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

[malahal at us.ibm.com]

Jonathan Brassow [jbrassow at redhat.com] wrote:
> My general feeling is that it is better to do in userspace, but this is 
> only because I think there is so much improvement to be done in the mirror 
> DSO - transient fault handling being one of those areas.  If you all can 
> get your benefits of multi-log, while I get my benefits of an improved DSO, 
> then I am very happy.
>
> That being said, there may also be merit in the kernel approach.  I haven't 
> tried to think through all the nasty cases where log devices and mirror 
> devices overlap.  For example, I want a 3-way mirror with a 2-way redundant 
> mirror log and I only have 3 physical disks.  If I get a failure on a 
> device that contains both log and leg, how are the failures going to be 
> handled?  It could get difficult with the layering...

The log mirror would be out-of-sync but should/would still continue to
operate.  The leg mirror would be out-of-sync as well. Depending on how
it is configured, the log mirror may go to linear/single device mode.

Thank you Jonathan for your comments.

--Malahal.