root@bitcorn:/mnt/md0# fsck -fCV /dev/md0p1
fsck from util-linux 2.31.1
[/sbin/fsck.ext2 (1) -- /dev/md0p1] fsck.ext2 -f -C0 /dev/md0p1
e2fsck 1.44.1 (24-Mar-2018)
ext2fs_open2: Bad magic number in super-block
fsck.ext2: Superblock invalid, trying backup blocks...
fsck.ext2: Bad magic number in super-block while trying to open /dev/md0p1

The superblock could not be read or does not describe a valid ext2/ext3/ext4
filesystem.  If the device is valid and it really contains an ext2/ext3/ext4
filesystem (and not swap or ufs or something else), then the superblock
is corrupt, and you might try running e2fsck with an alternate superblock:
    e2fsck -b 8193
 or
    e2fsck -b 32768

root@bitcorn:/mnt/md0# dmesg | grep md0
[  603.734416] md: md0 stopped.
[  603.752297] md/raid:md0: device sdb operational as raid disk 0
[  603.752298] md/raid:md0: device sdd operational as raid disk 2
[  603.752298] md/raid:md0: device sdc operational as raid disk 1
[  603.752538] md/raid:md0: raid level 5 active with 3 out of 3 devices, algorithm 2
[  603.766207] md0: detected capacity change from 0 to 8001302822912
[  603.771707]  md0: p1 p2
[  603.794854] EXT4-fs (md0): VFS: Can't find ext4 filesystem
[27284.994660]  md0: p1 p2
[75034.851792]  md0: p1 p2
[75312.365653]  md0: p1 p2
[76336.491130]  md0: p1 p2
[76355.758690]  md0: p1 p2