On 5/26/21 2:43 AM, Laurenz Albe wrote: > On Wed, 2021-05-26 at 08:57 +0900, Michael Paquier wrote: >> On Tue, May 25, 2021 at 03:17:37PM -0400, Andrew Dunstan wrote: >>> If we do decide to do something the question arises what should it do? >>> If we're to allow for it I'm wondering if the best thing would be simply >>> to ignore such a file. >> Enforcing assumptions that any file could be ready-only is a very bad >> idea, as that could lead to weird behaviors if a FS is turned as >> becoming read-only suddenly while doing a rewind. Another idea that >> has popped out across the years was to add an option to pg_rewind so >> as users could filter files manually. That could be easily dangerous >> though in the wrong hands, as one could think that it is a good idea >> to skip a control file, for example. >> >> The thing is that here we actually know the set of files we'd like to >> ignore most of the time, and we still want to have some automated >> control what gets filtered. So here is a new idea: we build a list of >> files based on a set of GUC parameters using postgres -C on the target >> data folder, and assume that these are safe enough to be skipped all >> the time, if these are in the data folder. > That sounds complicated, but should work. > There should be a code comment somewhere that warns people not to forget > to look at that when they add a new GUC. > > I can think of two alternatives to handle this: > > - Skip files that cannot be opened for writing and issue a warning. > That is simple, but coarse. > A slightly more sophisticated version would first check if files > are the same on both machines and skip the warning for those. > > - Paul's idea to try and change the mode on the read-only file > and reset it to the original state after pg_rewind is done. >
How about we only skip (with a warning) read-only files if they are in the data root, not a subdirectory? That would save us from silently ignoring read-only filesystems and not involve using a GUC. cheers andrew -- Andrew Dunstan EDB: https://www.enterprisedb.com
