SNMP server not reporting disk size

Thursday, February 29, 2024

I was setting up snmpd on an Ubuntu box, and noticed that it was reporting weird numbers for a couple of XFS filesystems I had setup for Minio.

An snmpwalk showed values like this:

HOST-RESOURCES-MIB::hrStorageType.51 = OID: HOST-RESOURCES-TYPES::hrStorageFixedDisk
HOST-RESOURCES-MIB::hrStorageType.52 = OID: HOST-RESOURCES-TYPES::hrStorageFixedDisk
...
HOST-RESOURCES-MIB::hrStorageDescr.51 = STRING: /minio/disk1
HOST-RESOURCES-MIB::hrStorageDescr.52 = STRING: /minio/disk2
...
HOST-RESOURCES-MIB::hrStorageAllocationUnits.51 = INTEGER: 0 Bytes
HOST-RESOURCES-MIB::hrStorageAllocationUnits.52 = INTEGER: 0 Bytes
...
HOST-RESOURCES-MIB::hrStorageSize.51 = INTEGER: 0
HOST-RESOURCES-MIB::hrStorageSize.52 = INTEGER: 0
...
HOST-RESOURCES-MIB::hrStorageUsed.51 = INTEGER: 0
HOST-RESOURCES-MIB::hrStorageUsed.52 = INTEGER: 0

So it was reporting the existence of the disks, but with all 0 values. The root ext4 filesystem showed up fine, was it something to do with XFS?

Turns out the answer was NO, it was the permissions of the /minio directory that the filesystems were mounted under. I figured this out when I noticed that df -h showed the disks when I was running as root

/dev/sdb1        60G  461M   60G   1% /minio/disk1
/dev/sdc1        60G  461M   60G   1% /minio/disk2

But when running as non-root, such as the Debian-snmp user, df -h didn't show the disks at all.

Turns out I had been too strict with the permissions on /minio, I had originally set that to

drwx------   4 minio-user root       4096 Feb 14 14:57 minio

But that apparently, and to my surprise, prevented snmpd from being able to read the size and usage information for the mounts under that directory. Changing that to 0755 fixed the problem, and I just made sure that the mountpoints had more strict permissions

drwxr-x--- 3 minio-user root 24 Feb 27 13:08 disk1
drwxr-x--- 3 minio-user root 24 Feb 27 13:08 disk2