[linux-lvm] LVM mirror questions

[Phillip Susi]

On 10/6/2010 1:40 PM, Petr Rockai wrote:
> Yes, (write) performance is a concern. Otherwise, what you describe is
> perfectly valid and achievable with LVM.

It is?  What happens when the log device fails?

>> I don't know how LVM handles issues of PVs that are not permanently dead
>> and come back on line again later.  The md driver uses a generation count in
>> the super block to determine which is newer. (How is that updated without loss
>> of efficiency?)
> 
> LVM has a generation counter as well. It's only updated on metadata
> writes though, so it doesn't cost anything. (Log is not part of metadata
> in this sense.) When you lose a leg (and use dmeventd), the metadata on
> the remaining PVs is updated to say that. The leg is also yanked from
> the mirror. You can add it as a fresh image (with full resync) if it
> ever comes back.

Why a full resync?  With mdadm, it just keeps flagging the dirty chunks
so it only has to copy those when the other disk returns.