Date: 28 Apr 1999 23:43:08 +0200 From: naddy@mips.rhein-neckar.de (Christian Weisgerber) To: freebsd-stable@freebsd.org Subject: Re: Does tar do sparse files these days Message-ID: <7g7vdc$3e6$1@mips.rhein-neckar.de> References: <199904280102.VAA02170@mailfw1.ford.com>
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Brian O'Connor. CF583173 HO 2nd Floor <boconno6@ford.com> wrote:
> In the dim dark days of yore(90-91) I was advised not to do full backups
> using tar. There were problems with sparse files, and device files etc.
There still are.
GNU tar, which FreeBSD uses, can't handle 32-bit dev_t. Apparently this
is a limitation of the archive format.
naddy@bigeye[~] tar cf /dev/null /dev
tar: Removing leading / from absolute path names in the archive.
tar: /dev/da0s3: minor number too large; not dumped
tar: /dev/da0s4: minor number too large; not dumped
tar: /dev/rda0s3: minor number too large; not dumped
tar: /dev/rda0s4: minor number too large; not dumped
Sparse files are a special case. You *cannot* reliably recognize sparse
files through the file API. The state of the art, which you get when you
invoke GNU tar with the -S option, is to compare stat.st_blocks with the
file size. If there are fewer blocks than you'd expect from the size,
there obviously must be holes somewhere, and you can now take guesses
which large ranges of null bytes might be holes. This should work for
most real world cases, but no way are you going to be able to exactly
reproduce a file that freely intermixes holes with actual blocks of null
bytes.
--
Christian "naddy" Weisgerber naddy@mips.rhein-neckar.de
carpe librum: books 'n' reviews <URL:http://www.carpe.com/buch/>;
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