Tune image publishing schedule

[simondeziel]

AFAIK, LXD images are cached locally for a certain period of time and refreshed either on a schedule or on demand.
I think the refresh behaviour is to always pull the newest image available from the remote server.

Assuming that refresh behaviour of always pulling the newest image is accurate, I think we should stop publishing images on weekend days. Here's a practical use case:

• User launches images:alpine/3.16 on Friday (LXD caches it locally)
• User suspends laptop on Friday evening for the weekend
• images:alpine/3.16 is rebuilt/republished Saturday, Sunday and Monday
• User resumes laptop on Monday, launches another images:alpine/3.16
• LXD tries to refresh the cached image but finds no vcdiff bridging Friday's cached image to Monday's. vcdiff only bridges yesterday's with today's images.
• LXD pulls a full images:alpine/3.16 (slow)

I think most distros don't publish updates during weekends (Ubuntu doesn't publish updates from Friday to Sunday) so rebuilding/republishing those works against the local cache that LXD manages.

Taking the same scenario but assuming we only publish from Monday-Friday:

• User launches images:alpine/3.16 on Friday (LXD caches it locally)
• User suspends laptop on Friday evening for the weekend
• images:alpine/3.16 is rebuilt/republished Monday
• User resumes laptop on Monday, launches another images:alpine/3.16
• LXD tries to refresh the cached image and finds a vcdiff bridging Friday's cached image to Monday's.
• LXD pulls a full images:alpine/3.16 vcdiff (fast)

@MusicDin does that make sense?

[tomponline]

It does make some sense. But I worry we may delay important updates by doing this.

I think this is a potential optimization we can look at a little later once we understand access patterns better.