i have moved from RAID level 10, to 5, to 6 and now back to 10. The rebuild and check times for RAID 6, in particular, are excruciating and stress the drives. With proper backups (which you should always have, regardless of the RAID level), RAID 10 offered speed (in build, recovery and usage) over capacity. Pick your trade offs.
to create a new 8 disk RAID10 array..
mdadm --create /dev/md0 --raid-devices==8 --level==raid10 --chunk==256 --assume-clean /dev/sdb1 /dev/sdc1 /dev/sdd1 /dev/sde1 /dev/sdf1 /dev/sdg1 /dev/sdh1 /dev/sdi1
mdadm with XFS (ext4 in 2.6.x kernels seemed to exhibit an annoying drive thrashing during idle).
mkfs.xfs -f -b size==4096 -d sunit==512,swidth==2048 /dev/md0
to scan array configuration..
mdadm --detail --scan
mdadm --detail --scan >>/etc/mdadm/mdadm.conf #.. to save configuration
To force a check of the RAID..
echo check > /sys/block/md0/md/sync_action
To monitor mdadm progress..
watch -d cat /proc/mdstat
to remove a disk from an array for replacement..
mdadm --fail /dev/md0 /dev/sda1
mdadm --remove /dev/md0 /dev/sda1
To add a new disk to array..
mdadm --add /dev/md0 /dev/sda1
to free up the disks to move to another mdadm RAID level from backups (migrating levels with realtime conversion is painfully slow and stresses the drives)..
mdadm --stop /dev/md0
mdadm --remove /dev/md0
unless using identical RAID certified hard disks, manually partition all drives to identical partition size so differing drive models and brands may be used within the RAID, as even the same series within a brand may have differing sector block counts for the drive across generations (e.g. create 931GB partitions with 1TB Samsung SpinPoint drives).
consumer grade drives should probably not be used for RAID usage due to their disabling of TLEF (Time Limited Error Recovery). Use the “WD Red” NAS drives for decent price/performance.
drives flagged as bad on powerup can result in the array appearing to have a bad superblock when the complete array cannot be assembled by mdadm. This can usually be rectified with the following..
mdadm --stop /dev/md0
mdadm --assemble --force /dev/md0 /dev/sdb1 /dev/sdc1 /dev/sdd1 /dev/sde1 /dev/sdf1 /dev/sdg1 /dev/sdh1 /dev/sdi1