Workload-based Unraid cache capacity planning
Unraid Cache Pool Size Calculator: NVMe and SSD Capacity
A cache pool should be large enough for everything that must remain fast plus every write that can accumulate before Mover succeeds. This calculator separates resident appdata and VM storage from temporary downloads and ingestion, then adds free-space and growth margin so the pool is not designed around an average day.
Quick answer
Size for the peak interval, not average daily traffic
Add permanent pool-resident data to the largest amount that can arrive between Mover runs, then reserve at least the largest expected file plus operating margin. If the pool contains important data that has not reached another backup, choose a redundant multi-device layout rather than treating one SSD as protected storage.
Live Amazon hardware
Current hardware for this Unraid decision
Products come from this sprint's dedicated catalogue. NVMe, SATA SSD, RAM, PCIe NIC, DAC, optical and switch classes stay separate. Conflicting capacities, external SSDs, enclosures, USB NICs, complete computers and obvious wrong product types are rejected.
Buying decision
Capacity, redundancy and endurance should be solved together
A 4TB single NVMe and two 2TB mirrored NVMe devices provide very different usable capacity and failure behavior. The calculator reports a usable-capacity target first, then a device-count direction and daily-write estimate so you can decide whether one large device, a mirror, or separate pools make more sense.
Interactive decision tool
Unraid Cache Pool Size Calculator
Use the inputs to narrow the hardware role before shopping. Results are planning guidance. Verify current Unraid behavior, motherboard lane sharing, exact SSD endurance, memory compatibility, NIC media and independent backups before deployment.
Compatibility checklist
Four checks before buying Unraid performance hardware
Cache and array are different paths
Fast cache can absorb writes at SSD speed, but Mover still has to drain data to the parity array later.
Measure before upgrading the network
2.5GbE or 10GbE only helps when the local pool, array or aggregate client workload can use the bandwidth.
Verify exact SSD and NIC details
NAND, DRAM, TBW, controller, connector and PCIe generation are shown only when the listing supports them.
Redundancy is not backup
Mirrored cache pools and parity protect specific hardware failures. Keep independent backups for valuable appdata, VMs and files.
Start with resident data that Mover should never remove
Appdata, Docker image files, VM disks, databases and working project files are commonly kept on a fast pool. These bytes form the fixed base of the capacity calculation because scheduling Mover does not make them disappear if the share is configured to remain on the pool.
Measure current usage and include snapshots, VM growth and application databases. A 300GB appdata share may be 500GB next year, so reserve a growth percentage instead of buying a cache device that is nearly full on installation day.
Then calculate temporary writes between Mover runs
Shares configured with the pool as Primary storage and the array as Secondary storage can accumulate new files until Mover transfers them. Multiply the expected daily writes by the maximum realistic number of days between successful moves.
If Mover normally runs nightly but a large job can keep files open or the server can be offline for maintenance, a two-day or three-day buffer may be more realistic than exactly 24 hours. The calculator lets the schedule represent the real failure window.
Downloads and unpacking can consume more space than final files
Download clients can temporarily hold incomplete archives, extracted files and the final payload at the same time. Media automation can also copy rather than move when source and destination cross filesystems.
Use peak temporary working space rather than only the final library size. A 100GB game or archive can briefly require substantially more than 100GB of pool space during download, verification and extraction.
Large files need a minimum-free-space reserve
Unraid documents minimum free space as the point where writes should stop using the pool and fall back to secondary storage, and recommends sizing that floor around the largest file expected. A zero value risks filling the filesystem.
The calculator adds a reserve independently from growth. If the largest regular file is 80GB, a 20GB leftover margin is not enough even if total average capacity appears adequate.
Mirrors trade raw capacity for availability
Two equal devices in a mirrored BTRFS or ZFS layout normally provide usable capacity roughly equal to one device while allowing one device failure without losing the pool. That is a major distinction from two drives used for striping or independent pools.
Choose redundancy from data importance, not from the desire to use every purchased terabyte. If important appdata and VMs reside on cache, a mirror plus backup is usually easier to recover from than a single very large device.
Separate pools can reduce overbuying
A server may need a large disposable download pool but only a smaller protected application pool. Unraid supports multiple named pools, so these workloads do not have to share the same redundancy and endurance level.
This can lower cost because high-endurance mirrored NVMe is reserved for critical data while a larger value SSD handles temporary files. The cost is more slots, cables and management.
