RE: system page size!

Good point, I neglected to mention that. cheers j. > -----Original Message----- > From: Art S. Kagel [mailto:kagel@erols.com] > Sent: Tuesday, February 06, 2001 11:15 PM > To: informix-list@iiug.org > Subject: Re: system page size! > > > Jack, pagesize os ONLY configurable by the DBA in XPS (8.xx). As > Jonathan stated in 6.x, 7.x, & 9.x it is fixed at 2K for all > platforms > except AIX and NT which are fixed at 4K. The ONLY other version of > Informix I am aware of that EVER had a pagesize difference > than 2K was > the old Turbo engine on Amdahl mainframes where it was 4K as well. > > In 5.xx there was an ONCONFIG parameter, PAGESIZE, which you > could set > at your own peril at init time to set the pagesize, however, > many folk > at Informix warned me that noone was sure that that parameter > was being > properly and consistently used everywhere in the code and > that trouble > including data loss might follow changing PAGESIZE from its default > value or 2048. > > Art S. Kagel > > "Parker, Jack" wrote: > > > > I would like to point out that even with 255 rows per > 'page' that translates > > to (don't forget header info): > > > > rowsize=7 for a 2K page > > rowsize=15 for a 4K page > > rowsize=31 for an 8K page > > > > Don't know about you, but 15 is an awfully small row > (granted I DO have a > > 10GB table with a rowsize of 11 and I would love to not be > wasting those 4 > > bytes per record - however that table used to be 75GB > before I redesigned it > > - I should be happy). > > > > You can also set your own page size with versions 7(?), 8 and 9. > > > > cheers > > j. > > (who backed off on the 8K page size for the 4K) > > > > > -----Original Message----- > > > From: Jonathan Leffler [mailto:jleffler@informix.com] > > > Sent: Thursday, January 11, 2001 5:18 PM > > > To: informix-list@iiug.org > > > Subject: Re: system page size! > > > > > > > > > Murat YILDIZ wrote: > > > > I am confused with the system page size, now? > > > > Where can I see what system page size informix is using? > > > > > > > > The last line of onstat -b tells me 2048 buffer size.... > > > > but on HP-UX dmesg shows this: > > > > physical page size = 4096 bytes, logical page size = 4096 bytes > > > > > > > > Is this physical page size something else? > > > > > > There are multiple pages sizes available to confuse the > unwary (or the > > > inquisitive). > > > > > > Informix is using logical pages of 2 KB (onstat -b), which is > > > the normal > > > Informix page size on most systems. At the time when it was > > > instituted > > > (1988?), most operating systems were using a 1 KB or 2 KB > > > page size, so > > > Informix's page size was a multiple of the system page > size, which was > > > good. Over time, the operating systems increased their page > > > size from 1 > > > or 2 to 4 KB, but Informix was unable to change its page > size to match > > > for a variety of reasons. These include the problems of > forwards and > > > backwards migration -- rewriting an entire database is a slow > > > process -- > > > and the fact that it is easier to waste space with internal > > > fragmentation on a 4 KB page. By that, I mean that if you > > > have a a row > > > size smaller than ((4096 - 28) / 255 - 4) or 11 bytes (eg a > > > cross-reference table with 2 INTEGER values, each > referencing rows in > > > another table), then you are stuck with just 255 rows per > page, even > > > though if you were not limited by the slot numbering, you > could fit in > > > more than 255 rows. So, I don't know of any platform > where there was > > > initially a 2 KB page version of OnLine which subsequently > > > changed to a > > > 4 KB page size. I believe XPS 8.3x does provide variable > size pages. > > > > > > What does the page size mismatch mean? Probably not very > much. ...I > > > don't like the conclusion I'm coming to, so I'm not sure > whether I'm > > > right... In the worst case (a cooked file), when OnLine > reads a 2 KB > > > page, the o/s reads 4 KB from disk and transfers 2 KB of that to > > > Informix's shared memory cache. If it needs the next 2 > KB before it > > > removes the (o/s) page from the (o/s) cache, then no > further disk read > > > is needed. If the user writes to the page after the o/s > > > clears the page > > > from its cache, then it has to reread the 4 KB page, > modify the 2 KB > > > from the Informix cache, and then write back the 4 KB > page. Similar > > > rules might apply to raw disk devices too; the o/s will read 4 KB, > > > return the relevant 2 KB to Informix, and discard (actually > > > discard) the > > > other 2 KB. Hmmmm, I wonder how much of a performance > boost would be > > > available if we resized the Informix pages to match (or > > > exceed) the o/s > > > page size. If I were redesigning the Informix page layouts, > > > I'd use big > > > pages (like 64 KB) with 8 byte rowids and 16-bit slot > > > numbers. For the > > > immediately foreseeable future, that would be bigger than > the o/s page > > > size, and therefore would not require the discarding of pages. > > > > > > And, going back to the original question, the fact that the > > > o/s uses one > > > page size and Informix uses another does not cause > confusion to either > > > piece of software because they are referring to different > > > sorts of pages > > > (though it may cause some extra work for the o/s). > > > > > > -- > > > Yours, > > > Jonathan Leffler (Jonathan.Leffler@Informix.com) #include > > > <disclaimer.h> > > > Guardian of DBD::Informix v1.00.PC1 -- http://www.perl.com/CPAN > > > "I don't suffer from insanity; I enjoy every minute of it!" > > > >