ZFS Compression versus Dedup: Space Savings and Operational Cost

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.

Contention occurs when workloads sharing CPU, memory, storage or a link make each other wait. Apparent spare total capacity can hide a hot core or single-queue bottleneck. Correlate backup jobs, antivirus scans, index maintenance and user traffic on a common timeline. Confirm the bottleneck before adding resources. Run a workload alone and with its usual competitors to separate shared-resource effects, and report peak-hour latency alongside average utilization.

Link capacity differs from useful application throughput. Protocol headers, encryption, retransmissions, small files and storage waits reduce net transfer speed. Keep bits and bytes distinct: 1 Gbit/s corresponds to a theoretical 125 MB/s, not an application performance guarantee. Estimate transfer time as data size divided by measured useful throughput. Observe the network, source reads and destination writes together to locate the bottleneck. Consider temporary slowdowns and competing workloads as well as average speed.

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.

SSD selection requires more than capacity and initial performance. Consider TBW, DWPD, warranty duration, power-loss protection and workload write patterns together. DWPD calculations must use the capacity and conditions defined by the manufacturer. Database logs, small random writes and temporary work files affect wear differently. Monitor SMART or vendor health data, considering temperature, media errors and remaining endurance together. Write amplification means application-written bytes may differ from the amount written to NAND.

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.