Re: Ext2fs and hashed table.

H. Peter Anvin (
15 Jun 1997 22:31:14 GMT

Followup to: <Pine.SGI.3.96.970615201424.29339C-100000@odin>
By author: Torbjorn Lindgren <>
In newsgroup:
> > >
> > > > I'd say "tries to handle"... gnu-tar can't distinguish between
> > > > allocated blocks all zero and sparse holes. in case where it's
> > > > important that some areas are really allocated, gnu-tar may break
> > > > your files. might not be a common problem but tar just can't deal
> > > > with sparse files perfectly; dump/restore can...
> > >
> >
> > Why would that matter?
> Because:
> 1. The files may well not *fit* on the disk if they aren't sparse! Take a
> couple sparse 2GB cores, which really take less than 100KB each, handled
> properly...
> 2. The file might *have* to be sparse, otherwise it won't work (rather
> uncommon, but it do exist).

[In case you weren't aware: GNU tar handles sparse files!]

True, but you're talking about the opposite case. The complaint was
that GNU tar can't tell when a file has blocks allocated all-zero,
i.e. is denser than it need be. My question is: why does *that*
matter? It will not look for holes (last I checked) in a file that
has no holes, so the argument that some files have to be 100%
allocated doesn't matter...

> > I would argue that dump can't deal with *ANY* file perfectly, since it
> > (in the typical configuration) is committing the utter no-no of
> > reading a non-quiescent r/w mounted filesystem from the raw block
> > device.
> At least it's *tries* to handle them, which is more than I would claim
> tar/cpio et al tries to do :-) Yes, another approach would be better, but
> there are lots of cases where tar/cpio isn't anywhere near enough.
> There *are* reasons why commercial UNIX backup systems doesn't usually use
> the 'read the raw-device' approach (one of them is the porting nightmare),
> but on the other hand the one's I know of does handle sparse files, it's
> really a necessity for serious backups.
> Wonder what it would take to make Legato (Networker) or Spectra Logic
> (Alexandria) to create a backup client for Linux. Both support SCO after
> all. The best would be to get them to port the Server too, but that might
> be considerably harder :-)

Legato *has* a Linux client, although it is unsupported. Of course,
we had to abandon Legato here because of repeated, severe failures
that Legato refused to give us competent help with.


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