Date: Wed, 21 Jul 2004 17:14:27 +0200 From: Daniel Lang <dl@leo.org> To: Jan Grant <Jan.Grant@bristol.ac.uk> Cc: current@freebsd.org Subject: Re: NEW TAR Message-ID: <20040721151427.GC54664@atrbg11.informatik.tu-muenchen.de> In-Reply-To: <Pine.GSO.4.61.0407211440210.28037@mail.ilrt.bris.ac.uk> References: <40F963D8.6010201@freebsd.org> <20040719060730.GA87697@nagual.pp.ru> <20040720081051.GB3001@cirb503493.alcatel.com.au> <B82A97D5-DA91-11D8-B0C4-000A95C893E4@lassitu.de> <Pine.GSO.4.61.0407211440210.28037@mail.ilrt.bris.ac.uk>
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Hi,
Jan Grant wrote on Wed, Jul 21, 2004 at 02:44:42PM +0100:
[..]
> You're correct, in that filesystem semantics don't require an archiver
> to recreate holes. There are storage efficiency gains to be made in
> identifying holes, that's true - particularly in the case of absolutely
> whopping but extremely sparse files. In those cases, a simple
> userland-view-of-the-filesystem-semantics approach to ideentifying areas
> that _might_ be holes (just for archive efficiency) can still be
> expensive and might involve the scanning of multiple gigabytes of
> "virtual" zeroes.
>
> Solaris offers an fcntl to identify holes (IIRC) for just this purpose.
> If the underlying filesystem can't be made to support it, there's an
> efficiency loss but otherwise it's no great shakes.
I don't get it.
I assume, that for any consumer it is totally transparent if
possibly existing chunks of 0-bytes are actually blocks full of
zeroes or just non-allocated blocks, correct?
Second, it is true, that there is a gain in terms of occupied disk
space, if chunks of zeroes are not allocated at all, correct?
So, from my point of view it is totally irrelevant, if a sparse file
is archived and then extracted, if the areas, which contain zeroes
are exactly in the same manner consisting of unallocated blocks
or not.
So, all I guess an archiver must do is:
- read the file
- scan the file for consecutive blocks of zeroes
- archive these blocks in an efficient way
- on extraction, create a sparse file with the previously
identified empty blocks, regardless if these blocks
have been 'sparse' blocks in the original file or not.
I do not see, why it is important if the original file was sparse
at all or maybe in different places.
Cheers,
Daniel
--
IRCnet: Mr-Spock - My name is Pentium of Borg, division is futile, you
will be approximated. -
Daniel Lang * dl@leo.org * +49 89 289 18532 * http://www.leo.org/~dl/
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