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On Fri, Oct 1, 2021 at 4:36 PM antlists <antlists@××××××××××××.uk> wrote: |
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> |
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> I now want to run lvm snapshot on the first boot of the weekend. Writing |
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> a unit to do the snapshot seems pretty easy, but obviously I don't want |
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> it firing every boot, if I stick the date in the volume name I don't |
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> want it colliding with an earlier run the same day, etc etc. |
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I'm not sure a timer is a great way to do this. Offhand I'm not sure |
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how the persistence on those works exactly - maybe you could get it to |
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fire off when you want it to. However, there is another issue you're |
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missing. |
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I assume you want to wait for the snapshot to complete before you |
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mount local filesystems. Normally you'd just do this with a |
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dependency - the local filesystems get an After dependency on your |
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snapshot, so they don't get mounted until the snapshot is complete. |
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The problem is that the service that does the snapshot won't be |
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enabled - so the dependency will not constrain mounting the |
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filesystems. An After dependency on its own doesn't cause the thing |
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it depends on to run - it just says that IF it is going to run it has |
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to run first. The service that does the snapshot is going to be |
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started by the timer, not a target, and so at the moment the |
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dependencies are analyzed it won't be taken into account. That |
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creates a race condition, since timers do not launch their services in |
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any sort of synchronization with the system startup. |
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Now, you could set a dependency on the snapshot such that it doesn't |
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let filesystems be mounted while it runs, but then when it gets run it |
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will force all your filesystems to unmount (which is basically going |
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to do a clean near-shutdown of your entire system to whatever degree |
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it has started up), then do the snapshot, then remount everything and |
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restart it all. That isn't what you want either. |
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I think the cleanest approach is just make the snapshot a regular |
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service and enable it and set up the dependencies so that it has to |
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complete before local filesystems are mounted. Then all you need to |
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do is put logic in that service that turns it into a no-op if there is |
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a recent enough snapshot already. |
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Timers are really useful for stuff that is asynchronous, whether |
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time-based or not. They're not really good for anything synchronous |
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with the boot process, unless there is some magic to this that I'm |
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unaware of. |
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-- |
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Rich |
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