A volume fills up quietly until it doesn’t. A log partition, a database’s data files, or a VMware datastore runs out of room, and whatever depends on it stops writing, stops responding, or fails outright, often with no warning beyond the outage itself. Nagios XI checks disk usage, mountpoint availability, and virtualization datastores against thresholds you set, so a volume crossing 90% full triggers a Warning or Critical alert well before it reaches zero.

Trend forecasting for storage growth, covered under Capacity Trend Forecasting below, requires a Nagios XI Enterprise Edition license.

Storage monitoring checks how much space a disk, mountpoint, or datastore has used against a threshold, and alerts when that threshold is crossed. It covers the volumes attached to a host directly, filesystems mounted from elsewhere, and the datastores backing virtual machines, and it depends on whichever mechanism, an installed agent, SNMP, or a hypervisor API, can report a value back to the monitoring server.

A full disk rarely announces itself until an application already depends on it.

• Prevent Application Outages: A database, log service, or file share that runs out of space usually stops accepting writes before anyone notices the disk itself.
• Plan Storage Purchases in Advance: Trend data on how fast a volume is filling gives lead time for a planned order instead of an emergency one.
• Catch Mount Failures Early: A share that silently unmounts can leave a host writing to the wrong local directory for days before anyone checks.
• Extend Visibility to Devices Without an Agent: SNMP reaches storage devices and appliances that can’t run a monitoring agent directly.

Disk Space Monitoring for Linux and Windows Hosts

Nagios XI ships with the check_disk plugin, though the default check only covers the root (/) filesystem, so each additional volume needs its own check. The Windows Server, Windows Desktop, and Linux Server wizards use the Nagios Cross-Platform Agent (NCPA) to add these checks with a Warning and Critical threshold per volume.

Documentation: Nagios XI 2024 And 2026 Best Practices | How To Monitor Windows Using NCPA In Nagios XI 2024 And 2026

Agentless Storage Monitoring Over SNMP

The check_snmp_storage.pl plugin reads a device’s storage MIB over SNMP, covering Linux, AIX, and other SNMP-enabled devices without installing an agent. The Linux SNMP Wizard handles device discovery and threshold setup, and SNMPv3 authentication and privacy passphrases are a more secure option than a plain-text SNMPv2c community string. This is the path for NAS appliances and network devices with no room for an agent; for host-level partitions you can reach directly, host disk space monitoring covers that in more depth.

Documentation: How To Monitor Linux Using SNMP In Nagios XI 2024 And 2026

Mountpoint and Filesystem Availability

The Mountpoint Wizard checks whether mountpoints on the Nagios XI server itself exist and are properly implemented, with an optional write test to confirm a mount accepts writes rather than just appearing in the list.

Virtualized Storage: VMware Datastore Usage

The VMware Wizard’s host monitoring mode includes Datastore Usage alongside other host metrics, with a Datastores tab to pick specific datastores. It connects to an ESX/ESXi host or a vCenter Server, and vCenter credentials cover every VM in the environment without re-running the wizard per host. The Nagios XI server needs the VMware Perl SDK installed first. For VMware monitoring beyond storage, the same wizard covers CPU, memory, and networking.

Documentation: How To Monitor VMware In Nagios XI 2024 And 2026

Capacity Trend Forecasting

Capacity Planning, under the Reports menu, projects future usage from historical performance data using Holt-Winters or polynomial extrapolation, and can alert a set number of days before a chosen value is projected to be exceeded. It requires a Nagios XI Enterprise Edition license.

Documentation: How To Use Capacity Planning In Nagios XI 2024 And 2026

Thresholds and Alerting

Every storage check compares a reported value, percent used or bytes free, against a Warning and Critical threshold you set per volume or datastore. Crossing either threshold triggers a notification through the contacts and escalations already configured for that host or service.

Security and Access to Storage Checks

User rights control who can see storage checks, reconfigure them, or run the wizards that create them, down to a read-only role for users who only need visibility. SNMPv3 authentication and privacy passphrases also keep storage-device credentials off the wire, unlike a plain community string.

Documentation: How To Understand User Rights In Nagios XI 2024 And 2026

RAID and Drive Health

Hardware RAID and S.M.A.R.T. drive health aren’t covered by plugins bundled with Nagios XI. Community plugins on Nagios Exchange, such as check_raid and a smartmontools-based check, add this, though they need separate installation and usually sudo or root access. S.M.A.R.T. catches some drive degradation before failure, but many drives fail without warning, so it’s worth pairing with RAID status and backups.

Fewer Emergency Storage Purchases: Capacity trend data flags a volume’s growth curve early enough to plan a purchase instead of reacting to one.

One Alert Path for Physical and Virtual Storage: Disk, mountpoint, and datastore checks land in the same contacts and escalations as the rest of a host’s checks, so a storage alert shows up next to the server monitoring already running for that machine instead of in a separate console.

Faster Root Cause on a Full Volume: An application failure traces back to a Critical disk check instead of a support ticket opened days later.

Coverage for Devices That Can’t Run an Agent: SNMP extends the same Warning and Critical logic to appliances and network storage that have no room for an installed agent.

• Network: Inbound UDP 161 for SNMP checks; NCPA listens on TCP 5693 between the Nagios XI server and each monitored host; VMware checks need HTTPS (443) access from the Nagios XI server to the ESXi host or vCenter Server.
• Accounts: A read-only SNMP community string or an SNMPv3 user for agentless checks; a VMware account scoped to read host and datastore statistics; an NCPA token configured on the agent and matched in the wizard.
• Software: NCPA installed on each Windows or Linux host checked directly; the SNMP daemon installed, configured, and enabled to start on boot on any device queried over SNMP; the VMware Perl SDK installed on the Nagios XI server before running the VMware Wizard.
• Licensing: A Nagios XI Enterprise Edition license for Capacity Planning’s trend forecasting.

1. Install the agent or protocol the target needs. Add NCPA to a Windows or Linux host, or enable and configure SNMP on a device that can’t run an agent.
2. Navigate to Configure > Configuration Wizards and search for the wizard that matches the target: Windows Server, Windows Desktop, Linux Server, SNMP Wizard, or VMware Wizard.
3. Enter the connection details the wizard asks for, an address and NCPA token, SNMP community string or v3 credentials, or a VMware host and account, and let it scan for available disks or datastores.
4. Select the volumes or datastores to monitor and set a Warning and Critical threshold for each one.
5. Complete the remaining wizard steps and click Finish & Apply to create the host and services and begin monitoring.

Documentation: Understanding And Using Configuration Wizards In Nagios XI 2024 And 2026 | How To Monitor Windows Using NCPA In Nagios XI 2024 And 2026 | How To Monitor VMware In Nagios XI 2024 And 2026

Nagios XI reads back whatever value the operating system, SNMP agent, or hypervisor already reports, disk percent used, mountpoint status, datastore free space, and compares it to the thresholds you set. It doesn’t manage a RAID controller, provision new storage, or replace an array’s own health telemetry; where an array exposes that data over SNMP or a plugin, Nagios XI can alert on it, but managing the array itself stays with the array’s own management interface.

For NAS and SAN arrays with a management console of their own, NAS and SAN monitoring covers the array-level checks that sit alongside what’s described here.

Deeper Disk and Drive Monitoring

• Disk Monitoring
• NAS and SAN Monitoring

Related Infrastructure

• Server Monitoring
• VMware Monitoring
• Windows Monitoring

Storage problems are easier to solve with a week of lead time than in the middle of an outage. Nagios XI puts disk usage, mountpoints, and VMware datastores next to the rest of the infrastructure you’re already watching, with the same Warning and Critical alerting.