[linux-lvm] Mirrored LV

Jonathan Brassow jbrassow at redhat.com
Sat Sep 20 03:39:52 UTC 2008

Just to answer some questions:

You can use just 2 disks to create a mirror and a disk log:
prompt> lvcreate --alloc anywhere -m1 -L 500M -n my_lv my_vg

I wish the allocation policy 'normal' (which is the default) would  
also do this... but instead you must choose a looser allocation policy  
to get what you want.

The reason that you want the disk-based log is to prevent all the  
extra work when you machine comes up.

The reason that LVM wants a separate disk for the log is so the head  
on the storage device doesn't need to go ping-ponging around the  
device.  (The log is touched for almost every write.)  So, its a  
performance thing... and possibly a not-wear-out-your-hardware thing  

It won't matter if the master disk with the log dies.  Your mirror  
will simply become a linear device composed of the last remaining disk  


On Sep 19, 2008, at 5:01 PM, Koen Vermeer wrote:

> Op vrijdag 19-09-2008 om 10:31 uur [tijdzone -0700], schreef
> malahal at us.ibm.com:
>> It needs it own PV. Create an extra partition on your master disk and
>> add the PV tou the volume group.  Then run lvcreate again with out
>> corelog option. By default the command would create a disklog. You  
>> can
>> also specify where you want your log.
> OK, did that! I guess I need one additional partition for each volume
> group, then.
> Now, what happens if the master disk (which includes the log) dies?
> Won't LVM complain that it cannot find the log? I'm not saying I  
> expect
> it too, but with this stuff I guess it's better to be safe than sorry!
> Best,
> Koen
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