From owner-freebsd-fs@FreeBSD.ORG  Thu Mar  8 17:50:52 2012
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Date: Thu, 8 Mar 2012 17:50:51 +0000
Message-ID: <CAFHbX1KkonKGyjgYBMxj=-Kn9GNdKr+T5GYDMvhNo-Deq6BUsA@mail.gmail.com>
From: Tom Evans <tevans.uk@googlemail.com>
To: Dan Rue <drue@therub.org>
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Cc: freebsd-fs@freebsd.org
Subject: Re: ZFS and mdconfig -t vnode - Unexpected behavior
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On Thu, Mar 8, 2012 at 4:19 PM, Dan Rue <drue@therub.org> wrote:
> I have discovered an unexpected behavior when using ZFS against a vnode.
> When running mdconfig -d against a vnode, there is a long delay before
> the blocks are fully written to the backing store file. Consider the
> following test script:
>
> [=E2=80=A6]
>
> Are there any ZFS tunables that could be related to this? What could be
> the cause of this behavior?
>

The tunable vfs.zfs.txg.timeout (in seconds) controls how bursty ZFS
is. I run with a high value (30), so my disks don't get constantly
hit, but if you lowered it, data will be written much more frequently.

Cheers

Tom