Il 2020-02-15 13:40 Zdenek Kabelac ha scritto:
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...)
Sure. But why having a single, valid filter set is not sufficient? In
other words, why/when I can not simply using global_filter, ignoring
"plain" filter?
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.
True. I myself saw some setup where hosts had direct visibility of
guest-created logical volumes. The obvious solution was to correctly set
global_filter. However, I have the impression that a good share of
complexity/issues/unexpected behaviors are due to LVM being able to be
nested (PV inside LV inside VG inside PV inside ...)
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
I never duplicate devices with mdraid, but BTRFS is so fragile that
taking a simple LVM snapshot of a BTRFS component device can lead to
data corruption.
I really think the gold standard here is ZFS.
Thanks.
--
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it [1]
email: g.danti@xxxxxxxxxx - info@xxxxxxxxxx
GPG public key ID: FF5F32A8
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