size of tar file

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From: "Fabio Giannotti" <fabiog@venmar.com>
References: <942dhp$cd9$1@nnrp1.deja.com> <20010116170637.L3112@jpradley.jpr.com> 
Subject: Re: tar much larger than contents
Date: Tue, 16 Jan 2001 16:30:42 -0600

Jean-Pierre Radley <jpr@jpr.com> wrote in message
news:20010116170637.L3112@jpradley.jpr.com...
> Nick Cole propounded (on Tue, Jan 16, 2001 at 09:13:38PM +0000):
> |
> | l *tar
> | -rw-------   1 manager  other     361984 Dec 20 03:51 bu1.tar
> | -rw-------   1 manager  other    3847680 Dec 20 03:51 bu2.tar
> |
> | tar tvf bu1.tar
> | tar: blocksize = 20
> | rw-rw-r--999/1      663 Dec 20 03:39
> | 2000 /usr/cheshire/credit/pbmerc.dat
> |
> | tar tvf bu2.tar
> | tar: blocksize = 20
> | rw-rw-rw-999/1   487696 Dec 20 03:39 2000
/usr/cheshire/credit/pbadmin/DB/pbadmain.dat
> | rw-rw-rw-999/1   169984 Dec 20 03:39 2000
/usr/cheshire/credit/pbadmin/DB/pbadmain.idx
>
> | Can anyone speculate why these tar archives are much larger than their
> | component files? I know blocking makes archives slightly larger, but
> | this is ridiculous.
>
> It isn't ridiculous, it's perfectly normal.  Once upon a time, you did
> need that many bytes in each tar-file to hold its specified contents.
> Using a 'tar cf 'file.tar ...' command does NOT first erase or null out
> file.tar; it never shrinks.  Try 'tar cvf bu1.tar /etc/profile', and
> bu1.tar will still be 361984 bytes.
>
Well I'll be darned.  You learn something new everyday.

Do you know the original reasoning behind this behavior?  It seems to me
that it *should* erase the file first...

Fabio

Macworld Mac Security Superguide $9.99

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