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Western Digital Purple Pro 22TB Hard Drive

WD Purple Pro 22TB Surveillance hard drive

re: Installing an 18TB Hard Drive into the OWC Mercury Elite Pro
re: Formatting a new Hard Drive or SSD in Apple Disk Utility
re: Western Digital 18TB Ultrastar DC HC550 Hard Drive

Questions on storage, RAID, backup, reliability? Consult with MPG Lloyd.

Seagate Iron Wolf Pro

This page evaluates the WD 22TB Purple Internal Surveillance Hard Drive (WEWD221PURP) for speed across the entire volume capacity as well as speeds for 32K to 512MB transfers.

Previously, MPG evaluated the Seagate Iron Wolf 20TB hard drive with excellent results. This review compares the relative performance and shows a very substantial speed difference as capacity utilization increases.

The OWC Thunderbay 6 enclosure was used to house the drive. Its Thunderbolt 3 interface offers 6 bays with 3 SATA channels. Similarly, the OWC Thunderbay 8 and OWC Thunderbay 4 would be equivalent test setups.

CLICK TO VIEW: Recommended Storage at B&H Photo


Choosing a drive capacity

Hard drives slow down as they fill up. A good way to evaluate hard drive is by its average speed across the capacity that will be used, as well as how it performs at around 80% capacity usage, plus its consistency across the capacity.

When purchasing hard drives, MPG recommends capacities that will keep actual storage used at no more than 80% of capacity, so as to maintain reasonably high speed. For example, anticipation of up to 16TB of storage needs suggests a 20TB hard drive (16TB is 80% of 20TB).


The 22TB Purple Pro 7200 rpm SATA III 3.5" Internal Surveillance Hard Drive from WD is designed for high-end AI-enabled recorders, video analytics appliances, deep-learning servers and cloud solutions that can benefit from high-capacity storage with high performance and additional workload capability. With support for up to 64 high-definition surveillance cameras, this 8TB hard drive features AllFrame AI technology, which helps reduce frame loss, improve video playback, enables the recording of multiple streams per camera, and supports up to 32 AI streams per drive for deep learning analytics within the system.

A SATA III 6 Gb/s interface, sustained data transfer speeds of up to 265 MB/s, and a 512MB cache all help to ensure uninterrupted data transfers. Furthermore, the drive's 600,000 load/unload cycles, annualized workload rating of 550TB per year, MTBF rating of 2.5 million hours, and <1 in 1015 non-recoverable read errors per bit read also help to ensure a strong operational life with enterprise-class performance, data integrity, and drive reliability, especially in larger-scale systems and enclosures where operational vibration is present.

  • 22TB Storage Capacity
  • 3.5" Form Factor
  • SATA III 6 Gb/s Interface
  • 512MB Cache
  • 7200 rpm
  • Up to 265 MB/s Sustained Data Transfers
  • 600,000 Load/Unload Cycles
  • 550TB per Year Workload Rating
  • Designed for 24/7 Surveillance
  • Supports up to 64 HD Cameras

About hard drive performance

Hard drives slow down as they fill up. The outer tracks deliver more data per rotation than more inward inner tracks due to constant data density, but a much greater circumference in the outer tracks—more data per revolution of the platters. Thus performance drops steadily as the slower and slower inner tracks are used. This is true of ALL hard drives, but not SSDs.

Use the fill-volume command of diglloydTools DiskTester to determine performance across drive capacity for hard drives or SSDs, single drives or RAID of any kind.

TIP #1: to sustain higher performance for same capacity utilization, buy larger hard drives.

Creating a volume using less than the full capacity bounds that volume’s storage area to the faster tracks (first volume created gets the outer and fastest tracks, next volume gets the next-fastest tracks, etc).

TIP #2: to guarantee minimum performance for a fixed-size capacity, the first partition/volume will be the fastest. Use less than the full capacity to guarantee a bounded minimum speed e.g., a 12TB partition on a 16TB total capacity. The extra space can be ignored, or used for a 2nd much slower volume.

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Test results: Speed across 22TB capacity

Test mule was the 2019 iMac 5K with the OWC Thunderbay 8 enclosure.

The fill-volume command of diglloydTools Disktester was used, with 128MB transfer size.

