From owner-freebsd-fs@freebsd.org  Wed Jul 25 22:55:33 2018
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From: Kirk McKusick <mckusick@mckusick.com>
To: "Mikhail T." <mi+thun@aldan.algebra.com>
Subject: Re: core dumps onto ZFS
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Is there any chance that your filesystem was more than 90% full?

Once ZFS gets past 90% full its performance drops precipitously.
As it approaches completely full it locks up for minutes per write
doing huge numbers of writes to its pool in an attempt to consolodate
what little free space it has left into something big enough for
it to use (which it sounds like is happening to you).

	Kirk McKusick