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Mismatch_cnt (/sys/block/md#/md/mismatch_cnt) is the number of unsynchronized blocks in the raid.

Not much of an issue if this is reported on raid-0 or raid-1 and can be ignored. See bugzilla report: Bug 566828.

The repair is to rebuild the raid:

This does not reset the count, but if you force a check after the rebuild is complete:

Count should return to zero. Verify with:

I have outlined below the steps taken to move from a normal single disk system and convert to a raid1 using an additional drive:

Note: /boot cannot be on lvm nor any other raids other than raid1 partition.

Don't forget to make the essential backups prior.

Convert / to raid/lvm

1. fdisk /dev/sdb and make clone of sda (convert partition type to "Linux raid autodetect")
   
2. add /dev/sdb1 -- /boot - 13 blocks (100Mb) - type raid
   
3. add /dev/sdb2 -- rest - type raid
   
4. Create raid partitions:
   
   partprobe
mdadm -C /dev/md0 --auto=yes -l 1 -n 2 missing /dev/sdb1
mdadm -C /dev/md1 --auto=yes -l 1 -n 2 missing /dev/sdb2
5. Create logical volume and copy / partition and /dev files over:
   
   pvcreate /dev/md1
vgcreate vg0 /dev/md1
lvcreate -L 4G -n lv0 vg0
mke2fs -j /dev/vg0/lv0
mkdir /mnt/lv0
mount /dev/vg0/lv0 /mnt/lv0
find / -xdev | cpio -pvmd /mnt/lv0
cp -aux /dev /mnt/lv0
6. Edit /mnt/lv0/etc/fstab to reflect the new root
   
   /dev/vg0/lv0            /     ;     ;     ;     ;   ext3    defaults        1 1
7. Chroot to new filesystem and create initrd with raid and lvm support
   
   mount --bind /dev /mnt/lv0/dev
chroot /mnt/lv0
mount -t proc /proc /proc
mount -t sysfs /sys /sys
vgscan
vgchange -ay
mkinitrd -v /boot/initrd-`uname -r`.lvm.img `uname -r`
umount /sys
umount /proc
exit
mv /mnt/lv0/boot/initrd-`uname -r`.lvm.img /boot
8. Edit grub.conf to point to new root /dev/vg0/lv0
   
9. reboot

Convert /boot to raid1