Hi Amit, On 2013-06-26 16:22:28 +0530, Amit Kapila wrote: > On Wednesday, June 26, 2013 1:20 PM Andres Freund wrote: > > On 2013-06-26 08:50:27 +0530, Amit Kapila wrote: > > > On Tuesday, June 25, 2013 11:12 PM Andres Freund wrote: > > > > On 2013-06-16 17:19:49 -0700, Josh Berkus wrote: > > > > > Amit posted a new version of this patch on January 23rd. But > > last > > > > > comment on it by Tom is "not sure everyone wants this". > > > > > > > > > > https://commitfest.postgresql.org/action/patch_view?id=905 > > > > > > > > > ... so, what's the status of this patch? > > > > > > > > That comment was referencing a mail of mine - so perhaps I better > > > > explain: > > > > > > > > I think the usecase for this utility isn't big enough to be > > included in > > > > postgres since it really can only help in a very limited > > > > circumstances. And I think it's too likely to be misused for stuff > > it's > > > > not useable for (e.g. remastering). > > > > > > > > The only scenario I see is that somebody deleted/corrupted > > > > pg_controldata. In that scenario the tool is supposed to be used to > > > > find > > > > the biggest lsn used so far so the user then can use pg_resetxlog > > to > > > > set > > > > that as the wal starting point. > > > > But that can be way much easier solved by just setting the LSN to > > > > something very, very high. The database cannot be used for anything > > > > reliable afterwards anyway. > > > > > > One of the main reason this was written was to make server up in case > > of > > > corruption and > > > user can take dump of some useful information if any. > > > > > > By setting LSN very, very high user might loose the information which > > he > > > wants to take dump. > > > > Which information would that loose? > Information from WAL replay which can be more appropriate by selecting > LSN.
Sorry, I can't follow. If wal replay still is an option you can just look at the WAL and get a sensible value way easier. The whole tool seems to only make sense if you've lost pg_xlog. > Also for a developer, guessing very high LSN might be easy, but for users > it might not be equally easy, and getting such value by utility would be > comfortable. Well, then we can just document some very high lsn and be done with it. Like CF000000/00000000. That would leave enough space for eventual writes caused while dumping the database (say hint bit writes in a checksummed database) and cannot yet be realistically be reached during normal operation. > One more use case for which this utility was done is as below: > It will be used to decide that on new-standby (old-master) whether a full > backup is needed from > New-master(old-standby). > The backup is required when the data page in old-master precedes > the last applied LSN in old-standby (i.e., new-master) at the moment > of the failover. That's exactly what I was afraid of. Unless I miss something the tool is *NOT* sufficient to do this. Look at the mail introducing pg_rewind and the ensuing discussion for what's necessary for that. Greetings, Andres Freund -- Andres Freund http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
