Cache Size, RPM, and Sequential Speed Demystified

RPM: Rotational Speed

RPM (Revolutions Per Minute) is how many times the platters spin per second. Most desktop and NAS hard drives spin at either 5,400 RPM or 7,200 RPM. Enterprise drives also come in 10,000 RPM and 15,000 RPM variants (SAS only).

RPM directly affects two things: rotational latency (how long the drive waits for the right data to spin under the read head) and, to a lesser degree, sequential throughput. A 7,200 RPM drive has roughly 4ms average rotational latency vs 5.5ms for a 5,400 RPM drive — a difference that matters in random I/O workloads but is negligible for sequential bulk transfers.

Cache: The Drive's DRAM Buffer

Every hard drive has a small amount of DRAM on the controller board used as a read/write cache. Typical sizes are 64MB, 128MB, 256MB, or 512MB. For NAS drives, 256MB is now standard at 8TB+ capacities.

The cache serves two functions: it holds recently read data for quick re-access (read caching), and it allows the drive to acknowledge writes to the host before those writes hit the platters (write buffering). Write buffering significantly improves burst write performance — but creates a brief window where data exists only in volatile DRAM, not on the magnetic medium.

For most home NAS workloads, the difference between 64MB and 256MB cache is negligible. The cache empties between file operations and rarely becomes a bottleneck unless the drive is being hammered with thousands of small random writes per second — a workload that SSDs handle better anyway.

Sequential Speed: What the Marketing Number Means

Sequential throughput — typically 200–280 MB/s for modern 7,200 RPM drives — is the speed at which the drive can read or write a large contiguous file in a single pass. This is the best-case scenario for a hard drive and the number manufacturers advertise.

Sequential speed is primarily determined by areal density (how densely data is packed on the platter) and RPM. A higher-density platter passes more data under the read head per revolution, which is why modern 8TB drives are actually faster sequentially than older 4TB drives despite using similar RPM.

What Actually Limits Real-World Performance

For the vast majority of home NAS use cases — Plex streaming, file backup, photo archiving — sequential speed is not the bottleneck. The real bottleneck is almost always the network connection or the random I/O characteristics of the workload.

Summary: What to Actually Look At