Created attachment 165721 [details]
config.gz

For some reason direct writes to /dev/sd* file of an usb drive is very slow. On the over hand write to a filesystem on the same drive are much faster.

How to reproduce:
dd if=image.iso of=/dev/sdj bs=512k
or
cat image.iso > /dev/sdj

Same also for partitions (sdj1)

As bwm-ng reports average speeds to sdj are 200 - 300 KB/s with rare spikes up to 1.5 MB/s, so dd reports following:
dd if=/dev/zero of=/dev/sdj1 bs=512k count=100
100+0 records in
100+0 records out
52428800 bytes (52 MB) copied, 118.525 s, 442 kB/s

sudo dd if=/dev/zero of=/dev/sdj bs=512k count=100
100+0 records in
100+0 records out
52428800 bytes (52 MB) copied, 90.7202 s, 578 kB/s

For the filesystem write the write speed is up to 4 MB/s and dd reports 2.6 MB/s:
sudo dd if=/dev/zero of=/mnt/cdrom/zero oflag=sync  bs=512k count=100
100+0 records in
100+0 records out
52428800 bytes (52 MB) copied, 20.2205 s, 2.6 MB/s

dmes output for the usb drive:
 6011.409625] usb 5-1: new high-speed USB device number 14 using ehci-pci
[ 6011.526322] usb-storage 5-1:1.0: USB Mass Storage device detected
[ 6011.526499] scsi host24: usb-storage 5-1:1.0
[ 6012.529497] scsi 24:0:0:0: Direct-Access     Ut165    USB2FlashStorage 0.00 PQ: 0 ANSI: 2
[ 6012.529779] sd 24:0:0:0: Attached scsi generic sg9 type 0
[ 6012.530519] sd 24:0:0:0: [sdj] 7897088 512-byte logical blocks: (4.04 GB/3.76 GiB)
[ 6012.531222] sd 24:0:0:0: [sdj] Write Protect is off
[ 6012.531226] sd 24:0:0:0: [sdj] Mode Sense: 00 00 00 00
[ 6012.531957] sd 24:0:0:0: [sdj] Asking for cache data failed
[ 6012.531960] sd 24:0:0:0: [sdj] Assuming drive cache: write through
[ 6012.597750]  sdj: sdj1
[ 6012.666217] sd 24:0:0:0: [sdj] Attached SCSI removable disk
[ 6405.334070]  sdj: unknown partition table
[ 6469.679507]  sdj: sdj1
[ 6469.683263]  sdj: sdj1

uname -a:
Linux bigpc 3.18.5 #108 SMP PREEMPT Fri Jan 30 12:47:34 CET 2015 x86_64 AMD FX(tm)-8120 Eight-Core Processor AuthenticAMD GNU/Linux

Distro: gentoo
Kernel vanilla, self compiled (config attached)