Date: Mon, 10 Aug 2009 19:09:21 +0200 From: Roland Smith <rsmith@xs4all.nl> To: Jay Hall <jhall@socket.net> Cc: freebsd-questions@freebsd.org Subject: Re: Backup Size Message-ID: <20090810170921.GC49364@slackbox.xs4all.nl> In-Reply-To: <20090810162528.GA49364@slackbox.xs4all.nl> References: <6206A242-7226-48E3-8D09-A1D3A651F2A8@socket.net> <20090810162528.GA49364@slackbox.xs4all.nl>
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On Mon, Aug 10, 2009 at 06:25:28PM +0200, Roland Smith wrote:
> On Mon, Aug 10, 2009 at 10:21:58AM -0500, Jay Hall wrote:
> > I am sure there is an easy explanation for this, but I cannot find it.
> >
> > I am backing up my /etc directory using the following command.
> >
> > tar -cvf - /etc | dd of=/dev/nsa1 obs=10240
>
> Why are you using dd? Tar was originally built to write to tape:
>
> tar -cvf /dev/nsa1 /etc
>
> > When the command completes, I receive the following message.
> >
> > 3080+0 records in
> > 154+0 records out
> > 1576960 bytes transferred in 0.179921 secs (8764740 bytes/sec)
> >
> > What concerns me is when running du -h /etc, the size of the folder is
> > reported as 1.7M.
>
> du rounds sizes up to the filesystem block size, which is 512 bytes by
> default. So you'll bound to see differences. And see below.
Oops, scratch that. Brain fart. du -h uses kilo-, mega- etc. bytes according
to du(1).
> > Is the number of bytes written to the tape less than the reported size
> > of the directory because of the way the files are written to the
> > tape? If so, how can the amount of space used be calculated?
>
> The fact that you are using tar also plays a part. Tar has some overhead to
> store information about the files it contains.
>
> If you want to know the total size of all files:
>
> find /etc -type f -ls | awk \
> 'BEGIN {t=0; c=0}; END {print t " bytes in " c " files"}; {t=t+$7; c++}'
>
> This returns '1320254 bytes in 362 files' in my case, while the tar/dd combo
> returns 1617920 bytes. The difference is the overhead for tar.
>
> If you really want to check if tar does the right thing, restore the backup to
> a different place (e.g. /tmp/etc) and check with diff:
>
> # rewind your tape to the correct position (not shown)
> cd /tmp; tar xvf /dev/nsa1
> diff -ru /etc /tmp/etc
>
> The diff command should give no output.
>
> Roland
--
R.F.Smith http://www.xs4all.nl/~rsmith/
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