On Mon, Feb 6, 2012 at 4:48 AM, Bruce Momjian <br...@momjian.us> wrote: > On Sun, Feb 05, 2012 at 08:40:09PM +0000, Simon Riggs wrote: >> On Sun, Feb 5, 2012 at 3:59 AM, Bruce Momjian <br...@momjian.us> wrote: >> > On Sat, Dec 24, 2011 at 03:56:58PM +0000, Simon Riggs wrote: >> >> > Also, as far as I can see this patch usurps the page version field, >> >> > which I find unacceptably short-sighted. Do you really think this is >> >> > the last page layout change we'll ever make? >> >> >> >> No, I don't. I hope and expect the next page layout change to >> >> reintroduce such a field. >> >> >> >> But since we're agreed now that upgrading is important, changing page >> >> format isn't likely to be happening until we get an online upgrade >> >> process. So future changes are much less likely. If they do happen, we >> >> have some flag bits spare that can be used to indicate later versions. >> >> It's not the prettiest thing in the world, but it's a small ugliness >> >> in return for an important feature. If there was a way without that, I >> >> would have chosen it. >> > >> > Have you considered the CRC might match a valuid page version number? >> > Is that safe? >> >> In the proposed scheme there are two flag bits set on the page to >> indicate whether the field is used as a checksum rather than a version >> number. So its possible the checksum could look like a valid page >> version number, but we'd still be able to tell the difference. > > Thanks. Clearly we don't need 16 bits to represent our page version > number because we rarely change it. The other good thing is I don't > remember anyone wanting additional per-page storage in the past few > years except for a checksum. > > I wonder if we should just dedicate 3 page header bits, call that the > page version number, and set this new version number to 1, and assume > all previous versions were zero, and have them look in the old page > version location if the new version number is zero. I am basically > thinking of how we can plan ahead to move the version number to a new > location and have a defined way of finding the page version number using > old and new schemes. > > I don't want to get to a point where we need a bit per page number > version.
Agreed, though I think we need at least 2 bits set if we are using the checksum, so we should start at version 2 to ensure that. -- Simon Riggs 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
