Re: strange zfs size allocation data

[Greg Troxel <gdt%lexort.com@localhost>]

Mouse <mouse%Rodents-Montreal.ORG@localhost> writes:

>> This is a test case, to see if backing up and restoring a sparse file
>> results in a sparse file.  I realize that this probably requires a
>> logging fuse driver and a lot of complexity to do 100% right.
>
> Is bup zfs-specific?  Because, if you're not doing something

No, it is a general backup program.  I just happen to have sources for
it on zfs.  Which people tell me is a great filesystem and is now not
odd on NetBSD....

> filesystem-specific, I actually think you will have trouble even
> _defining_ what "100% right" is for this test, since everything about
> sparseness, right down to whether it's even a thing, is
> filesystem-dependent.

True.  the point is to try to verify that the backup program, when
restoring a sparse file, writes it in such a way that the normal
implementation of sparse files works, meaning results in a file without
blocks storing all the zeros.

What you are missing, and everybody else too, is that the fact that this
is theoretically impossible is irrelevant to it being useful in the real
world, to detect regressions, even if it also occasionally detects
bizarre behavior.

A better test would be 'fuse-sparsetest' that makes metadata available
for inspection later about the writes it sees.  But that's hard to
write.