Re: [BackupPC-users] Too many links : where is the problem ?

Hi,

I wrote on 2011-08-10 00:44:43 +0200 [Re: [BackupPC-users] Too many links : 
where is the problem ?]:
> [...]
> you *might* be able to simply mount your pool file system as ext4.
> [...]
> I've heard you can mount any ext3 FS as ext4, but you might not be able to
> mount it as ext3 again later (which makes sense, considering you might have
> files with > 32000 links, for example).

I just did a small test. I created an ext3 FS on an empty partition (logical
volume, actually), mounted it, created a file with 32000 links with the Perl
command line posted before, umounted the FS, mounted it as ext4, proceeded to
create further links (starting at 32001; the command stopped with "Too many
links" after the 65000th link as expected), umounted the FS again, and tried
to mount it as ext3 again. That worked without complaining. An 'ls' of the
file shows 65000 links:

% ls -aln 1
-rw-r----- 65000 1000 1000 0 Aug 10 04:14 1
% rm 65000
% ln 1 65000
ln: creating hard link to `1': Too many links
% ls -aln 1
-rw-r----- 64999 1000 1000 0 Aug 10 04:14 1

Please note that this test was done on an empty FS without real data on it. If
I had lost the FS, no damage would have been done. Mileage on your populated
and valuable FS may vary. All it is meant to show is that - to a certain
extent - ext3 and ext4 seem to be interoperable. While you may not be able to
revert to ext3 in all cases (I don't know), mounting an ext3 as ext4 does seem
to principally work and give you the higher max link count. In any case, it is
an option worth investigating.

This experiment was done on Linux 2.6.32 with e2fsprogs 1.41.12. The results
might be specific to these versions of kernel and user space tools, though I
would expect them to apply at least to newer versions also.

Regards,
Holger

P.S.: The FS even passes an 'fsck -t ext3 -f' cleanly, but that might only
      apply to ext4-aware e2fsck versions.

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