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Backup retention interval is unstable #38

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Urth opened this issue Aug 31, 2020 · 0 comments
Closed

Backup retention interval is unstable #38

Urth opened this issue Aug 31, 2020 · 0 comments

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@Urth
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Urth commented Aug 31, 2020

The interval between backups in the same retention period is unstable. For example when a daily backup is selected as the best candidate for a weekly retention period it does not consider the previous weekly backup. As a result the interval between weekly backups can vary between 6-8 days.

Select planb-20200622T0014Z as fresh match for 20200621T0013Z (planb.storage.base)
Select planb-20200621T0004Z as best match for 20200621T0013Z with diff:540 (planb.storage.base)
Select planb-20200614T0005Z as best match for 20200614T0004Z with diff:-60 (planb.storage.base)
Select planb-20200609T0012Z as best match for 20200607T0005Z with diff:-173220 (planb.storage.base)
Select planb-20200604T0012Z as best match for 20200602T0012Z with diff:-172800 (planb.storage.base)

Because planb-20200622T0014Z is the closest it will be preserved and cause planb-20200621T0004Z to be removed even though it is the perfect match compared to the previous backup planb-20200614T0005Z.
@Urth Urth closed this as completed in ac71f8b Sep 2, 2020
Urth added a commit that referenced this issue Sep 2, 2020
@Urth
Fix unstable snapshot interval
5740dc3 
Fix #38 by always clamping dates to a sane value for the given period.
Previously the targetted desired date for a given retention period would
depend on the day the backup was performed. As a result it shifted each
day by one day causing unstable intervals in between.

So a weekly retention period interval could be vary between 6 to 8 days.
For example weekly backups are now clamped to sundays (hardcoded, #39)
and as a result the best matching snapshot no longer shifts day by day.

This uses dateutil to perform the weekday clamping.
Add Django 3.1 as a supported version in the setup requirements.
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