Re: The benefits of lvmlockd over clvmd?

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Hi David,

Thanks for your explanations!

On 01/10/2018 12:06 AM, David Teigland wrote:
On Tue, Jan 09, 2018 at 11:15:24AM +0800, Eric Ren wrote:
Hi David,

Regarding the question of the subject, I can think of three main benefits of
lvmlockd over clvmd:

- lvmlockd supports two cluster locking plugins: dlm and sanlock. sanlock
plugin can supports up to ~2000 nodes
that benefits LVM usage in big virtulizaton/storage cluster,
True, although it's never been tried anywhere near that many.  The main
point hiding behind the big number is that hosts are pretty much unaware
of each other, so adding more doesn't have any affect, and when something
happens to one, others are unaffected because they are unaware.

The comments above is only talking about lvmlockd with sanlock, and it's
because the different protocols/algorithms used by them: sanlock with Paxos,
dlm with corosync, right?


while dlm plugin fits HA clsuter.

- lvmlockd has better design than clvmd. clvmd is command-line level based
locking system, which means the
  whole LVM software will get hang if any LVM command gets dead-locking
issue. However, lvmlockd is *resources* based
cluster locking. The resources to protect is VG and LV so that the deadlock
issue will be isolated inside the resource and
operations on other VG/LV can still proceed.

Is this point roughly true?


- lvmlockd can work with lvmetad.

But, I may be wrong in some points. Could you please help correct me and
complete the benefit list?
To me the biggest benefit is the design and internal implementation, which
I admit don't make for great marketing.  The design in general follows the
idea described above, in which hosts fundamentally operate unaware of

Sorry, "the idea described above" by me?

others and one host never has any effect on another.  That's diametrically

For example, with clvmd the command "lvchange -ay VG/LV" will try to activate the LV on every hosts, but with lvmlockd, we need to perform "lvchange -asy" on each host :)

opposite to the original clvm "single system image" design in which
everything that happens is in theory meant to be happening everywhere.

Got it. Thanks again.

Regards,
Eric

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