Question: I have two different computers with different RAID5 arrays. When I run the command 

```
mdadm --detail /dev/md0

```

one drive reports "active" while the other reports "clean" for the "state" field. What is the difference? Should I worry about either state? Both seem to work fine.

Answer: 
From [the RAID arrays documentation](https://www.kernel.org/doc/html/latest/admin-guide/md.html) in the Linux kernel:

> 
> 
> ```
> clean - no pending writes, but otherwise active.
>     When written to inactive array, starts without resync
>     If a write request arrives then
>       if metadata is known, mark 'dirty' and switch to 'active'.
>       if not known, block and switch to write-pending
>     If written to an active array that has pending writes, then fails.
> active
>     fully active: IO and resync can be happening.
>     When written to inactive array, starts with resync
> 
> ```
> 
> 

So, no, you don't need to worry about either state; both are normal operation. If you saw `inactive`, that you'd have to worry about.

`mdadm` includes a raid monitor daemon that will alert you (via email by default) of any conditions you need to be aware of, in particular a failed disk. You should make sure it is configured & running.
The metadevice (md0) should report `clean` and the physical disks (f.e. /dev/sda2 and /dev/sdb2 for a simple mirror) should be `active sync`. Then all is fine.