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From: "Harik A'ttar" <harik@chaos.ao.net>
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Subject: Re: [linux-lvm] Error setting up a mirrored lv
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Laurenz, Dirk wrote:
> Hi,
> 
> i'm doing this:
> 
>>> hgest3301:~/LVM2.2.02.06 # lvcreate -m1 -n lvmirror -L 10G 
>>> vgtest /dev/dm-55 /dev/dm-56
>>>   Not enough PVs with free space available for parallel allocation.
>>>   Consider --alloc anywhere if desperate.

> 
> what's wrong? 
> Is there any documentation out there?
> Why do we need three pvs? for a mirror?
> Are there any other options?

As you noticed, you need a log partition.  dm-mirror is a bit smarter 
then the basic MD raid1 in that it uses a log disk to keep track of 
write completions for synchronization.  I've only recently got a kernel 
that supports modern mirror, but it looks like the log file sticks at a 
single extent even with a 20gb mirror (all my free space).

Is there a reason that the mirror is on a separate volume rather then a 
serialized metadata chunk at the end of each?  Just for seektime 
performance reasons?