TrueNAS and ZFS: Pools, Vdevs, RAIDZ and Dataset Design

Raw capacity is not the capacity available to applications. RAID or erasure coding, filesystems, reserved space, metadata, snapshots and growth headroom are separate deductions. TB and TiB representations also change the displayed number. Write calculations with units, establish protected usable capacity, then subtract operating reserves. Track growth rate as well as current utilization. The projected exhaustion date should leave enough time to procure and deploy additional capacity.

An application directory, mount point, logical volume and physical device belong to different layers. Inode exhaustion, read-only mounts and filesystem errors can stop writes even when space appears available. A snapshot often shares the same storage failure domain and is not an independent backup. Mount options and expansion methods depend on the filesystem. Verify mounts after reboot: writing to an unmounted directory can fill the wrong device.

IOPS measures operations per second, whereas MB/s measures transferred data. Identical IOPS values produce very different bandwidth at different block sizes. For example, 10,000 operations/s at 4 KiB is approximately 39.1 MiB/s. Read/write mix, random versus sequential access and protection mechanisms affect the result. Measure actual block distributions and concurrency rather than transferring a benchmark result directly to production. High disk IOPS does not prove good application response; latency must be examined alongside it.

Latency is the time between starting an operation and receiving its result. An average can hide a small number of very slow operations; medians and p95/p99 percentiles answer different questions. Network RTT, storage waits, processor queues and application processing contribute to end-to-end time. State whether measurements come from the client or server. Check whether increases coincide with traffic growth, maintenance or capacity limits. Record normal and peak-hour baselines before selecting an alert threshold.

Retention defines which recovery points are kept and for how long. Daily, weekly and monthly points do not represent identical change patterns; full-copy creation and chain dependencies affect physical capacity. Retention decisions combine business requirements, applicable obligations and technical capacity. Longer retention does not automatically provide better recovery: the right point must be discoverable and readable. When changing a policy, test whether existing points are deleted immediately or handled differently by the product.

High availability aims to keep service running through specified failures with a short interruption; backup recovers lost or corrupted data from an earlier point. A cluster can replicate an accidental deletion to another node. HA therefore does not replace backup. Consider DNS, identity, network, storage and power dependencies together. Successful node failover is insufficient by itself: measure user sessions, application writes and external integrations after the transition as well.