I wonder about acronyms in specification of Seagate Exos 7E8 8TB SAS (ST8000NM0075) and Seagate Exos X10 10TB SAS (ST10000NM0096) disks. In Performance part I can see:

  • Random Read/Write 4K QD16 WCD (IOPS) - Can you please tell me what's "4K QD16 WCD"?
  • Max. Sustained Transfer Rate OD (MB/s, MiB/s) - Can you please tell me what's "OD"?


3 Answers 3


The results of IOPS performance tests depend for a large part on the settings used in the benchmarking tool. For any meaningful comparison with other drives (from the same manufacturer or not) you need to know what parameters were used in the test.

In addition to shodanshok's anwser :

4K = 4 kilobyte = the size of the read/write IO requests used in the test.
Aligning the IO request size with the native sector size used on a drive results in optimal (test) performance.

QD16 = the Queue Depth used in the test.
By using the native command queuing a disk can optimize the order in which read/write commands are executed on the actual permanent storage. A high queue number is mainly of interest with SSD's as with their parallel channels maximizing the utilization of each channel requires sufficiently high queue depths, but even with spinning rust you see performance improvements by allowing the disk controller to optimize the order of requests.

  • QD stands for "Queue Depth": it describe how many concurrent operation were issued to disk;

  • 4K describes the size of the outstanding requests. In this case, it means 4 KB sized requests;

  • WCD stands for "Write Cache Disabled": it is a method to disable the disk DRAM writeback cache for enhanced data safety. Note that if WCD is set, HDDs without a NVC (non volatile cache) will have much lower random write performance. On the other hand, HDDs with NVC often have similar performance to a fully enabled writeback cache. Give a look here for more details. As a side note, this probably is the reason behind the significant random write performance difference between hyperscaler vs standard Exos model;

  • OD stands for "Outer Disk": as modern HDDs have a constant angular velocity, the outer disk tracks read/write faster than the inner one.

  • 4K QD16 WCD means 4-kilobytes sector's size Queue Depth 16 commands Write Cache Disabled.

  • OD stands for Outer Disk tracks as described in the second answer.

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