On Fri, 14 Feb 2003, Bruce Momjian wrote: > OK, once we have PITR, will anyone want incremental backups?
Well, I'm not entirely clear on how PITR will work, so I may be off-base here, but it seems to me that offering incremental backups that back up only changed pages might not be all that big a win, given how postgres writes its pages. On DBMSs that don't use MVCC, if you change a particular item in a row ten times, one page is changed. If you do it in postgres, you could well be changing ten pages, as the system writes the two copies of the entire row wherever it can find space. So in databases where a lot of rows are changed, where an incremental backup would normally be a win because it would be much smaller than the logs over a given period, it isn't going to be with postgres. But you know, if we could get rid of redundant changes in the logs we're using for backup, that could save a lot of space in a situation like the one I described above. If a particular row and column is changed fifty times over the course of a month, it's going to be recorded fifty times in the log. But there's really no need for all fifty of those, if you don't mind not being able to restore to any time before the current time. You can reduce the size of the logs you need to store for backup by throwing away the first forty-nine of those changes, and keeping only the most recent version. There shouldn't be any worries about referential integrity, because when you do a restore, you start with a full backup that is ok, and once you've successfully applied all the transactions in the log, you know it will be ok again, so any intermediate states during the restore where integrity is not maintained are not a problem. cjs -- Curt Sampson <[EMAIL PROTECTED]> +81 90 7737 2974 http://www.netbsd.org Don't you know, in this new Dark Age, we're all light. --XTC ---------------------------(end of broadcast)--------------------------- TIP 2: you can get off all lists at once with the unregister command (send "unregister YourEmailAddressHere" to [EMAIL PROTECTED])
