Thank you for such a detailed answer.

I'm still working out what our policies should be, but:
- most of our data is 'host specific', so when a host is removed, its data is 
nearly entirely useless (not 100% true, but nearly)
- we're quite tight with disk space
- we'd love to have 13 months of data available

I may end up putting the important data into specific shards, and then all the 
other stuff into shorter-lived shards, but until then, I thought it might be 
useful to delete the old data.

One use case where deleting the old data is useful is in Grafana templated 
dashboards. If you have a template variable called 'host' which does something 
like:

SHOW TAG VALUES FROM system WITH KEY=host

...then it'll show all the hosts that have ever reported stats. If you then 
select one that's been dead a while, you (seem to) get no results, which 
confuses people. Since the data is of low value, deleting the host's data means 
no more confusion. I'd agree that a better solution would be to somehow filter 
those hosts from the template variable (but I can't see how we could do that).

Having looked at this some more and your answer, I'm thinking I'll write some 
sort of batch job which will:

- Get a list of all known hosts
- query each one for any stats in the last (say) week
- For all hostnames that don't return any data, delete them

Since deletion isn't cheap, I'm wondering if I could do them a chunk at a time. 
If I can, then I could just delete them slowly with multiple small deletes. A 
quick look at it suggests DROP SERIES is my only choice, so at off-peak times 
is probably my only option.

Thanks again for your help :-)

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