Re: [RFC][PATCH 0/4] dm-log: support multi-log devices

[malahal@us.ibm.com]

Takahiro Yasui [tyasui@redhat.com] wrote:
> Hi Malahal,
> > A while back IBM posted a patch to LVM that constructs a log device with
> > a mirror and then creates the real mirror using such a mirrored log
> > device. I think this may solve your problem. It was completely written
> > in LVM and Stefan refreshed it to the latest LVM.
> 
> Thank you for the comment and information. It seems that your
> approach seems to address my problem, too. Here I have a concern
> about write performance because an additional mirror mapping might
> introduce additional delay and overhead. In addition, error for
> log devices is better to be handled by the simple way, and a basic
> error handling would work.

In theory yes, but I doubt it would be user visible that much. We expect
transient failures under some circumstances, so we would like to handle
them. In other words, a failed device is expected to come back and the
mirror target should re-integrate it automatically when it comes back.
Can your multi-log code handle re-synchronizing a log device?

With our user level only implementation, the log device handling would
be as good as the real mirror *leg* handling. We get all the benefits of
the mirror without doing any code! Wouldn't it be nice?


> I couldn't find any discussion after you posted the patch.
> Could you tell me if IBM also have the same background as I have,
> or do you have another issue to solve? I would also like to know
> if my approach solves your problem.

Jonathan, Alasdair and I had discussions about the patch. I can send
them to you if you want.

As I said, we want to handle transient device failures. Can your patch
work with such log devices?

--Malahal.