Low Disk Space - Thresholds

The Low Disk Space alert monitors available disk space on volumes to
prevent system crashes and database outages. It evaluates two thresholds
together: minimum free space (MB) and minimum free percentage (%).

An alert fires only when both conditions are breached at the same
time. If only one threshold is met, no alert is sent.

Important: Because both thresholds must be met simultaneously, one
threshold will always be the controlling factor — and which one
controls depends on the size of the drive. Understanding this helps
you set meaningful thresholds for your environment.

How the AND Logic Works

Think of the two thresholds as a gate with two locks. Both locks must
open before the alert fires.

The threshold that requires more free space at a given drive size is the
one that controls when the alert fires. The other threshold becomes
irrelevant until the controlling one is already met.

Default Tolerances

WISdom includes three built-in tolerance levels. Each can be modified,
and custom tolerances can be added to fit your environment.

Tip: Lower tolerance means a more relaxed alert — it takes a more
critical condition to trigger. Higher tolerance means the alert fires
sooner, giving you more advance warning.

Crossover drive size is the point at which both thresholds align
exactly. Above this size, the MB threshold controls when the alert fires.
Below it, the percentage threshold controls it.

Examples

Small Drive — 100 GB (Low Tolerance)

With Low tolerance defaults (50 GB and 20%):

• 20% of 100 GB = 20 GB free
• At 20 GB free, the MB threshold (50 GB) is already met — both
  conditions are true
• The alert fires at 20 GB free (20%)
• The percentage is the controlling threshold

On small drives, free space in MB drops below 50 GB well before the
drive reaches 20% free, so the percentage threshold is always the last
to be met and controls when the alert fires.

Mid-Size Drive — 500 GB (Low Tolerance)

With Low tolerance defaults (50 GB and 20%):

• 20% of 500 GB = 100 GB free
• At 100 GB free, the MB threshold (50 GB) is not yet met — only one
  condition is true, so no alert fires
• The alert fires when free space drops to 50 GB (~10% free) —
  both conditions are finally met
• The MB threshold is the controlling threshold

This drive is above the 250 GB crossover point, so the MB threshold
takes control. The alert fires at ~10% free rather than 20%.

Large Drive — 2 TB (Low Tolerance)

With Low tolerance defaults (50 GB and 20%):

• 20% of 2 TB = ~400 GB free
• At 400 GB free, the MB threshold (50 GB) is not yet met — no alert fires
• The alert fires when free space drops to 50 GB (~2.4% free)
• The MB threshold is the controlling threshold

On a 2 TB drive, the default Low tolerance alert won't fire until the
drive is over 97% full. For drives this size, consider raising the MB
threshold significantly. See
Recommendations by Drive Size.

Very Large Drive — 10 TB (Low Tolerance)

With Low tolerance defaults (50 GB and 20%):

• 20% of 10 TB = ~2 TB free
• The alert fires when free space drops to 50 GB (~0.5% free)

At 0.5% free, the drive is essentially full before the alert fires. The
default thresholds are not designed for drives this size. A custom
tolerance with a much higher MB threshold is strongly recommended.

Recommendations by Drive Size

Monitoring Only One Threshold

Neither threshold field can be fully disabled, but you can make one
threshold irrelevant by setting it to a value that can never be breached.

To monitor only free percentage (effectively disable the MB
threshold): Set the MB threshold to a value larger than any disk in your
environment. For example, 10,485,760 MB (10 TB) ensures the MB condition
is always true and never becomes the controlling factor.

To monitor only free space in MB (effectively disable the percentage
threshold): Set the percentage to 100%. A volume can never have more
than 100% free space, so this condition is always true — leaving the MB
threshold as the sole controlling factor.

Note: Both fields are required. These are workarounds for
environments where one dimension of disk monitoring is not meaningful
for your needs.

Deep Dive: Calculating the Controlling Threshold

For environments with a wide range of drive sizes, it helps to calculate
exactly when an alert will fire before deploying a tolerance.

Crossover point formula:

Crossover (GB) = MB Threshold (GB) ÷ (% Threshold ÷ 100)

Using Low tolerance defaults (50 GB, 20%):

50 ÷ 0.20 = 250 GB crossover

• Drive size < crossover: % controls; alert fires at
  Drive Size × (% Threshold ÷ 100)
• Drive size > crossover: MB controls; alert fires at the
  MB Threshold value regardless of drive size

Low Tolerance — Alert Fire Points by Drive Size

(50 GB MB threshold · 20% percentage threshold · ~250 GB crossover)

Med Tolerance — Alert Fire Points by Drive Size

(50 GB MB threshold · 5% percentage threshold · ~1 TB crossover)

Note: Med tolerance holds the percentage threshold at 5%, so the
percentage is the controlling factor on drives up to ~1 TB. Above that,
behavior is identical to Low tolerance — the 50 GB MB threshold takes
over and the alert fires very late on large drives.

High Tolerance — Alert Fire Points by Drive Size

(12.5 GB MB threshold · 20% percentage threshold · ~62.5 GB crossover)

Note: High tolerance has the smallest MB threshold (12.5 GB) and
the lowest crossover point (~62.5 GB). This makes it the most
aggressive for small drives but the least useful for large ones —
12.5 GB free on a 2 TB drive is ~0.6% free. If your environment
includes drives larger than 250 GB, High tolerance alone is not
sufficient. Pair it with a custom tolerance sized for your larger
volumes.

Tip: For environments with a wide mix of drive sizes, consider
creating separate custom tolerances scoped to specific volume types or
size ranges rather than applying a single global threshold across all
volumes.

Related Topics

• Alert Rules
• Custom Alerts
• Notification Channels
• Profiles — Alerts