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

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...

And speaking of layering... If we made LVM capable of generic layering (e.g. ability to stack targets, like RAID10 or snapshots of mirrors) and we improved the DSO, wouldn't we get everything we want? Stacking is already high on the list of priorities... so another good place to focus attention would be the mirror DSO. :)

Perhaps others have a stronger opinion on kernel vs. userspace.


On Dec 31, 2008, at 2:15 PM, malahal us ibm com wrote:

Alasdair, Jonathan: Any comments regarding kernel module vs
implementing it entirely with in LVM to support this multi-log feature?

Thanks, Malahal.

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