[linux-lvm] LVM mirroring questions...

[Stuart D. Gathman]

On Tue, 29 Apr 2008, Morten Torstensen wrote:

> Marcin Ka³u¿a wrote:
> > Why software RAID is not an option? It works with partitions as well as with
> > whole disks (actually it doesn't care what it is working on as long as it's
> > a block device)
> 
> Because doing mirroring in the LVM is much easier as you don't have to 
> set up the md layer. Simple LE to multiple PE mapping is easy to do and 
> makes LVM mirroring easy and flexible.

Using md with LVM was a work around for LVM mirroring not being available /
reliable at first.  With LVM, you can mirror some LV and not others.
For instance, a /work LV used only for software builds might not get mirrored.
(If the underlying PV crashes, the build gets interrupted, but you just
start over after replacing the disk.)

With md, fully taking advantage of new disks added to the system often
requires repartitioning and solving knapsack problems (moving partitions
can take place online with live filesystem using md).  With LVM, 
many equal sized PE make this automatic.  (Although the write intent
table needs a 3rd drive.)

-- 
	      Stuart D. Gathman <stuart at bmsi.com>
    Business Management Systems Inc.  Phone: 703 591-0911 Fax: 703 591-6154
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