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From:   David Sterba <dsterba@suse.cz>
To:     Greed Rong <greedrong@gmail.com>
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Subject: Re: BTRFS: Transaction aborted (error -24)
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On Mon, Jun 15, 2020 at 08:50:28PM +0800, Greed Rong wrote:
> Does that mean about 2^20 subvolumes can be created in one root btrfs?

No, subvolume ids are assigned incrementally, the amount is 2^64 so this
shouldn't be a problem in practice.

> The snapshot delete service was stopped a few weeks ago. I think this
> is the reason why the id pool is exhausted.
> I will try to run it again and see if it works.

The patches to reclaim the anon bdevs faster is small enough to be
pushed to older stable kernels, so you should be able to use it
eventually.

As a workaround, you can still delete the old subvolumes to get the
space back but perhaps at a slower rate and wait until the deleted
subvolumes are cleaned. That there's no way to get the number of used
anon bdevs makes it harder unfortunatelly.