How do you know software RAID is up and running?

Note that, upon requests, I may translate this document into traditional Chinese.
How do you know that your SATA RAID arrays are up and running? It was tough for me to find out the answer even googled thru many web pages. I actually had once found that the RAID arrays I created were only partially running. There are two places that you can be ensured that the SATA RAID arrays are up and running. In my system, it looks like these:

1. check your filesystem disk space usage by issuing df -k and verify that the filesystems are /dev/md*.

   Filesystem             1K-區段      已用     可用 已用% 掛載點
   /dev/md0                255799     66585    175566  28% /
   tmpfs                   453368         0    453368   0% /dev/shm
   /dev/md5              80089452    124612  79964840   1% /home
   /dev/md4                393452     32840    360612   9% /tmp
   /dev/md1              48827944    391588  48436356   1% /usr
   /dev/md2              29293500     84732  29208768   1% /var
   

2. check your /proc/mdstat and make sure both partitions are U which means up and running. For example, in my system, md0 have two partitions which are sda1 and sdb1 and both partitions are [UU]. If any partition is [_], then it means one partition is not running.

   Personalities : [raid0] [raid1] [raid5]
   md1 : active raid1 sda5[0] sdb5[1]
         48829440 blocks [2/2] [UU]
   
   md2 : active raid1 sda6[0] sdb6[1]
         29294400 blocks [2/2] [UU]
   
   md4 : active raid1 sda8[0] sdb8[1]
         393472 blocks [2/2] [UU]
   
   md5 : active raid1 sda9[0] sdb9[1]
         80091904 blocks [2/2] [UU]
   
   md3 : active raid1 sda7[0] sdb7[1]
         1951744 blocks [2/2] [UU]
   
   md0 : active raid1 sda1[0] sdb1[1]
         272960 blocks [2/2] [UU]
   
   unused devices: