Leaderboard

USB so-called DAS/JBOD/etc units usually internally use a SATA Port multiplier setup, and is likely the source of your issues. A SATA Port multiplier is a way of connecting multiple SATA Devices to one root port, and due to the way that the ATA Protocol works, when I/O is performed it essentially takes the entire bus that is created from that root port hostage until the requested data is returned. it also is important to know that write caching will skew any write benchmarks results if the enclosure uses UASP or you have explicitly enabled it. these devices perform even worse with a striping filesystem (like raidz, btrfs raid 5/6 or mdraid 5/6), and having highly fragmented data (which will cause a bunch of seek commands that again, will hold the bus hostage until they complete, which with spinning media does create a substantial I/O burden) honestly, your options are either accept the loss of performance (it is tolerable but noticeable on a 4 drive unit, no idea how it is on your 8 drive unit), or invest in something like a SAS JBOD which will actually have sane real world performance. USB isn't meant for more than a disk or two, and between things like this and hidden overhead (like the 8/10b modulation, root port and chip bottlenecks, general inability to pass SMART and other data, USB disconnects, and other issues that aren't worth getting into) it may be worth just using a more capable solution