Re: On markers of changed data

On Fri, Oct 6, 2017 at 11:22 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Andrey Borodin <x4mmm(at)yandex-team(dot)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