RAID 0 Performing slow after a while

60 MBps isn’t too bad for HDDs with some I/O contention.

RAID-0 striping? Presumably software RAID-0?

Which macOS?

Downhill with a tailwind and 15K RPM HDDs, you might get 150 to 200 operations per second.

4.2K RPM drives do rather worse.

SSDs routinely do well past 100,000 IOPS, if not a million or more.

Look up the specs for your particular HGST HDDs, and for the enclosure(s?), and see what the theoretical performance might be.

Background: Why is my hard disk drive iMac so slow? - Apple Community

"RAID 0 Performing slow after a while: I have three 4TB HGST spinning HDD's in some generic Amazon enclosures (Maiwo brand) rated at USB 3.1. Whenever I disconnect, and re-mount these drives, they perform up to 300MB/s. Tested using Blackmagic Disk Speed Test. After a while, I notice that when performing another test, their performance can drop to 60MB/s. What might be the cause of this?"

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Lagging Hard Drive RW:



Run First Aid:

Your drive's appear to be overrunning. So, try running First Aid while these are open in Disk Utility.

Generic items from Amazon is almost always trouble. I barely trust I will actually get a name brand item when ordering from Amazon these days. Who knows what that generic enclosure is doing or its quality. If you want compatible & reliable, then stick with respected brands with a long history. Generally the low end & generic items use the cheapest USB chipsets which tend to have more issues.

Hitachi hard drives get really hot, so it is possible those drives are overheating within that enclosure or are causing that enclosure to overheat. Check the temperature of those hard drives are below their max rated temperature and make sure the enclosure is not too hot. Also, make sure to use NAS rated drives or Enterprise grade drives so that they can handle the vibrations, otherwise those vibrations can severely impair & even damage the hard drives.

You should also check the health of those hard drives....maybe one of them is failing. The drive's SMART health report may also provide some clues for other issues as well. You can try using DriveDx (free trial period) to attempt to check the health of those hard drives, but you will need to install a special USB driver. Even with that special USB driver, it is possible the generic enclosure will not allow the necessary communication with the hard drive to access their health information.

Is the enclosure connected directly to the Mac? If not, then connect it directly to the computer. In fact disconnect all other external devices in case one of them is causing a problem. If the drives are connected directly to the Mac, then try connecting them to a USB3 hub to see if that makes a difference. Try using each other USB-C ports as well and a different USB-C cable.

For best USB3 performance, use USB3 devices that support the UASP protocol (every device in the chain must support it....the drive enclosure as well as the hub/dock/adapters that drive enclosure connects). It is doubtful generic items or low end items will support UASP.

DevonStanczyk wrote:

Yes RAID 0 created with MacOS Disk Utility. Configured as RAID 0 one should expect 3x read and write speeds, which is true for me when I first mount them.

Single-stream RAID-0 read and write within the chunk size gets the speed and latency of the HDD, not 3x.

That 3x is an aggregate, and occurs when enough streams and enough chunks are all in flight. But not too many.

Whether those HDDs are overheating, or the HDDs have errors slowing I/O (see the “SMART” data), or your I/O pattern is smaller and ending up contending for fewer or one spindle, or those sleds are somehow throttling performance, or the HDDs are switching from elevator to direct seeks, or the USB itself is limiting your I/O, or whatever else?

An SSD will dust this configuration.

Might the outer HDD tracks be that much faster than the inner tracks?