On Wed, 2019-09-04 at 12:44 -0700, Gordon Messmer wrote: > On 9/4/19 8:20 AM, Patrick O'Callaghan wrote: > > For a USB drive it probably doesn't make much difference. Output will > > be buffered and speed is limited by the USB interface. > > If you aren't specifying a block size, the default block size tends to > involve more round-trips through the kernel and through the USB bus. In > that case, it isn't the USB interface bandwidth that causes slow > transfers, but the latency involved in each tiny request. > > I'd test this, but I seem to have left my bag of USB drives at home > today. :) > > Feel free to 'dd' a drive to /dev/null with and without a specified > large block size to demonstrate the difference. Maybe I'm wrong.
This is for an otherwise unused 4.5GB partition on an SSD. The CPU is an i7-3770 with 8GB of RAM: [poc@bree ~]$ sudo time dd if=/dev/sda2 of=/dev/null 8787968+0 records in 8787968+0 records out 4499439616 bytes (4.5 GB, 4.2 GiB) copied, 11.925 s, 377 MB/s 4.48user 7.26system 0:11.92elapsed 98%CPU (0avgtext+0avgdata 2092maxresident)k 8787968inputs+0outputs (0major+89minor)pagefaults 0swaps [poc@bree ~]$ sudo time dd bs=4096 if=/dev/sda2 of=/dev/null 1098496+0 records in 1098496+0 records out 4499439616 bytes (4.5 GB, 4.2 GiB) copied, 8.42924 s, 534 MB/s 0.59user 2.43system 0:08.43elapsed 35%CPU (0avgtext+0avgdata 2216maxresident)k 8788104inputs+0outputs (2major+89minor)pagefaults 0swaps (increasing the block size to 10MB gets 549 MB/s, i.e. almost no difference) Given that the (theoretical) speed of USB-2 is 60 MB/s, the drive would be a bottleneck in both cases. For USB-3 (10 or 20 times faster, depending on version) there would be a difference. poc _______________________________________________ users mailing list -- users@lists.fedoraproject.org To unsubscribe send an email to users-le...@lists.fedoraproject.org Fedora Code of Conduct: https://docs.fedoraproject.org/en-US/project/code-of-conduct/ List Guidelines: https://fedoraproject.org/wiki/Mailing_list_guidelines List Archives: https://lists.fedoraproject.org/archives/list/users@lists.fedoraproject.org
