Re: [linux-lvm] New features for using lvm on shared storage

[David Teigland <teigland@redhat.com>]

On Tue, Jan 10, 2017 at 09:02:36PM +0800, Eric Ren wrote:
> Hi David,
> 
> Sorry for faking this reply because I'm not in the maillist before I noticed
> this email (quoted blow) you posted for a while.
> 
> I have a questions about "lvmlockd":
> 
> Besides clvmd cannot be used together with lvmetad, is there any other
> main differences between "lvmetad" and "clvmd"? Do you recommend we replace
> "clvmd" with "lvmetad"?

Hi, I think you're looking at the differences between lvmlockd and clvmd.
They have a completely different design and implementation, with lvmlockd
being much simpler and more obvious in what it does and how it works.
clvm was sort of designed around the old concept of a "single system
image" across a cluster, and tried hard to hide any difference between
local/shared VGs.  lvmlockd just inserts lock/unlock around disk
modifications when the VG is shared.  And as mentioned before, lvmlockd
can use sanlock which opens it up to new uses where a cluster doesn't fit.
I recommend replacing clvm with lvmlockd; I think it works better, and
clvm will eventually be phased out.

(lvmetad, the metadata caching daemon, can indeed be used with lvmlockd,
and not clvmd, but this is not a big advantage.  The metadata caching has
not turned out to be much of a benefit.)

> """
> 
> For people using LVM (or clvm) on shared storage, I encourage you to take
> a look at two recent additions that you may find to be an improvement:
> 
> 1. System ID
> 
> is for static ownership of VGs by hosts:
> http://man7.org/linux/man-pages/man7/lvmsystemid.7.html
> 
> The nearest equivalent to system ID in the past has been configuring
> lvm.conf filters uniquely on each host.
> 
> 2. lvmlockd
> 
> is for dynamically sharing VGs among hosts:
> http://man7.org/linux/man-pages/man8/lvmlockd.8.html
> 
> Using lvmlockd with dlm is similar to what clvmd has done in the past.
> Using lvmlockd with sanlock has no prior equivalent.
> 
> (For distribution packaging, see the "lvm2-lockd" rpm.)
> 
> Dave
> """