On Fri, Oct 6, 2017 at 11:22 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: > Andrey Borodin <x4...@yandex-team.ru> writes: >> Is it safe to use file modification time to track that file were changes >> since previous backup? > > I'd say no: > > 1. You don't know the granularity of the filesystem's timestamps, at least > not without making unportable assumptions. > > 2. There's no guarantee that the system clock can't be set backwards. > > 3. It's not uncommon for filesystems to have optimizations whereby they > skip or delay some updates of file mtimes. (I think this is usually > optional, but you couldn't know whether it's turned on.) > > #2 is probably the worst of these problems.
Or upwards. A simple example of things depending on clock changes is for example VM snapshotting. Any logic not depending on monotonic timestamps, with things like clock_gettime(CLOCK_MONOTONIC) is a lot of fun to investigate until you know that they are not using any monotonic logic... So the answer is *no*, do not depend on FS-level timestamps. The only sane method for Postgres is really to scan the page header LSNs, and of course you already know that. -- Michael -- Sent via pgsql-hackers mailing list (firstname.lastname@example.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers