On Monday, June 18, 2012 09:19:48 PM Heikki Linnakangas wrote: > On 18.06.2012 21:45, Andres Freund wrote: > > On Monday, June 18, 2012 08:32:54 PM Heikki Linnakangas wrote: > >> On 18.06.2012 21:13, Andres Freund wrote: > >>> On Monday, June 18, 2012 08:08:14 PM Heikki Linnakangas wrote: > >>>> The page header contains an XLogRecPtr (LSN), so if we change it we'll > >>>> have to deal with pg_upgrade. I guess we could still keep XLogRecPtr > >>>> around as the on-disk representation, and convert between the 64-bit > >>>> integer and XLogRecPtr in PageGetLSN/PageSetLSN. I can try that out - > >>>> many xlog calculations would admittedly be simpler if it was an > >>>> uint64. > >>> > >>> I am out of my depth here, not having read any of the relevant code, > >>> but couldn't we simply replace the lsn from disk with > >>> InvalidXLogRecPtr without marking the page dirty? > >>> > >>> There is the valid argument that you would loose some information when > >>> pages with hint bits are written out again, but on the other hand you > >>> would also gain the information that it was a hint-bit write... > >> > >> Sorry, I don't understand that. Where would you "replace the LSN from > >> disk with InvalidXLogRecPtr" ? (and no, it probably won't work ;-) ) > > > > In ReadBuffer_common or such, after reading a page from disk and > > verifying that the page has a valid header it seems to be possible to > > replace pd_lsn *in memory* by InvalidXLogRecPtr without marking the page > > dirty. > > If the page isn't modified the lsn on disk won't be changed so you don't > > loose debugging information in that case. If will be zero if it has been > > written by a hint-bit write and thats arguable a win. > > We use the LSN to decide whether a full-page image need to be xlogged if > the page is modified. If you reset LSN every time you read in a page, > you'll be doing full page writes every time a page is read from disk and > modified, whether or not it's the first time the page is modified after > the last checkpoint. Yea, I somehow disregarded that hint-bit writes would make a problem with full page writes in that case. Normal writes wouldn't be a problem... This should be fixable but the result would be too ugly. So its either converting the on-disk representation or keeping the duplicated representation.
pd_lsn seems to be well enough abstracted, doing the conversion in PageSet/GetLSN seems to be easy enough. Andres -- 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
