On Tue, Dec 22, 2015 at 12:10 PM, Adam C. Emerson <aemer...@redhat.com> wrote:
> Comrades,
>
> Ceph's victory is assured. It will be the storage system of The Future.
> Matt Benjamin has reminded me that if we don't act fast┬╣ Ceph will be
> responsible for destroying the world.
>
> utime_t() uses a 32-bit second count internally. This isn't great, but it's
> something we can fix. ceph::real_time currently uses a 64-bit bit count of
> nanoseconds, which is better. And we can change it to something else without
> having to rewrite much other code.
>
> The problem lies in our encode/deocde functions for time (both utime_t
> and ceph::real_time, since I didn't want to break compatibility.) we
> use a 32-bit second count. I would like to change the wire and disk
> representation to a 64-bit second count and a 32-bit nanosecond count.
>
> Would there be resistance to a project to do this? I don't know if a
> FEATURE bit would help. A FEATURE bit to toggle the width of the second
> count would be ideal if it would work. Otherwise it looks like the best
> way to do this would be to find all the structures currently ::encoded
> that hold time values, bump the version number and have an 'old_utime'
> that we use for everything pre-change.

Unfortunately, we include utimes in structures that are written to
disk. So I think we're stuck with creating a new utime_t and
incrementing the struct_v on everything that contains them. :/

Of course, we'll also then need the full feature bit system to make
sure we send the old encoding to clients which don't understand the
new one, and to prevent a mid-upgrade cluster from writing data on a
new node that gets moved to a new node which doesn't understand it.

Given that utime_t occurs in a lot of places, and really can't change
*again* after this, we probably shouldn't set up the new version with
versioned encoding?
-Greg

>
> Thank you!
>
> ┬╣ Within the next twenty-three years. But that's not really a long time in the
>   larger scheme of things.
>
> --
> Senior Software Engineer           Red Hat Storage, Ann Arbor, MI, US
> IRC: Aemerson@{RedHat, OFTC, Freenode}
> 0x80F7544B90EDBFB9 E707 86BA 0C1B 62CC 152C  7C12 80F7 544B 90ED BFB9
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