warm standby implementation -- it's also a valid backup method, i.e. archive the transaction logs along with a snapshot of the data directory. In addition, the copy of a file into the pg_xlog directory needs to be atomic and there's no guarantee that any given archive_command will use a tool that does atomic copies. Erik Jones, Database Administrator Engine Yard Support, Scalability, Reliability 866.518.9273 x 260 Location: US/Pacific IRC: mage2k
