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Date:      Mon, 12 Dec 2016 20:25:18 -0500
From:      Shawn Webb <shawn.webb@hardenedbsd.org>
To:        javocado <javocado@gmail.com>
Cc:        freebsd-virtualization@freebsd.org, freebsd-questions@freebsd.org, FreeBSD Filesystems <freebsd-fs@freebsd.org>
Subject:   Re: Re-sparse a file-backed IO device + zfs
Message-ID:  <20161213012518.GA77233@mutt-hardenedbsd>
In-Reply-To: <CAP1HOmQ6zXOqeD=DYe0rtnR6TkzyndLZHyutgynBXWNMVuiksA@mail.gmail.com>
References:  <CAP1HOmQ6zXOqeD=DYe0rtnR6TkzyndLZHyutgynBXWNMVuiksA@mail.gmail.com>

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On Mon, Dec 12, 2016 at 05:20:21PM -0800, javocado wrote:
> Hi,
>=20
> I'm setting up a bhyve wherein:
>=20
> host #  truncate -s 1T vol.file
> host #  du -ah vol.file
> 200K    vol.file
>=20
> host #  /usr/sbin/bhyve ... -s 4,ahci-hd,vol.file ...
>=20
> Then inside the bhyve I create a zpool (ada0 =3D vol.file):
>=20
> bhyve #  zpool create -O devices=3Doff -O atime=3Doff -O compression=3Don=
 -m
> /mnt/data1 data1 ada0
>=20
> And I put a bunch of stuff in the zpool ... and the vol.file grows in siz=
e:
>=20
> host #  du -ah vol.file
> 100G    vol.file
>=20
> Then I remove the files from the zpool and the zpool usage returns to 0 b=
ut
> of course the vol.file size does not shrink, the data is still there (but
> not referenced?)
>=20
> Normally I'd just write zeros to a file inside the zpool until the pool
> fills up, then maybe cp --sparse vol.file for good measure, but with
> compression on in the zpool the zeroing doesn't really fill up space or
> seem to overwrite anything. In my testing the zero file grew larger than
> 100G with no change to vol.file  I did not let it run forever, however.
>=20
> Any other ideas how to scrub off or clear out deleted data from a zpool
> and/or this kind of file-backed device?

Instead of dd'ing /dev/zero, try /dev/random. All zeros compress
extremely well, [pseudo-]random data does (or, ideally, should) not.

--=20
Shawn Webb
Cofounder and Security Engineer
HardenedBSD

GPG Key ID:          0x6A84658F52456EEE
GPG Key Fingerprint: 2ABA B6BD EF6A F486 BE89  3D9E 6A84 658F 5245 6EEE

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