On Wed, Sep 7, 2016 at 4:49 PM, Jim Nasby <jim.na...@bluetreble.com> wrote:
> On 9/7/16 6:10 PM, David Gibbons wrote: > >> That is NOT safe. The problem is it allows rsync to use mtime alone >> to decide that a file is in sync, and that will fail if Postgres >> writes to a file in the same second that the first rsync reads from >> it (assuming Postgres writes after rsync reads). You need to add the >> --checksum flag to rsync (which means it will still have to read >> everything that's in /var/lib/pgsql). >> >> >> The checksum flag as you mention is not performant, >> > > Definitely not. :/ > > If this is a concern, you're much better using the *--modify-window *flag: >> When comparing two timestamps, rsync treats the timestamps as being >> equal if they differ by no more than the modify-window value. This is >> normally 0 (for an exact match), but you may find it useful to set this >> to a larger value in some situations. >> >> Hence, rsync -va --modify-window=1 would remove your concern about a >> same second race condition without forcing the sync to read through all >> the files. >> > > Very interesting and useful! Isn't this heading in the wrong direction? We need to be more precise than 0 (since 0 is computed off of rounded/truncated time stamps), not less precise than 0. Cheers, Jeff
