From owner-freebsd-fs@freebsd.org  Thu Apr 20 14:30:45 2017
Return-Path: <owner-freebsd-fs@freebsd.org>
Delivered-To: freebsd-fs@mailman.ysv.freebsd.org
Received: from mx1.freebsd.org (mx1.freebsd.org
 [IPv6:2001:1900:2254:206a::19:1])
 by mailman.ysv.freebsd.org (Postfix) with ESMTP id CF49FD469DE
 for <freebsd-fs@mailman.ysv.freebsd.org>; Thu, 20 Apr 2017 14:30:45 +0000 (UTC)
 (envelope-from mckusick@mckusick.com)
Received: from chez.mckusick.com (chez.mckusick.com [70.36.157.235])
 (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits))
 (Client did not present a certificate)
 by mx1.freebsd.org (Postfix) with ESMTPS id B7B2D820;
 Thu, 20 Apr 2017 14:30:45 +0000 (UTC)
 (envelope-from mckusick@mckusick.com)
Received: from chez.mckusick.com (localhost [IPv6:::1])
 by chez.mckusick.com (8.15.2/8.15.2) with ESMTP id v3KEVN59061503;
 Thu, 20 Apr 2017 07:31:23 -0700 (PDT)
 (envelope-from mckusick@chez.mckusick.com)
Message-Id: <201704201431.v3KEVN59061503@chez.mckusick.com>
From: Kirk McKusick <mckusick@mckusick.com>
To: Maxim Sobolev <sobomax@freebsd.org>
Subject: Re: UFS snapshot "file" is slightly bigger than underlying disk
 partition
cc: FreeBSD Filesystems <freebsd-fs@freebsd.org>
In-reply-to: <CAH7qZfs4oBdtvJEir95eHkTeVm+TPO59+z2L1n0pXj4o3fB9Ww@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-ID: <61501.1492698683.1@chez.mckusick.com>
Content-Transfer-Encoding: quoted-printable
Date: Thu, 20 Apr 2017 07:31:23 -0700
X-BeenThere: freebsd-fs@freebsd.org
X-Mailman-Version: 2.1.23
Precedence: list
List-Id: Filesystems <freebsd-fs.freebsd.org>
List-Unsubscribe: <https://lists.freebsd.org/mailman/options/freebsd-fs>,
 <mailto:freebsd-fs-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-fs/>
List-Post: <mailto:freebsd-fs@freebsd.org>
List-Help: <mailto:freebsd-fs-request@freebsd.org?subject=help>
List-Subscribe: <https://lists.freebsd.org/mailman/listinfo/freebsd-fs>,
 <mailto:freebsd-fs-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Thu, 20 Apr 2017 14:30:45 -0000

> From: Maxim Sobolev <sobomax@freebsd.org>
> Date: Thu, 20 Apr 2017 02:39:12 -0700
> Subject: UFS snapshot "file" is slightly bigger than underlying disk par=
tition
> To: FreeBSD Filesystems <freebsd-fs@freebsd.org>,
>         Kirk McKusick <mckusick@mckusick.com>
> =

> Hi Kirk,
> =

> I've noticed that the snapshot file is slightly bigger than underlying d=
isk
> partition. First I thought it's some kind of header attached to the end,
> but the size of difference is actually dependent on the disk size. Is it=
 by
> design, or some sort of "off by x" error? What's annoying about that is
> that the size is not multiple of SECTOR_SIZE. Also looks like if I just =
cut
> that junk out resulting FS image is just as usable.
> =

> Attached script illustrates that. The first column is size of the
> partition, the second column is the size of the difference, both in byte=
s.
> =

> 1048576 48
> 2097152 56
> 4194304 72
> 8388608 72
> 16777216 72
> 33554432 72
> 67108864 72
> 134217728 72
> 268435456 72
> 536870912 72
> 1073741824 72
> 2147483648 72
> 4294967296 96
> 8589934592 152
> 17179869184 256
> 34359738368 464
> 68719476736 880
> 137438953472 1720
> 274877906944 3392
> 549755813888 6744
> =

> Please advise, thanks.
> =

> -Max
> (P.S. This is 11.0-RELEASE-p9)

The extra space is for auxilary information that is used to track
changes made in the snapshot. Removing it will cause your snapshot
maintainance to slow down by 10-100x and in the worst case can
cause it to become corrupt. In short, don't mess with it.

	Kirk McKusick