From owner-freebsd-fs@freebsd.org  Thu Aug 18 14:58:59 2016
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To: FreeBSD FS <freebsd-fs@freebsd.org>
Cc: freebsd-stable <freebsd-stable@FreeBSD.org>
From: "Eugene M. Zheganin" <emz@norma.perm.ru>
Subject: cannot destroy '<dataset name>': dataset is busy vs iSCSI
Message-ID: <57B5CD2F.2070204@norma.perm.ru>
Date: Thu, 18 Aug 2016 19:58:55 +0500
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Hi.

I'm using zvol clones with iSCSI. Perdiodically I renew them and destroy
the old ones, but sometimes the clone gets stuck and refuses to be
destroyed:

(I'm showing the full sequence so it's self explanatory who is who's parent)

[root@san2:/etc]# zfs destroy esx/games-reference1@ver5_6       
cannot destroy 'esx/games-reference1@ver5_6': snapshot has dependent clones
use '-R' to destroy the following datasets:
esx/games-reference1-ver5_6-worker111
[root@san2:/etc]# zfs destroy esx/games-reference1-ver5_6-worker111
cannot destroy 'esx/games-reference1-ver5_6-worker111': dataset is busy

The only entity that can hold the dataset open is ctld, so:

[root@san2:/etc]# service ctld reload                                    
[root@san2:/etc]# grep esx/games-reference1-ver5_6-worker111 /etc/ctl.conf
[root@san2:/etc]# zfs destroy esx/games-reference1-ver5_6-worker111      
cannot destroy 'esx/games-reference1-ver5_6-worker111': dataset is busy

As you can see, the clone isn't mentioned in ctl.conf, but still refuses
to be destroyed.
Is there any way to destroy it without restarting ctld or rebooting the
server ? iSCSI is vital for production, but clones sometimes holds lot
of space.

Thanks.
Eugene.