File-system choice affects usable capacity and expansion
BTRFS and ZFS both support multi-device pools but have different allocation profiles and expansion rules. A capacity estimate should therefore be interpreted alongside the intended filesystem and redundancy layout rather than as a promise that every two-drive combination produces the same usable bytes.
Before buying mixed capacities, review how the chosen pool profile handles the smaller device. Usable space can be limited by mirror pairing or vdev geometry even when the raw sum looks large.
Daily writes are the endurance input
The same capacity can experience radically different write volume. A 2TB VM pool with several busy databases may write far more data per day than a 4TB media-staging pool used twice a week.
Track actual host writes when possible. For a new build, estimate downloads, VM writes, backups, transcodes and application churn separately, then compare the resulting multi-year total with verified SSD endurance.
Mover bandwidth determines how quickly headroom returns
A cache pool can ingest data faster than the parity array can absorb it later. If nightly writes regularly exceed what Mover can send to the array during the available window, the pool grows even though the schedule runs successfully.
Estimate the slow downstream path as well as incoming network speed. A 10GbE client can fill a small cache rapidly, while a parity-protected HDD array may drain it much more slowly.
Parity checks and maintenance can compete with Mover
Unraid parity checks read all array drives and can reduce foreground performance because they contend for disk resources. Running Mover during the same window can therefore extend how long cached data remains on the pool.
Schedule heavy maintenance deliberately and keep enough cache capacity to survive overlap. A capacity plan that assumes perfect nightly draining can fail during the exact weeks when the array is busiest.
Free space protects performance as well as reliability
SSDs and copy-on-write filesystems generally behave better when not driven to absolute zero free space. Unraid also notes free-space requirements for pool operations such as BTRFS balance.
The calculator therefore reserves capacity rather than targeting 99 percent utilization. That margin also makes unexpected uploads and temporary VM growth less likely to stop services.
Revisit the calculation after a month of real use
The first sizing pass is a planning model. After deployment, compare actual pool occupancy, peak daily writes, Mover duration, SSD health and growth with the assumptions used here.
If the pool never exceeds 25 percent usage, future expansion can wait. If it routinely approaches the reserve floor or Mover cannot catch up, increase capacity or split workloads before the pool begins causing service interruptions.
Questions people ask
Unraid cache sizing questions
How much Unraid cache do I need for daily downloads?
Multiply peak daily writes by the longest realistic interval between Mover runs, then add resident data, largest-file reserve and growth margin.
Should cache be larger than the biggest file?
Yes. Minimum free space should also leave room for the largest expected file while other resident and temporary data are present.
Does a two-drive mirror double usable cache capacity?
No. A mirror primarily adds redundancy; usable capacity is generally closer to one member device than the raw sum.
Can I use different-size SSDs in an Unraid pool?
Some layouts allow mixed sizes, but usable capacity and expansion behavior depend on BTRFS or ZFS topology. Verify the specific profile before purchasing.
Should Docker appdata count as permanent cache usage?
Yes if the share is intended to remain on the pool. Measure current appdata and leave growth room.
How often should Mover run?
Choose a schedule that fits your workload and downstream array speed. Off-peak operation is common, but capacity should tolerate a delayed or missed run.
Why is my cache filling even though Mover runs?
Open files, incorrect Primary/Secondary storage settings, insufficient array drain speed, large bursts or minimum-free-space behavior can all contribute.
Can I use one pool for downloads and VMs?
Yes, but separate pools can reduce contention and let temporary data use different redundancy or endurance characteristics from critical VMs.
Does 10GbE mean I need a huge cache?
It can increase how quickly data arrives, so burst capacity becomes more important. The array’s Mover drain rate still determines how fast that headroom returns.
Is cache redundancy a backup?
No. Mirroring protects against a device failure, while backup protects against logical loss and broader system failures.
Official references and methodology
Verify the exact Unraid and hardware behavior
The calculator separates resident pool data, temporary writes, Mover interval, minimum-free-space reserve and growth. It then applies redundancy as a capacity factor instead of assuming raw SSD capacity equals usable capacity. It uses Unraid’s current Primary/Secondary storage, Mover and cache-pool model and avoids claiming exact filesystem overhead.
- Unraid Cache Pools
- Unraid Shares and Mover
- Unraid File Systems
- Unraid Array Configuration and Write Modes
As an Amazon Associate, Cloudzat may earn from qualifying purchases. Prices, model revisions, memory compatibility, SSD endurance, NIC firmware and marketplace conditions can change. Verify the exact part before purchase.