From owner-freebsd-fs@freebsd.org  Wed Oct  4 16:43:40 2017
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Date: Wed, 4 Oct 2017 17:43:37 +0100
From: Gary Palmer <gpalmer@freebsd.org>
To: javocado <javocado@gmail.com>
Cc: FreeBSD Filesystems <freebsd-fs@freebsd.org>
Subject: Re: lockup during zfs destroy
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On Wed, Oct 04, 2017 at 09:15:27AM -0700, javocado wrote:
> My questions:
> 
> - is this what it appears to be, a memory exhaustion?
> - if so, why isn't swap utilized?

Kernel memory generally isn't pushed to swap as it could lead to deadlock
situations way too easily.

> - how would I configure my way past this hurdle?
> - a filesystem has a DELETE_QUEUE ... does the zpool itself have a destroy
> queue of some    kind?  I am trying to see if I can see the zpool working
> and how far along it is, but I do not know what to query with zdb

Yes, it does, I believe behind the feature@async_destroy flag on
the pool.  "zpool get feature@async_destroy" to see the enabled
status.  Not sure if you can query the queue to see how it is
progressing.  I haven't destroyed any pools, but with snapshots
you can check the free space on the pool using "zpool list"
and it gradully increases in the background.

Regards,

Gary