On Sun, Feb 23, 2014 at 9:49 PM, Tom Lane <tgl@xxxxxxxxxxxxx> wrote: > Jeff Janes <jeff.janes@xxxxxxxxx> writes: >> On Sunday, February 23, 2014, Scott Marlowe <scott.marlowe@xxxxxxxxx> wrote: >>> I'm guessing that this is so that it can be rolled back. Unlink is >>> likely issued at commit; > >> I would hope that ftruncate is issued at commit as well. That doesn't >> sound undoable. > > It's more subtle than that. I'm too lazy to look at the comments in md.c > right now, but basically the reason for not doing an instant unlink is > to ensure that if a relation is truncated and then re-extended, open file > pointers held by other backends will still be valid. The ftruncate is > done to ensure that allocated disk space goes away as soon as that's safe > (ie, at commit of the truncation); but immediate unlink would require > forcing more cross-backend synchronization than we want to have. > > If memory serves, the inode should get removed during the next checkpoint. I was moments away from commenting to say that I had traced the flow of the code to md.c and found the comments there quite illuminating. I wonder if there is a different way to solve the underlying issue without relying on ftruncate (which seems to be somewhat expensive). -- Jon -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
