[Bug 232152] unrecoverable state after zroot is filled with data to 100%, can't boot, can't import

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=232152

            Bug ID: 232152
           Summary: unrecoverable state after zroot is filled with data to
                    100%, can't boot, can't import
           Product: Base System
           Version: 11.1-RELEASE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: petr.fischer at me.com

My experience on virtual server:

I accidentally filled the zroot zpool with data to the 100% of capacity
(available ZFS space exactly 0).

Then, it's not possible to boot, just because it's not possible to import zroot
pool anymore.

When I booted from FreeBSD 11.1-RELEASE Install CD ROM (Live CD option), I
can't import the zroot pool with: zpool import zroot
Command never ends, zpool process is in "tx->tx" state forever, CPU utilization
is 0%, interrupts 0%, idle 100%.

When I try to import filled zroot in readonly mode, yes, that works! But in
readonly mode, you can't delete data or snapshots.

So it looks like game over. Maybe ZFS is not able to do any commit records
anymore on 100% filled pool, I don't know...

There is only 512MB (+ 1GB swap) on my virtual server, but 2 years without any
problem with ZFS (hourly snapshots, zfs send for backup, everything works like
a charm).

Is it really that dangerous to fill the zpool with data to 100%?

Such a situation will be simple to simulate:

1) install FreeBSD on the ZFS
2) fill some dataset with random data to the 100% of capacity
3) check with zfs command, if there is really 0 bytes available space
4) reboot
5) gameover

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