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Date: Fri, 19 Feb 2016 14:50:21 -0800 (PST)
From: Don Lewis <truckman@FreeBSD.org>
Subject: Re: Zfs heavy io writing | zfskern txg_thread_enter
To: n.corvini@gmail.com
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On 19 Feb, Niccolò Corvini wrote:
> Hi, first time here!
> We are having a problem with a server running FreeBsd 9.1 with ZFS on a
> single sata drive. Since a few days ago, in the morning the system becomes
> really slow due of a really heavy io writing. We investigated and we think
> it might start at night, maybe correlated to to crondaily (standard) but we
> are not sure.  After a few hours the situation returns to normal.
> Any help is much appreciated
> The machine is a Intel Xeon E5-2620 with 36GB of RAM, the HDD is a 2TB an
> is half full.

The only way that you should get a lot of write traffic during the daily
periodic runs is if atime updates are enabled.  I don't know what the
default was for FreeBSD 9.1, but recent versions of FreeBSD disable
atime updates on ZFS everywhere except /var/mail.  What does
	zfs get -o all atime
say?

The other thing I was going to ask is how full the disk is.  ZFS
performance degrades quite a bit when the disk gets close to full.
At 50% of capacity, you shouldn't be running into that problem.