LVM Snapshot Size Calculator
Size an LVM snapshot volume based on expected change rate and snapshot lifetime.
About LVM Snapshot Size Calculator
A classic (non-thin) LVM snapshot only needs to store copy-on-write blocks — the original contents of any block on the source LV that gets overwritten after the snapshot was taken — not a full copy of the source. Its required size therefore scales with how much data changes during the time the snapshot is kept, not with the source LV's total size. Undersizing it is dangerous: if a COW snapshot fills up, LVM automatically drops it and it becomes permanently invalid, which is why a safety margin above the raw estimate is standard practice.
Formula
Snapshot space needed
snapshot_space = source_LV_size × daily_change_rate% × days_kept × (1 + safety_margin%)
- L
- source LV size
- r
- daily change rate fraction
- d
- days the snapshot is retained
- m
- safety margin fraction
Worked example
With the default values below, LVM Snapshot Size Calculator returns Recommended Snapshot Size (GB) of 22.50. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Source LV Size (GB)minimum 0
- 200
- Daily Change Rate (%)accepts 0–100
- 3
- Days Snapshot Will Be Keptminimum 0.1
- 3
- Safety Margin (%)accepts 0–200
- 25
Results
- Recommended Snapshot Size (GB)
- 22.50
- Overflow Risk
- Moderate — monitor snapshot usage during the retention window.
- Base Estimate (no margin)
- 18.00
- Snapshot Size as % of Source LV
- 11.25%
Step by step
- 1Values used
- 2Snapshot space needed
- 3Recommended Snapshot Size (GB)= 22.50
- 4Overflow Risk= Moderate — monitor snapshot usage during the retention window.
- 5Base Estimate (no margin)= 18.00
- 6Snapshot Size as % of Source LV= 11.25
Assumptions and limits
- The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
What happens if an LVM snapshot runs out of space?
The kernel automatically invalidates (drops) the snapshot once its copy-on-write space is exhausted, and it becomes permanently unusable — you cannot resume or top it up after the fact. This is why over-provisioning with a safety margin matters more for snapshots than most other storage sizing decisions.
Can I resize an LVM snapshot after creating it?
Yes, with lvextend on the snapshot volume itself, and it's safe to do proactively if you notice usage climbing faster than expected (via lvs -o snap_percent). Extending before it fills is far preferable to losing the snapshot.
Does a snapshot slow down writes to the source LV?
Yes, somewhat — every write to the source LV after a snapshot exists must first copy the original block to the snapshot's COW area before the new data is written, adding overhead proportional to the number of active snapshots. This is one reason snapshots are typically kept short-lived (hours to a few days) rather than as long-term backups.