Raid started with 4 out of 5 drives

Hello.

My raid went suddently missing.. I googled some commands regarding a missing raid and I managed to start the raid again with mdadm --assemble /dev/md0. But..

mdadm: /dev/md0 has been started with 4 drives (out of 5).

My sdf drives wont start. mdadm: no RAID superblock on /dev/sdf

SMART says nothing is wrong with the drive. How can I get superblocks on again and what is it?

Thanks in advance.

Okey, I found out that I could "Recover" in OMV GUI under Raid management. Now it's clean again and running with 5 drives. But I keep getting emails from the NAS about the raid and my NAS is running slow. The connection is falling out sometimes and my Plex is lagging.

How can I fix that?

"A SparesMissing event had been detected on md device /dev/md0."

Output from mdadm.conf file:

Is the definition correct or why do I keep getting emails from server?

Quote from henfri

Do you have a spare?
Do you know, what this means?

A short search revealed:
Remove the "SparesMissing event"

Thanks for your answer. I guess I should have searched better before creating last post.

I guess a spare means another HDD ready to kick-in, in case of another HDD failure? I do not have this, so I changed the spare=1 to 0.

I guess the problem is fixed now. But my performance issue for the server in general is likely not.

Quote from henfri

What kind of raid was it by the way?
How many devices?

It is a raid5 with 5 HDD. Suddently one day one drive was missing, but I could just recover in OMV GUI with the same drive. I guess thats why the spare=1 appeared.

Quote from henfri

Correct for the spare.
For the performance issue, you should search for guides.
Isolate the problem. CPU (top), IO (iotop or copy a file on the command line), network (iperf), ...

Yes I did see it. I will search for guides.

Long story short. The server used to run without problems, but suddenly when I put "pressure" on the nas by adding torrents and moving files, I lose connection to the GUI and bittorrent for a while. But my plex server keeps running without problems even that I cannot log in or change page. I did some updates in the GUI a while ago. Other than updating and fixing the raid, I havent done anything else..? I also have added more RAM to the motherboard because I only had 3GB. But that was already when the problem was here.