_alloc_init calculates the number of necessary log extents via 'mirror_log_extents'.  'mirror_log_extents' takes 3 arguments: region_size, pe_size, and size of the mirror LV.  Unfortunately, _alloc_init is guessing at the mirror size by using 'ah->new_extents / ah->area_multiple' - the number of extents that the mirror images have.  However, this is /always/ wrong when allocating the log separately.  Further, the log is always allocated separately unless we are up-converting the mirror at the same time.  It was by luck alone that a default value of '1' reflects what we want in most cases.

In order to get a decent value computed, we need to pass in the 'lv' argument to allocate_extents.  This would normally imply a desire for cling/contiguous allocation to the given LV, but since we are not allocating any parallel extents and only log extents, it works fine.

BEFORE:
[root@bp-01 LVM2]# vgs -o name,vg_extent_size vg
  VG   Ext  
  vg   1.00k
[root@bp-01 LVM2]# lvcreate -m1 -L 20M -n lv vg
  device-mapper: reload ioctl failed: Invalid argument
  Failed to activate new LV.


AFTER:
[root@bp-01 LVM2]# vgs -o name,vg_extent_size vg
  VG   Ext  
  vg   1.00k
[root@bp-01 LVM2]# lvcreate -m1 -L 20M -n lv vg
  Logical volume "lv" created
[root@bp-01 LVM2]# lvs -a vg
  LV            VG   Attr     LSize  Poo Origin Snap%  Move Log     Copy%  Convert
  lv            vg   mwi-a-m- 20.00m                        lv_mlog 100.00        
  [lv_mimage_0] vg   iwi-aom- 20.00m                                              
  [lv_mimage_1] vg   iwi-aom- 20.00m                                              
  [lv_mlog]     vg   lwi-aom-  2.00k                                              

Fix checked in upstream in Version 2.02.89.

git commit IDs:
bfd774a20a22f8224a8bc3b507c4564fe6f54fc6 -- original
2d8a2f35c77fdeef1dbe0ef791db8530d07826eb -- follow-up correction

I was unable to reproduce this issue with the latest scratch built rpms.

2.6.32-195.el6.x86_64

lvm2-2.02.87-2.1.el6    BUILT: Wed Sep 14 09:44:16 CDT 2011
lvm2-libs-2.02.87-2.1.el6    BUILT: Wed Sep 14 09:44:16 CDT 2011
lvm2-cluster-2.02.87-2.1.el6    BUILT: Wed Sep 14 09:44:16 CDT 2011
udev-147-2.38.el6    BUILT: Fri Sep  9 16:25:50 CDT 2011
device-mapper-1.02.66-2.1.el6    BUILT: Wed Sep 14 09:44:16 CDT 2011
device-mapper-libs-1.02.66-2.1.el6    BUILT: Wed Sep 14 09:44:16 CDT 2011
device-mapper-event-1.02.66-2.1.el6    BUILT: Wed Sep 14 09:44:16 CDT 2011
device-mapper-event-libs-1.02.66-2.1.el6    BUILT: Wed Sep 14 09:44:16 CDT 2011
cmirror-2.02.87-2.1.el6    BUILT: Wed Sep 14 09:44:16 CDT 2011


[root@taft-01 ~]# pvcreate --setphysicalvolumesize 1G /dev/sdb1 /dev/sdc1 /dev/sdd1 /dev/sde1 /dev/sdf1 /dev/sdg1
  Writing physical volume data to disk "/dev/sdb1"
  Physical volume "/dev/sdb1" successfully created
  Writing physical volume data to disk "/dev/sdc1"
  Physical volume "/dev/sdc1" successfully created
  Writing physical volume data to disk "/dev/sdd1"
  Physical volume "/dev/sdd1" successfully created
  Writing physical volume data to disk "/dev/sde1"
  Physical volume "/dev/sde1" successfully created
  Writing physical volume data to disk "/dev/sdf1"
  Physical volume "/dev/sdf1" successfully created
  Writing physical volume data to disk "/dev/sdg1"
  Physical volume "/dev/sdg1" successfully created
[root@taft-01 ~]# vgcreate -s 1K mirror_sanity  /dev/sdb1 /dev/sdc1 /dev/sdd1 /dev/sde1 /dev/sdf1 /dev/sdg1
  Volume group "mirror_sanity" successfully created
[root@taft-01 ~]# lvcreate -m 1 -n mirror_on_1Kextent_vg -L 20M mirror_sanity
  Logical volume "mirror_on_1Kextent_vg" created

Fix verified in the latest official rpms.

2.6.32-198.el6.x86_64

lvm2-2.02.87-3.el6    BUILT: Wed Sep 21 09:54:55 CDT 2011
lvm2-libs-2.02.87-3.el6    BUILT: Wed Sep 21 09:54:55 CDT 2011
lvm2-cluster-2.02.87-3.el6    BUILT: Wed Sep 21 09:54:55 CDT 2011
udev-147-2.38.el6    BUILT: Fri Sep  9 16:25:50 CDT 2011
device-mapper-1.02.66-3.el6    BUILT: Wed Sep 21 09:54:55 CDT 2011
device-mapper-libs-1.02.66-3.el6    BUILT: Wed Sep 21 09:54:55 CDT 2011
device-mapper-event-1.02.66-3.el6    BUILT: Wed Sep 21 09:54:55 CDT 2011
device-mapper-event-libs-1.02.66-3.el6    BUILT: Wed Sep 21 09:54:55 CDT 2011
cmirror-2.02.87-3.el6    BUILT: Wed Sep 21 09:54:55 CDT 2011


SCENARIO - [create_mirror_on_1Kextent_vg]
Create a mirror on a VG with an extent size of only 1K
Recreating PVs/VG with smaller (1K) extent size

taft-01: pvcreate --setphysicalvolumesize 1G /dev/sdd1 /dev/sdd2 /dev/sde1 /dev/sde2 /dev/sdf1 /dev/sdf2 /dev/sdg1 /dev/sdg2 /dev/sdh1 /dev/sdh2

taft-01: vgcreate -s 1K mirror_sanity /dev/sdd1 /dev/sdd2 /dev/sde1 /dev/sde2 /dev/sdf1 /dev/sdf2 /dev/sdg1 /dev/sdg2 /dev/sdh1 /dev/sdh2

taft-01: lvcreate -m 1 -n mirror_on_1Kextent_vg -L 20M mirror_sanity
Deactivating mirror mirror_on_1Kextent_vg... and removing
Restoring VG to default extent size

    Technical note added. If any revisions are required, please edit the "Technical Notes" field
    accordingly. All revisions will be proofread by the Engineering Content Services team.
    
    New Contents:
Do not document.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

http://rhn.redhat.com/errata/RHBA-2011-1522.html