On Sat, Jul 27, 2019 at 2:27 PM Andres Freund <and...@anarazel.de> wrote: > Note that neither of those mean that it's not a good idea to > posix_fallocate() and *then* write zeroes, when initializing. For > several filesystems that's more likely to result in more optimally sized > filesystem extents, reducing fragmentation. And without an intervening > f[data]sync, there's not much additional metadata journalling. Although > that's less of an issue on some newer filesystems, IIRC (due to delayed > allocation).
Interesting. One way to bring back posix_fallocate() without upsetting people on some filesystem out there would be to turn the new wal_init_zero GUC into a choice: write (current default, and current behaviour for 'on'), pwrite_hole (write just the final byte, current behaviour for 'off'), posix_fallocate (like that 2013 patch that was reverted) and posix_fallocate_and_write (do both as you said, to try to solve that problem you mentioned that led to the revert). I suppose there'd be a parallel GUC undo_init_zero. Or some more general GUC for any fixed-sized preallocated files like that (for example if someone were to decide to do the same for SLRU files instead of growing them block-by-block), called something like file_init_zero. -- Thomas Munro https://enterprisedb.com
