Dne 14. 02. 20 v 21:40 David Teigland napsal(a):
On Fri, Feb 14, 2020 at 08:34:19PM +0100, Gionatan Danti wrote:
Hi David, being filters one of the most asked questions, can I ask why we
have so many different filters, leading to such complex interactions and
behaviors?
Don't get me wrong: I am sure you (the lvm team) have very good reasons to
do that, and I am surely missing something? But what, precisely? How should
we (end users) consider filters? Should we only use global_filter?
You're right, filters are difficult to understand and use correctly. The
complexity and confusion in the code is no better. With the removal of
lvmetad in 2.03 versions (e.g. RHEL8) there's no difference between filter
and global_filter, so that's some small improvement. But, I think filters
should be replaced or overhauled with something easier to use and more
useful at a technical level.
I've created a bz about that and welcome thoughts about what a replacement
should or should not be like. With input the work is more likely to be
prioritized.
One of the 'reason' for having 2 sets of filter was the presence of universal
'scanning' tool (aka udev) - which is assessing & reading devices in a system
and its combination with various 'VM' environments where actual device are
passed to guest systems on your hosting machine.
So there are many different combinations where different commands may need to
see different subset of devices - so i.e. your guest machine should not have
an impact on correctness of your 'hosting' machine no matter what guess will
write (i.e. duplicating signatures...)
While in many cases for many single home users with single set of devices this
can be seen maybe as an 'overkill' solution - in the more generic world where
there is unfortunately not yet any widely used/accepted solution solving the
core problem: 'who is the owner of a device' having several sets of filter
was the only solution we were able to create.
It's worth to note lvm2 is solving way more issues then other similar device
technology (i.e. mdraid, btrfs....) where it's very simple to cause big
confusion and data corruptions (even unnoticed) once duplicates appears in
your system...
Zdenek
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