Date: Wed, 28 Jan 2026 09:50:17 -0800 From: "Jin Guojun[VFF]" <jguojun@gmail.com> To: questions@freebsd.org Subject: Re: external USB disk: 400MB/s transfer, but get only 40MB/s Message-ID: <1a30cb21-5b80-42b4-b908-f2e0960b77e6@gmail.com> In-Reply-To: <aXoypNa5LYrleVfS@c720-1400094>
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On 1/28/26 08:00, Matthias Apitz wrote: > Hello, > > I did tests on two laptops (ASUS and C720) with different USB disks, and > writing with > > # dd if=file of=/mnt/file bs=8m > > a big file of many GBytes, see below. The disk was always announced in > /var/log/messages as a 400.000MB/s transfers devices by the driver, but > the measured transfer was only between 40MB/s and 50MB/s. The file > system on the USB disks was NTFS. > > Why is this? > > matthias > > > Jan 25 10:57:41 ASUS-150 kernel: da0: <WD Elements 2620 1023> Fixed Direct Access SPC-4 SCSI device > Jan 25 10:57:41 ASUS-150 kernel: da0: 400.000MB/s transfers > 82594649636 bytes transferred in 2005.078575 secs (41192725 bytes/sec) > > > Jan 25 11:57:26 ASUS-150 kernel: da0: <WD Elements 2621 1026> Fixed Direct Access SPC-4 SCSI device > Jan 25 11:57:26 ASUS-150 kernel: da0: 400.000MB/s transfers > 82594649636 bytes transferred in 1492.881431 secs (55325659 bytes/sec) > > > Jan 25 13:08:13 c720-1400094 kernel: da0: <WD Elements 2621 1026> Fixed Direct Access SPC-4 SCSI device > Jan 25 13:08:13 c720-1400094 kernel: da0: 400.000MB/s transfers > 46271915961 bytes transferred in 1081.647154 secs (42779122 bytes/sec) > 400MB/s is bus speed, the actual speed of USB drives are different, you need to check your disk models and specs. "WD Elements external hard drives generally provide reliable, standard performance for backups and media storage, with typical read/write speeds ranging between 100–130 MB/s for portable HDD models via USB 3.0." You are doing read and write on disks so that the performance is one half of the disk speed (or one half of average of two drives). Try following (assume your drive is ad0) dd if=/dev/zero of=/mnt/file bs=8m for testing writing speed dd if=/dev/ad0 of=/dev/null bs=8m for testing reading speed -Jinhome | help
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