Re: Why copy_relation_data only use wal when WALarchiving is enabled

Heikki Linnakangas wrote:
> I wrote:
>> Unfortunately I don't see any easy way to fix it. One approach would be
>> to avoid reusing the relfilenodes until next checkpoint, but I don't see
>> any nice place to keep track of OIDs that have been dropped since last
>> checkpoint.
> 
> Ok, here's one idea:
> 
> Instead of deleting the file immediately on commit of DROP TABLE, the
> file is truncated to release the space, but not unlink()ed, to avoid
> reusing that relfilenode. The truncated file can be deleted after next
> checkpoint.
> 
> Now, how does checkpoint know what to delete? We can use the fsync
> request mechanism for that. When a file is truncated, a new kind of
> fsync request, a "deletion request", is sent to the bgwriter, which
> collects all such requests to a list. Before checkpoint calculates new
> RedoRecPtr, the list is swapped with an empty one, and after writing the
> new checkpoint record, all the files that were in the list are deleted.
> 
> We would leak empty files on crashes, but we leak files on crashes
> anyway, so that shouldn't be an issue. This scheme wouldn't require
> catalog changes, so it would be suitable for backpatching.
> 
> Any better ideas?
Couldn't we fix this by forcing a checkpoint before we commit the transaction 
that created the new pg_class entry for the clustered table? Or rather, more 
generally, before committing a transaction that created a new non-temporary 
relfilenode but didn't WAL-log any subsequent inserts.

Thats of course a rather sledgehammer-like approach to this problem - but at 
least for the backbranched the fix would be less intrusive...

regards, Florian Pflug