On Wed, Jun 26, 2013 at 8:57 PM, Amit Kapila <amit.kap...@huawei.com> wrote: > On Wednesday, June 26, 2013 4:40 PM Andres Freund wrote: >> 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. > > Originally 2 parts were proposed, one was to get LSN from data pages and > other from data pages. > Original proposal is: > http://www.postgresql.org/message-id/6C0B27F7206C9E4CA54AE035729E9C382851FFA > 1@szxeml509-mbs > > The second part for looking into WAL was written but due to xlogreader > patch, it was postponed and I didn't get time after that > to pursue it. > > >>The whole tool >> seems to only make sense if you've lost pg_xlog. > > The tool's initial intent was if pg_controldata is lost and this idea is > originated in below mail chain: > http://www.postgresql.org/message-id/4274.1340084...@sss.pgh.pa.us > > >> > 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. > > Can we be ultra sure, that this LSN is not reached. I think it will take > vary long to reach such LSN, but still theoretically it can be possible. > I don't have any evidence. > >> > 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. > > You mean to say if user knows the max LSN of data pages in old-master and > last applied LSN in new master, he cannot decide whether > Full backup is needed? It should be straightforward decision that skip a > backup if that old-master LSN is less than the new-master (i.e., last > applied LSN, IOW, timeline switch LSN). > It was proposed as a usecase in this below mail: > http://www.postgresql.org/message-id/CAHGQGwHyd1fY0hF0qKh0-uKDh-gcbYxMOFBYVk > kh4jzji-f...@mail.gmail.com
I guess he meant the commit hint bit problem. Regards, -- Fujii Masao -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