For over 13 years, DiskTester has been the best means available to ascertain how a drive performs across its capacity. When working with RAID, graphing each hard drive individually can reveal performance irregularities that can cause hiccups in RAID performance when multiple drives are used—problem drives can be swapped out for more consistent ones. But here, only a single drive was available for testing.

  • On the fastest part of the volume, speed is very fast for a hard drive, around 275 MB/sec.
  • At 50% capacity speed is down to about 230 MB/sec—still very fast for a hard drive.
  • At 80%, about 175MB/sec—superb, since that is ~16TB of capacity usage.
  • Past 80% capacity, speed falls off to ~123 MB/sec at the very limits of the capacity.
  • At the 18TB mark, substantially faster than 18TB hard drives (~150MB/sec vs ~127MB/sec).


The Western Digital 18TB Ultrastar is nominally faster over the capacity by about ~7MB/sec. But that has no significance to real usage, and the Seagate 20TB has 2TB more capacity. Moreover, at the 18TB mark, the Seagate 20TB is still doing ~150MB/sec, about 18% faster than the Western Digital 18TB near its capacity.

Average Write speed:  222.4 MiB/sec = 233.2 MB/sec
Average Read speed:   223.8 MiB/sec = 234.7 MB/sec

Vertical scale is MB/sec. Horizontal scale is percentage of volume capacity eg 50% of 20TB volume is 10TB.

22TB WD Purple Pro hard drive: speed across 22TB volume capacity

vs Seagate Iron Wolf 20TB

Below, the WD Purple 22TB is compared against the Seagate Iron Wolf 20TB hard drive. Both drives were filled with 20TB of data.

The increased capacity of the WD Purple Pro 22TB results in up to 30% faster performance as the capacity utilization approaches 20TB. That’s a huge difference in real performanceUsed as a single drive and to 20TB of capacity utilization, the WD Purple 22TB is a clearly superior performer.

In simple terms, if the goal is the fastest 20TB of storage capacity, the WD Purple Pro offers clearly superior performance to the Seagate Iron Wolf 20TB, a principle that applies in general when comparing any higher-capacity hard drive to a lower-capacity hard drive.

22TB WD Purple Pro hard drive: speed across 20TB volume capacity, vs Seagate Iron Wolf 20TB

30% difference in throughput as capacity utilization increases

The graph below shows the performance difference in relative performance of the WD Purple Pro 22TB vs the Seagate Iron Wolf 20TB, for capacity utilization from 0...20TB.

By the time 20TB capacity is used (Seagate full), the WD Purple Pro is 30% faster eg ~159 MiB/sec vs 118 MiB/sec for the Seagate 20TB. At just 10TB full, the difference is only about 8% but the speed differential climbs rapidly after that.

22TB WD Purple Pro hard drive: speed across 20TB volume capacity, vs Seagate Iron Wolf 20TB

Test results: speed vs transfer size

Test mule was the 2019 iMac 5K with the OWC Thunderbay 8 enclosure.

The run-sequential-suite command of diglloydTools Disktester was used for this data.
disktester run-sequential-suite

Transfer performance for 32K and 64K transfers is noticeably inferior to the Seagate Iron Wolf 20TB for 32K, 64K, 128K transfers. At 256K and larger transfers, the WD Purple is 6% or so faster. Depending on usage (I/O transfer sizes), which drive is better will vary, but off hand it looks like the Seagate would be a better general-purpose performer.

22TB WD Purple Pro hard drive hard drive: speed vs transfer size


Questions on storage, RAID, backup, reliability? Consult with MPG Lloyd.

The WD 22TB Purple Internal Surveillance Hard Drive is an outstanding choice for ultra high capacity storage as a single drive. Its higher capacity means higher performance than any 16/18/20TB hard drive for large transfers, up to 30% faster as capacity utilization approaches 20TB.

For workloads of small transfers (32K to 128K), the Seagate Iron Wolf 20TB may run faster. But such workloads are a poor choice for hard drives; an SSD should be used instead.

Speed differences for single drives being what they are, a dual drive setup will eclipse either. For example, a RAID-0 stripe of 2 X 20TB will be far speedier and with 40TB. Installed in something like the OWC Thunderbay 4 with only a modest price difference at the time this was written. For example, 2 X 20TB for $660 or 2 X 22TB for $1000 or 1 X 22TB for $500—the value proposition strongly favors a 2 X 20TB setup.

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